G.M. Darrow, The Strawberry: History, Breeding and Physiology
IT IS VERY DIFFICULT to assess the value to breeders of the particular knowledge which is available concerning the morphology and physiology of the strawberry. This knowledge ranges from facts about minute states and processes to general descriptions of the larger complexes in which these facts exist. The difficulty lies in making decisions as to which facts are germane, and which facts are not. It would seem that much of this problem persists because choices have to be made concerning for what sort of people the facts are intended and what uses this information could have for them.
The material in this chapter is supplied either to answer, or to point the way to answering questions that strawberry breeders or investigators might find important. This chapter describes the strawberry plant and its fruit, presenting in an orderly way much of the information which intensive studies have supplied. Such an ordering of information provides a general structure for those who pursue specialized investigations, while, at the same time, the data of special study is included for those who wish it.
Figure 19-1 shows the plant of a strawberry, a bit of its root, its crown, leaves, and a fruit cluster with eight fruits, plus three flowers that did not set. There is a great deal more to a strawberry plant however than is revealed in this photograph. In all its parts it is one of the most changeable of all crop plants, and for this reason it is one of the most widely adapted and widely raised of all crops. The following pages contain a survey of some of our knowledge of the strawberry's root, crown, leaves, flowers, and fruit as they affect our understanding of what to breed for a variety.
Different species and varieties have very different-sized root systems, depending largely on whether they make runners freely and express their vigor in number of runner plants, or whether they make few runner plants and express their vigor in making large individual plants. Within limits, each form can be changed into the other-by restricting runners in the one case, and, in the other, by forcing more runners with nitrogen and by other means.
At the Iowa Experiment Station plants of different ages and with different root systems were compared. Succulent young plants with few lateral rootlets were not anchored as well as the older ones with a large root system, and were injured much more by heaving and by winter's cold. Figure 19-2 shows the two types of roots-the large primary ones which originate in the crown can be seen best in the youngest plant at the right, and, on the other plants, the small secondary lateral roots that make up the mass of the root system and which arise from the primary roots. There are usually 20 to 35, but there may be up to 100 or more primary roots and thousands of small rootlets in a good root system. Though the root is elastic, stretching and contracting as much as a centimeter (Kerner and Oliver, 1895), roots are often seriously broken when alternate freezing and thawing occurs. Mulching with pine needles, straw, or hay helps to prevent this sort of damage.
In general, root development is rapid in the fall and spring when there is not too great a demand for water by the leaves. Darrow (1929) found extensive root development as far north as North Carolina during December and January. Lineberry (1944) showed that ample available nitrogen greatly increased root growth during this period.
The primary roots push out rapidly from the crown and may become several inches in length before they branch. Van Tieghman and Douliot (1888) state that the primary roots arise only at the two sides of a median leaf trace, and White (1927) reports that they arise from the younger portion of the crown just outside the vascular cone. Proper depth of planting is the depth at which the plant formerly grew; if too deep, the crown may rot and the leaves fail to push through; if too shallow, the roots may dry out and be cold-injured in winter. After planting, both new primary and new secondary roots appear.
PARTS OF A ROOT. The wide adaptation of the strawberry is due partly to its variable root system, which is perennial, but perennial only in part. Along the beaches of the Pacific Coast, chiloensis grows in sand dunes and sandy beaches, and roots of these plants may live for many years. In eastern states, virginiana has many primary roots, most of which live for but one or two years. The leaves of virginiana die with cold weather and in the spring new leaves appear from whose base new roots can grow. In general, cultivated varieties follow the pattern of virginiana. Nelson and Wilhelm (1957) describe the development of secondary tissues in the fall or early winter in California. First, vascular cambial strands are laid down, next a complete vascular ring develops, and finally a cork cambium. These outer cambium layers develop into a polyderm, the outer part of which is composed of a protective covering of dead cells, and the inner part of living storage tissue.
The macroscopic parts of a root are (1) the root tip, which is the region of very active growth (Fig. 19-3), (2) the white rootlets which take in most of the water and nutrients, often through root hairs, and (3) the corky-covered region which absorbs some water but is mostly a conducting part. Though Nelson and Wilhelm found starch in primary roots, none was found by White (1927) in Maryland, but only in the secondary roots which he considered to be the overwinter storage organ. A very short exposure-less than a minute in sunny, dry air-may kill all roots not partially corked over. A plant with only old roots absorbs water so slowly that if planted in dry, sunny weather most, or all, leaves must be removed. Figure 19-3 shows a root tip and Figs. 19-4 and 5 are of cross sections of young roots. Figure 19-4 shows a branch root arising opposite a vascular bundle in a tetrarch primary root. The vascular bundles become heavily lignified and fill the center of secondary roots. White noted that the pericycle of primary roots is usually several layered and that of secondary roots few layered. The endodermis is usually distinguished only in very young roots. The root hairs mostly arise from fibrous rootlets, but some may arise from the primary roots.
SYMBIOTIC RELATIONS. Many investigators have found a certain type of fungus (Endogne sp.) in the roots of the strawberry and have suggested that it is mycorrhizal in nature (Figs. 19-6, 19-7 19-8), that is, the fungus may furnish nutrients to the host plant. The mycorrhizal fungus is commonly called Rhizophagus or the Phycomycetous mycorrhizal fungus. White (1929) noted the restricted distribution of chiloensis and its association with mycorrhizal plants as an indication of a possible special need for the fungi. The possibility of the mycorrhizal fungi becoming parasitic under some conditions also was noted by him.
LIFE OF ROOT SYSTEM. In general strawberry roots grow one year
and die the next, during the fruiting season. But when all flower
clusters are removed from a muture plant, most of its root system
does not die at fruitng time. Conditions are extremely variable
and some roots may die when a few weeks old while some at least
may become woody and live many years. Plantations of the Ambato
strawberry in Ecuador in a volcanic soil are very old and under
such conditions the individual roots live for many years
USE OF PLASTIC. Roots of the strawberry grow chiefly downward in well-drained sandy soils and a few roots may be found as deep as twenty-four inches. In clay soils they spread more horizontally. Ball and Mann (1927) found 90 percent of the roots in the top six inches of soil. In late fall, when the water table rises and the oxygen in the deeper layers becomes low, root growth is shallow. The oxygen content of the air in the soil where root growth is active is nearly that of the air above the soil, but where soil is water-logged, it may be as little as 1/10.000 the normal. Black plastic in Florida and clear plastic in Japan and southern California are used to cover the soil over thousands of acres of strawberries. The sun warms the soil, and the heat does not radiate so rapidly off the soil under plastic, so that with several degrees warmer soil for several months, more extensive root and crown development occurs (Plates 19-1a and b). Root growth continues much later in the fall than does leaf growth. The plastic serves also to conserve soil moisture, and prevent soil washing and weed growth.
WHERE PRIMARY ROOTS ARISE. Under favorable condi ions new primary roots grow from the crown at the base of each new leaf. About fix such roots grow from each leaf base, three from each side. However, if the crown of the plant is above the ground, the new roots may not start or may dry up before they reach the soil. If soil is drawn up around the crowns, new primary roots can grow to supplement or replace old roots If a new runner plant (Fig.19-10) lies on moist soil, roots quickly push out; if the soil dries out, the root tips die (Fig. 19-11). Root distribution around the plant occurs in a characteristic fashion because the leaf arrangement is in a 2/5 spiral and the roots come from the leaf bases (Fig. 19-12).
Top Growth of Plant
ANNUAL CYCLE OF GROWTH. In the North when a plant is set in the spring, new leaves appear with a bud in the axil of each leaf. During the summer some of the buds stay dormant, some develop into runners, and occasionally one develops into a branch crown. In the fall, depending on variety and conditions, the buds in leaf axils develop more often into branch crowns and into flower buds. In Howard 17, development of buds into runners ceases about the middle of August and bud growth from then until winter is by development of runners already initiated, by branch crowns, or by flowerbud initiation. Varieties like Howard 17 develop very large individual plants with many branch crowns that are especially productive, but make their runner plants relatively early in the season only. In northern states no additional flower-bud initiation takes place in the spring. In Maryland, a little spring flower-bud initiation occurs but it is of small value, if any. In eastern North Carolina and southward the days are short enough when spring comes, so that extensive initiation occurs and good fruit develops from winter- and spring-formed buds.
DEVELOPMENT OF PLANT AND RUNNERS. The development of a clone of the Howard 17 is shown in Fig. 19-13. The mother plant was set April 1, started its first runner May 27, had four runners on June 10, and by September 15 had a clone with 11 runners and 109 runner plants. The mother plant was not as large on September 15 as on June 10; its energy went to make new plants of the clone. A mother plant making as large a clone (Fig. 19-13) does not produce as much fruit as one which has had its runners kept off, or which does not normally produce many runners, such as plants of Midland or Earlidawn.
RUNNER PLANTS. The swollen end of a runner normally develops into a runner plant with roots from the under side and leaves and growing point at the tip. When plants are set in early spring in Maryland, runners begin to appear by the end of May and are produced throughout the summer and with most varieties until September and October. Plant propagators can often make counts of the number of runners about September 1 and expect to double the number by the end of the growing season. Shoemaker (1929) in Ohio and Morrow (1937) in North Carolina obtained the yields of runner plants that were rooted at different times in the summer. Plants of the Howard 17 rooted in June produced over fifteen times as many berries as did those rooted in late October and November, in Ohio. In North Carolina, plants of three varieties, Blakemore, Klondike, and Missionary, responded very much alike, with the June-rooted runner plants having three and one-half times as many berries as the November-rooted ones.
THE STRAWBERRY CROWN. The crown is actually the very shortened stem of the plant. In the sand dunes of the Pacific Coast the crown of F. chiloensis may become as much as two feet in length and the nodes several inches apart. Fig. 19-14b shows an actual crown with all but the woody vascular tissue dissolved out. Its structure is shown in Fig. 19-14a. This vascular tissue forms a cylinder with strands running spirally in both directions. The most striking feature is the leaf-trace connections to the vascular cylinder, so that if the roots are cut off any side the leaves of that side do not wilt, but are supplied by free cross-transfer from any point throughout the circumference. Thus, reduction of any part of the root system affects the plant as a whole, not just one side. The central part, the pith, is made up of large cells which are easily injured and turned brown by the formation of ice crystals in late fall and winter (Figs. 19-15 and 19-16). The narrow cambium layer outside the pith does not seem to be injured quite so readily by freezing as the pith, but if browned, the water and food conducting tissue of the plant is destroyed and the plant may die. Freezing injury is easily seen by cutting the crowns length wise. Uninjured pith at the center is entirely white. With slight injury to the crown, but not measurable in its effect on the plant, browning of the lower part of the pith occurs; with more severe injury, deeper browning, and with real damage to the plant, browning and blackening of the outer cambium occurs. Enormous differences in hardiness of strawberries exist, from the Ambato and the Red Chilean that are extremely susceptible to freezing, to the Dunlap, Ogallala, and the native strawberries of the far North, where temperatures of below-40 are withstood, even without snow cover.
THE STRAWBERRY LEAF. As stated above, and shown in Fig. 19-12, the leaves are arranged in a 2/5 spiral, each 6th leaf being just above the first, for maximum light exposure. Leaves of vesca are thin, those of chiloensis thick, and of virginiana intermediate (Figs. 19-17 and 19-18). The thin leaves of vesca are characteristic of the humid woodland plant which vesca is, while those of chiloensis, with thick cuticles and deep-set stomata, are characteristic of a dryland plant. Measurements of leaf thickness, at Corvallis, Oregon, were F. chiloensis, 220 m ; F. virginiana, 143 m ; F. vesca, 99 m ; Marshall, 163 m ; Blakemore, 192 m , and F. nilgerrensis, 163 m (unpublished).
The leaves of F. chiloensis are characteristically evergreen and live through relatively cold winters, those of virginiana die soon after severe frosts occur in the fall. Leaf characters of varieties and hybrids range from those like chiloensis to those like virginiana. Leaves of most varieties of the eastern United States are nearer those of virginiana. In the spring the embryonic leaves, enfolded by stipules, push out with warm weather, both by cell enlargement and to some extent by cell division, and in two to three weeks of warm weather reach full size. The individual leaves live for one to three months-those of Howard 17 in Maryland averaging 54 days with a range of 21 to 77 days. Though frequently killed by fungi, the leaves usually die in sequence. In Fig. 19-13 the mother plant, Howard 17, had a leaf area of 530 sq. cms. May 27, 1311 sq. cms. July 8, but only 880 sq. cms. on September 15. If runners had been kept off, the leaf area would have been many times this. Overwintering leaves may be scarlet, purple, or green without a trace of purple, or intermediate. Most leaves are trifoliate, but some varieties have four or five leaflets; usually the latter are most recently derived from or most closely related to F. chiloensis.
ANATOMY. The structure of a young leaf of F. chiloensis is illustrated in Fig. 19-17
(see also Fig. 8-12), showing the stomata, the air spaces, the large epidermal cells, and the thick cuticle. A view of the lower surface of a leaf with its many stomata is shown in Fig. 19-18. Their number, their placement, their response to high rate of water loss the thickness of the cuticle, and possibly the extent of the air space within the leaf, together determine the water loss and the drought resistance of the species or variety. Plants in humid atmosphere have much thinner, much larger leaves with fewer, but two to three times larger, vessels. In the shade there may be but one palisade layer with the green chloroplasts and in the open, two layers with the leaves much deeper green.
Measurements of the cell surfaces, within the strawberry leaf, that were exposed to air spaces showed from less than two times to over four times as much inner leaf surface as outer; a Marshall leaf having 4.4 to 1, chiloensis 4.1 to 1, and nilgerrensis ranging from 2.2 to 3.1 to 1. The water loss by the leaves, mostly by the stomata, in one test averaged 7.6 cc. per 100 sq. cms. and ranged from 5.3 cc. to 10.8 cc. depending on the variety, species, and weather condidons (Darrow & Sherwood l931). The strawberry has rnore stomata per square millimeter than many plants, 300 to 400 versus 246 for the apple, by way of example.
The amount of water a plant uses in a day is dependent on its leaf area, the extent of its root system, the water supply, the temperature, the intensity and duration of light period, and the humidity. On sunny days in August, a plant with ten leaves may use a third of a pint of water, but on cloudy days not over half as much. With loss of water greater than the intake, the plants wilt and if it continues for several days the older leaves may die. When wilting is this severe the smaller roots are in dry soil and die. Such a plant has a restricted root system for taking in water and nutrients, and a restricted top for manufacturing food. Many weeks may be required for such a plant to regain lost leaves or roots.
The chlorophyll of the leaf manufactures the food just as long as the sun furnishes energy and as long as water supplies the nutrients and carbon dioxide to the leaf and carries the food and waste away. When it is dark, the chlorophyll stops food manufacture. Also, if the supply of carbon dioxide from the air stops, as when the stomata close, food manufacture stops; so also when the nutrient or water supply ceases, or the sugar is not carried off, or the temperature is too high or low, or poisons injure the chlorophyll. Carbon dioxide in the air goes into the interior of the leaf, chiefly through the stomata. The extensive air spaces of the interior of each leaf make possible the circulation of the carbon dioxide, which forms about three parts per 10,000 of the air volume, to much of the plant. The carbon dioxide is dissolved in the water in the cell walls which makes it available for plant processes.
LEAF NUMBER AND YIELD. The number of leaves per plant in late fall is used as a measure of leaf area, which in itself is directly related to the number of fruits borne by the plant the next year. Many of the buds in leaf axils turn into flower buds, and usually, under average conditions within a variety, the more leaves, the more flower clusters. The older runner plants in general have the most leaves and greatest leaf area and produce the most fruit. A two-leaf plant in October may have one small fruit cluster of three to five berries, while a fifty-leaf plant may bear a quart of berries. Different varieties have different numbers of fruit clusters per crown and different types of flower clusters.
In the spring the embryonic leaves within the bud of each crown develop as soon as growth commences. By the time the first willows and narcissus are in bloom, one or more of the overwintering leaves have unfolded. Leaf growth and production of new leaves is rapid from then on.
DEVELOPMENT OF GROWING POINTS. A drawing of a plant as it appears at the blossoming season is given in Fig. 19-19. This plant developed from a runner tip during the previous summer. In the fall the growing point at the end of its short stem was transformed into a flower bud which, in turn, in the spring developed into the flower cluster shown. Because its growing point became a flower bud, no further vegetative development of the plant could take place except as new growing points appeared, and, except through such new growing points, no leaves in addition to those already initiated could develop.
In the plant shown, the three leaves P, Q, and R had already been initiated in the fall before a flower bud formed. In the spring these quickly unfolded and reached full size at the time the drawing was made. The oldest leaf labeled P is the lower one, the next younger Q and the youngest R. It should be noted that the broad petiole bases of each of these three leaves encircle the entire stem or crown of the plant. Before unfolding, these petiole bases together with their stipules cover and protect the growing-point. In winter this protection is especially important.
The plant illustrated in this figure had suflficient vigor in the fall to start the bud in the axil of leaf R. which is just below the terminal. This developed to such an extent that a fourth leaf S has now expanded. The base of this leaf, however, encircles only the bud A. Growing-points B and C may be seen in the axils of leaves Q and R. If growing point A also turned into a flower bud then growing-point B would continue the growth of the plant. This has actually occurred, as can be seen in the longitudinal section of the crown in the upper right-hand corner of the drawing. Bud A is seen to consist of a rudimentary leaf and the small flower buds of a second inflorescence. Still other growing-points can be found under the old leaf bases shown circling the crown below the leaf P. If the plant should be given exceptionally good growing conditions, growing-points B and C might both develop into additional crowns. If growing points develop during the summer they are runner tips, but in the fall with shortening days and lower temperatures they become flower buds.
Guttridge (1959) exposed one plant each of pairs of runner plants joined by runners to long daily light periods. The effect of the long light period on one plant of each pair was to increase petiole lengths and leaf size and to delay flower initiation in the other plant of each pair exposed to a short day, and he concluded that there was good evidence for the existence of a growth-regulating substance(s) that promoted vegetative growth and inhibited flower initiation and that this substance controlled the vegetative-fruiting responses of the strawberry. By cutting the tops off in August soon after the previous harvest and so removing the flower inhibition produced by leaves, yields were increased from 10 percent to about 100 percent.
RUNNERS. Runners are produced all summer from buds in the axils of new leaves, and in succession as the leaves develop. Guttridge suggests that the first axillary bud to differentiate in the spring becomes the first runWer. These runners are two nodes and two internodes in length. The more rapidly the plant grows the more runners are produced. Their size and final length depend on growth conditions and varietal characteristics. Long runners may be advantageous in placing the runner plants that develop at their tips at a distance from the mother plant. Leading varieties have runners of medium length. The two internodes making up the runner are on the average about equal in length. The bud at the first node is usually dormant, but may develop into a runner; such branch runners are usually much smaller than the primary runner; if the runner tip is cut off, a plant may form instead of a branch runner. In the spring, plants with no flower buds start leaves and runners before those with flower buds, and those with few flower buds before those with many. Plants producing runners as early as the first fruit ripens, usually produce less than those starting runners after harvest. The virginiana normally starts runners much earlier than chiloensis; and the runners of virginiana are much more slender than those of chiloensis. The runners of both are variable in length; both have long or short runners, depending on conditions of growth. Most runners survive until winter, though in the southern United States many die earlier. Runners of chiloensis tend to survive winter's cold, and often do on the Pacific Coast. Its runners may even become part of the stem of an old clone and live for many years (Fig. 19-9). Also, in mild climates, by forcing them to grow erect, the runners of ordinary varieties may live for a full year. (The ridiculous "climbing" everbearing strawberry!)
Because flower buds, runner buds, and branch crowns all arise as buds from leaf axils, intermediate structures might be expected and are known (Fig. 19-20). Under moist conditions the inflorescence may root at one or more of its nodes and even produce plants. Etter reported a variety having inflorescences that produced runners. Roots are often produced by inflorescences when the first internode is short and the first node has contact with soil under the leaves. F. orientalis in Maryland often had long inflorescences that rooted.
ANATOMY. The runner has a thick cortical layer surrounding a cylinder of very large vessels arranged in bundles separated by rays; the cylinder in turn surrounds a central pith of thin-walled cells. The whole structure is well adapted for carrying the large amounts of water and nutrients necessary to establish runner plants. Food and water may be carried freely in either direction, and the parent plant may support, or be supported by, a large clone of runner plants for months.
Gay (1857) first described the development of the runner series. At the runner tip a new plant is formed, the first leaf of which is a scale, or bract-like structure, but whose leaf traces arise in the crown of the new plant. The bud in the axil of the first leaf is well placed to receive water and nutrients, and is most likely to become a runner and to continue a runner series. It is, however, a new runner, not a continuation of the original runner. In everbearing varieties the bud in the first leaf axil may be a flower bud. Gay noted that in F. viridis of Europe the first runner may be two nodes long, but that the next runners are one node long.
FLOWER-BUD DEVELOPMENT. In Maryland, the first visible change of a strawberry plant's growing point into a flower bud is a broadening of the very tip as shown in Fig.l9-21, in this case occurring September 1. In seven to ten days this has developed as shown in Fig. 19-22. By October I the parts of flowers can be distinctly seen (Fig. 19-23) even with the unaided eye, and by November 20 most of the fall development has taken place (Fig. 19-24). Crowns of Howard 17 showing flower buds natural size are shown in Fig.19-25. In the spring the parts of each flower in a cluster enlarge but most of the differentiation has already taken place the previous fall. In southern England (Robertson, 1954) early-rooted runners began to form flower primordia as follows: Auchincruive Climax in early August, Royal Sovereign early to mid-September, and Sir Jos. Paxton in late September.
Development of Inflorescence
In Fig. 19-19, a basal-branching flower cluster is shown, the details of the branching at the base being illustrated in the longitudinal section in the upper right-hand corner. The method of development of such a cluster may be understood best by comparing it with a flower bud as it develops in the fall in the crown of a plant. Such a flower bud is shown in Fig. 19-22. Here the primary flower has clearly developed from the terminal stem growing-point while the secondary flowers developed from lateral buds. As shown in Fig. 19-22, the pedicel of the primary flower L does not often elongate to equal those of the branches, especially when the branching is basal.
In the strawberry buds arise only from leaf axils and the branches of the inflorescence arise only from bract axils, the leaves being modified into bracts. In Fig. 19-19, branch W arose from the axil of bract E and branch X from the axil of bract D. On branch W the flower M is terminal, but two branches had originated-the first in the axil of bract F and the second in the axil of bract G. Each of these branches has terminated in a flower N. but each has also branched.
In the inflorescence shown each branch has three internodes-a relatively long, a very short, and a second relatively long one. Thus branch W terminates with flower M. At W is a long internode, between the bases of F and G is a very short internode, and between flower M and bract G a second relatively long internode. Branch Y terminates in flower N. It has a relatively long internode at Y. a very short one between H and I, and another relatively long one between I and N. Though the inflorescence in the drawing has a very short peduncle (from V to the base of E in the longitudinal section) in most varieties of the strawberries a fairly long peduncle is usually produced. Typically, then, the primary axis and all of its branches have the long, short and long internodes. Where the peduncle is very short as in Fig. 19-19, the inflorescence is said to be basal-branching.
The inflorescences of many strawberry varieties, however, are quite irregular. Instead of two, several branches may start out at the base or at any point on the peduncle. The primary axis may then have one or two long internodes and several very short ones. Occasionally the primary axis may have several long internodes, but this is not common, at least in most varieties.
Flower Buds, Light, and Temperature. With most varieties, when in late summer and fall the photoperiod shortens to eleven to thirteen hours, the bud in a leaf axil instead of becoming a branch crown or a runner becomes a flower bud. Van den AA (1942) found that at least six days and up to 14 days of short photoperiods (six to twelve hours) were necessary for flower bud initiation, with six to eight weeks needed for the beginning of flowering in the Deutsch Evern variety. Commercial growers of Deutsch Evern shortened the days for four to five weeks from the last of May to July 1 to start flower bud intiation and to initiate enough flower buds for a crop. Experimentally Moore and Hough (1962) were able to obtain flower bud initiation in the Sparkle variety by shortening the day from sixteen to eight hours at 70 F day and 65 night temperature, after twelve to fifteen daily cycles. They had previously obtained initiation within twenty-one days. If large plants are dug September 1 and placed in a warm greenhouse with supplementary electric light for four to five hours the flower buds already initiated develop, but no new ones do. Studies by Waldo (1930) and others have shown that for the latitude of Maryland the first flower buds of Howard 17 are inidated in the latter part of August and additional ones are initiated later. All continue to develop until freezing weather. Actually, flower-bud initiation results when either the daily light periods are shortened, or the temperature is lowered. Along the coast of California with its cool climate, many varieties initiate flower buds all summer long. Such varieties are especially sensitive to temperature. However, up north, along the coast of Oregon and Washington where the temperatures are as cool, the light periods of midsummer are enough longer so that the same varieties do not produce as well. Likewise, southward at Irapuato, Mexico, the light periods are enough shorter in midsummer, and the elevation high enough for moderate temperatures, so that varieties like Elorida Ninety, Klondike, Missionary, and others make flower buds and produce a crop the year through. From Mexico south to the equator and to about 15 to 20 degrees south of that at elevations above 3500 ft., most strawberry varieties bear the year through. However, the elevation must be sufficient to make the temperature cool enough for the particular variety. Missionary grows and fruits probably best of any variety in the warmest temperatures under short days. Florida Ninety may do as well
in some tropical areas.
Flower buds initiate in the short days of fall, winter, and early spring whenever the temperatures are high enough. The map (Fig. 19-26) shows where in southern states initiation continues all winter, or intermittently all winter. In Florida it continues all winter, in southern Louisiana nearly all winter, and in eastern North Carolina it occurs when warm periods occur. In Louisiana and North Carolina temperatures are high enough for plant growth to occur often in late February and through March, when days are short enough for flower-bud initiation. What is called a "crown crop" is harvested, usually, in May. To increase this crop growers use plastic covers to warm the soil and increase root and crown development. Further north in Maryland, flower bud initiation in March, and probably in cool Aprils, may occur, but never enough to be worth harvesting; flower clusters from such buds are mostly sterile and those that do set, produce small berries. Along the coast of California, with low temperatures that are still high enough for flower bud initiation, commercial crops are produced by many varieties all summer.
Darrow and Waldo (1929) after testing about 140 varieties, suggest that the response to daily light periods, to temperature, and to a rest period is so characteristic that the regional adaptation of new originations could be determined by growing them in the winter in the greenhouse. Later (1934) they state that "the Blakemore has not grown as well as Missionary in the short days of low light intensity of winter and this may be interpreted to indicate that it will not succeed as well in Florida as the Missionary. In actual tests (and by grower experience) in Florida this seems to be borne out." Northern varieties do not grow under the short days of winter even under a high tenperature unless first given a rest period. Southern varietiey may grow so vigorously so late in the season in the North that few flower buds refomed, and they are relatively unproductive.
Downs and Piringer (1955) reported on responses of everbearing and June-bearing strawberries to photoperiods in summer. When grown at photoperiods of eleven, thirteen, fifteen, and seventeen hours, all three everbearers produced flower clusters but produced more under the longer light periods. Red Rich produced 5.0 clusters at thirteen hours and 2S.2 at seventeen hours They tended to produce the most runners at thirteen-hour photoperiods. Three June bearers produced their most runners at the fifteen-hour photo period. Climax produced a flower dusters at the eleven- and thirteen-hour photoperiods, but none at fiffteen- and seventeen-hour ones.
The response to daily light periods and to temperature is so characteristic that it has been suggested that by growing varieties in the winter in the greenhouse their regional adaptation would be indicated.
RATE OR FLOWER BUD DEVELOPMEWT. Waldo (1930) found that in Maryland different varieties inidated flower buds at different times, but that in general the degree of development at the end of the growing season was correlated with the time of initiation. A physiological test of initiation (referred to above) consists of placing plants in a warm greenhouse at stated intervals such as September 1 and 15, October 1 and 15, and lengthening the daily light period with artificial lights. Under these conditions only flower buds already develop flowers. In Maryland, in one comparison, took 15 days to develop from initiation to a stage showing primary, secondary and tertiary flower buds, and fifty-five days to fully developed buds, while for Dunlap the periods were six and thirty-five days. In Oregon the Marshall begins to initiate flower buds by September 1, but Ettersburg 121 not until about November 1. However, the latter was more evergreen and continued development at lower temperatures than Marshall (Waldo and Darrow, 1932).
FLOWER-BUD DEVELOPMENT AND SIZE OF PLANT. Davis (1922) first called attention to the relation of time of runner-plant formation to yield. Runnerplants formed in October produced less fruit than those produced in August. Morrow (1931) extended this study, and Sproat and others (1935) showed that the yield per plant was related to the number of leaves as a measure of leaf area; the older runner plants having the most leaves and the most fruit the following year. However, if the older plants are crowded, they may have few leaves and few fruits. Leaf number in the fall has been found a good measure of possible yield the following spring.
FLOWER BUD DEVELOPMENT AND MOTHER PLANTS. In both Florida and California plants are set and handled so that large individual plants having no runner plants are fruited. These produce maximum crops. In one study (Darrow, 1929) mother plants with runners kept off produced 132 fruits while mother plants with runners allowed to root to September 1, produced 43 fruits. This relation also holds true for northern regions, but so far it has been difficult in the Northeast to bring to fruiting a full stand of fully developed plants. Losses of plants from insects and diseases and weakening of piants from these and unknown causes have so far prevented this sytem from being generally adopted in northern states, even though ideal varieties for this system-Earlidawn and Midland-are available for some areas.
INFLORESCENCE TYPES. The number of crowns, inflorescen es, and flowers per plant in matted rows produced by 48 varieties at Glenn Dale, Md., averaged 1.6 crowns, 2.4 inflorescences, and 23 flowers under one set of conditions (Darrow, 1929). The inflorescence is really a modified stem and at each node of the inflorescence a bract replaces the leaf, while the bud in the axil of the bract develops into a branch of the inflorescence. The bract at the first node is often as large as a leaflet of a true leaf. Sometimes it consists of three leaflets. Bracts at the second, third, and later nodes are progressively smaller. The branching of a typical inflorescence has one primary, two secondary, four tertiary, and eight quaternary flowers. However, different varieties have different types of inflorescences and even any one variety may have many types depending in part on where it is grown. Each branch has three internodes: a long, a very short, and a long one. The effect of the very short internode is to make it appear that there are opposite branches at the nodes; however, the lower branch has the larger bract and the flower and berry on
FLOWER STRUCTURE. The flower arrangement of the strawberry is typically five-parted as shown in Figs. 19-27 and 28. In vigorous plants extra flower parts are common both in wild and cultivated kinds (Plates 19-2c to d). Under unfavorable conditions, as in poor light and at low temperatures, flower parts are suppressed in a regular pattern, first the stamens, next petals, then sepals, and epicalyx, and finally the pistils. When growth is slow at flowerbud development, the calyx and epicalyx may become foliar. How wide petals, sepals, and epicalyx open is in part genetical, in part environmental.
FLOWER TYPES. Flower types have already been referred to as male or staminate, perfect-flowered or hermaphrodite, and female or pistillate; all the higher chromosome numbered (hexaploid and octoploid) species have these types in the wild. Pistillates tend to set all flowers. Males set none, but the fertility of flowers with stamens grades from pure males with no pistils through all degrees of pistil development and fertility to those setting practically all flowers as in Rockhill (Fig. 11-7). Longworth (1854) reported that on the average not one in 10 flowers of hermaphrodites set fruit. Today hermaphrodites develop far more of their flowers into fruit than that.
Several records for the average set of flowers of perfect and imperfectflowered varieties have been made: in Minnesota, 67 percent vs. 72 percent; at Salisbury, Md., 66 percent vs. 88 percent; at College Park, Md., 64 percent vs. 82 percent; and at Glenn Dale, Md., 72 percent vs. 94 percent, 64 percent vs. 95 percent, and 61 percent vs. 90 percent in different years and conditions. At Glenn Dale, Md., one season, seven pistillate varieties averaged 31.7 fruits per plant and 1.7 flowers not set. Twenty-one perfect-flowered varieties averaged 10.2 fruits and 8.8 per plant did not set. If allowance is made for non-setting equal to that of the pistillate, then perfect-flowered varieties averaged about 70 percent fertile and 30 percent infertile.
Change of locale affects flower set also. At Glenn Dale, Maryland, the European variety La Constante set three fruits, and 39.8 flowers did not set; and the European White Pineapple set 1.2 fruits, while 18.4 flowers did not set. Also, Ettersburg 121, which succeeds in Oregon, set 0.2 flowers, while 17.2 did not set. The Howard 17, which replaced many varieties, set 16.4 and 2.7 did not set. It is not yet known why a variety like Ettersburg 121 that was so productive on clay soils in Oregon is so unproductive in Maryland and the same is so for the affected European varieties. Often the earlier formed flower buds on a plant develop flower clusters that set more fruit than do the later flower buds on the same plant.
Under some conditions varieties called pistillate develop a few stamens with pollen. Duchesne observed this. Meehan stated that in moist air and under favorable surroundings pistillate varieties often become hermaphrodite. Hovey and Crescent were apparently such varieties. Some
times varieties like Glen Mary that seem to have good stamens under some conditions do not set well by themselves-and set far better when cross-pollinated. Its flowers are functionally pistillate part of the time. By growing Dunlap in pure sand Gardner obtained 91 pistillate and only 2 hermaphrodite plants. Fig. 19-29 shows two flower clusters from one plant with pistillate flowers on one and hermaphrodite flowers on the other.
STAMENS. The stamens in multiples of five, commonly 20 to 35, are usually arranged in three whorls (Figs. 19-27 and 19-28), although Shaffner shows but two (Fig. 19-27>). They differ in size and length and are of a deep golden color when they contain good pollen (Fig. 19-28 and Plate 19-2a). When pollen degenerates at a late stage the anthers are not full sized and are pale yellow (Plate 19-2f). Poorly developed stamens-"staminoclia"-and stamens with good pollen may be found in the same flower (Plate 19-2e).
Pollen is mature before the flower or the anthers open, but usually the anthers do not crack until after the flower opens and the anthers dry a little. The anthers open at the sides (Figs. 19-30 and 19-31), sometimes under tension so that pollen is thrown onto pistils and petals; the pollen is at first heavy and sticky but later becomes dry and is carried by air currents. Pollen (Plate 19-1) remains viable for several days under ordinary conditions but if dry can be kept in a refrigerator for weeks. Valleau found abortive pollen in anthers of all 120 varieties he examined, the range being from less than 1 percent to 100 percent and the average being a little over one-third. No self-incompatability was found in cultivated varieties.
STERILITY AND PARTIAL POLLINATION. When the first flowers of perfectflowered varieties open and set well, but the later flowers only partially set or do not set at all, natural sterility is the primary cause. But if the first flowers develop into nubbins, and yet the later flowers produce good berries, the poor development is probably due to partial pollination (Fig. 19-32). Petals and calyx may cover some of the pistils and prevent pollination. Frost and insect injury may also cause nubbins and both as well as fungi may kill the flowers. Sterile flowers are soon infected by fungi which makes diagnosis as to the cause of non-setting often difficult.
Pollen Production and Climate
Those who have worked with the strawberry have learned that plants of the same variety vary in their pollen production in any one area, as well as in different areas. The first flowers to open in the spring of many varieties may develop good anthers with abundant pollen in some seasons, but almost none in other years. Pistillate flowers that have abortive stamens may under other conditions have good stamens. Castle (1904) noted that Crescent and Stirling Castle were pistillate in the United States, but perfect-flowered in England. Hovey, Longworth, and others noted this response to conditions, in the early 1800's.
PISTILS. The pistils are arranged in a regular spiral on the stem end of the receptacle and the seeds also, as may be noted by examining well-shaped berries (Fig. 19-34). The general structure of a pistil is figured by Winston as shown in Fig. 19-33. The stigma is rough and sticky. Strasburger (1939) pollinated flowers of F. virginiana with pollen of moschata and found typical fertilization twenty-four to forty-eight hours later. The pistil base, the achene, commonly called the seed, contains one ovary. The ovary contains one ovule. The achenes are attached on the underside to the receptacle by fibro-vascular strands as well as by the epidermal layer. The style is also on the underside of the achene near its attachment to the receptacle. The style commonly persists until the berry is ripe. The achene is fully developed several days before the berry is mature. Each achene contains a single seed; as described by Winston (1902), there is a many-layered outer hard pericarp, next a soft thin testa, and then a single-layered endosperm enclosing the embryo. The food is stored almost entirely in the two cotyledons in the form of protein and fat with no starches.
No true after-ripening period at low temperatures is necessary for most of the seed and seed may be sown as soon as the berries are ripe, or held in the refrigerator dry for years. However, quicker and more uniform, but no greater, germination is obtained if the seed is stored moist for a month in an ordinary refrigerator at 32 to 40°F. Seed of the different varieties and species vary greatly in speed of germination beginning in four days and continuing for three to four weeks. Henry (1934) reported that 25 C. (= 77 F.) gave the best germination; the amount of germination of hybrid seed ranging mostly from 58 to 94 percent, with one cross as low as 22 percent. Scott (1948) reported that treating with sulfuric acid for fifteen minutes hastened, but slightly lowered germination, and soaking in chlorine solution for eight hours also hastened germination (1955). Bringhurst and Voth (1957) in California obtained maximum germination after two to four months' storage moist at 32 to 34 degrees F.
THE BERRY, SIZE, POSITION, AND NUMBER OF SEED. After fertilization, the ovules develop rapidly. A hormone is produced quickly and the flesh around reach fertile seed starts swelling (Fig. 19-32). Valleau (1918) and Gardner (1923) both pointed out that the primary berry of a cluster is the largest and that the secondary, tertiary, and quaternary are progressively smaller (Figs. 19-1 and 19-34), in their tests averaging 80 percent, 47 percent, and 32 percent of the primary. Darrow (1929) reported 48 percent and 33 percent as compared with the primary for the average of secondary and tertiary of one variety. However, Darrow (1929) also reported that for basal branching clusters secondary berries were 91 percent of the primary by weight. In general, the berries on basal branching clusters average far larger than on high branching ones (Fig. 19-36). No basal branching was obsened in Klondike in Maryland, yet it regularly produces basal branching in the deep South. For some varieties high branching clusters indicate that they are being grown north of the area where they succeed best. Vigorous plants of varieties with many basal branching inflorescences bear more large fruit than similar plants of varieties with chiefly high branching clusters (Figs. 19-35 and 19-36).
Primary berries are not only largest and ripen first, but have the most seeds. Valleau (1918) found 382 seeds for the primary berry, and 224, 151, and 92 successively for the secondary, tertiary, and quarternary berries of one variety. Gardner (1923) reported an average for Gandy of 518 pistils for the primary, and 83 for the last flowers. Not only is it important for the breeder to cross the first flowers that open because they have far more seed than the last flowers, but also because the primary flowers are the most likely to set. As stated above, in nearly all varieties some of the later flowers and in some varieties most of the later flowers do not develop into berries. They are functionally males.
BERRY DEVELOPMENT. The primary flower, the first flower of those varieties that open in the spring, may have little pollen, less than later flowers of the same variety. However, the stamens are so placed that as they crack open they readily scatter pollen onto many of the pistils; and, as pistils remain receptive for several days, bees usually can get pollen from the later flowers to complete the fertilization of these primary ones. Pollination of all pistils of the flower are necessary for maximum berry size. Although blossoms may remain receptive for even ten days in cool weather if not pollinated, the reaction to fertilization is quite rapid, usually resulting in petal fall and the drying up of the pistils, sometimes within as short a time as twenty-four to forty-eight hours. However, flowers pollinated after the pistils have been receptive for one to several days develop into berries quicker than those pollinated as soon as the flowers open, and, as a result, they mature nearly at the same time as those pollinated when the pistils are first receptive.
GROWTH PERIOD-FLOWER TO RIPE FRUIT. At the beginning of the strawberry season in Maryland, the average period from flower opening to berry maturity is about 31 days, and at midseason, with longer days and higher temperatures, five to six days less. In Oregon for three years (1912,1913, and 1914) the average periods for many varieties were twenty-nine, thirty-three, and thirty-five days. Everbearing varieties that mature their fruit in twenty to twenty-five days in the long days and high temperatures of mid-summer take 60 days in the autumn. Though Kerner and Oliver (1895) gave 2671 as the total daily hour-degrees after January 1 to ripe fruit of F. vesca in Germany, for a true index it would be necessary to establish the relative effect of different degrees of temperature on plant and fruit development and how much those effects are modified by other conditions. Records at College Park, Maryland, indicate the variation in seasons. the effect of environment and of varieties:
The approximate ripening period in the United States is best shown by the map (Fig. 19-37) based on the time when shipments are made from each section. Along the Atlantic Coast for central and south Florida this period is four months, one and one-half months for eastern North Carolina, and 1 month for New Jersey. This difference in ripening period is based on the length of time of flower-bud formation in the different regions due to temperature and photoperiod differences and interaction.
BERRY GROWTH. Tschiercke (1886) has given the detailed structure and development of the berry (see Figs. 19-38 and 19-39). The flower base, which is the receptacle, develops into the edible part of the strawberry and consists of a fleshy pith at the center; next a ring of vascular bundles with branches leading to the acher es, the seeds; then a fleshy cortex outside the ring and an epidermis bearing a few hairs and connected to the superficial achenes. From the flower stage the pith cells increase in size, especially in length, and the narrow intercellular spaces already present increase in width, especially toward maturity. The cells of the cortical layer are similar to those of the pith, but are thinner walled and increase in size about twice as fast as the pith (Havis, 1943). The meristematic tissue, next to the epidermis, has no intercellular spaces, but the cells continue dividing after those of the pith and cortex cease. The reason for differences in shape and size of different varieties depends not only on their early development when flower buds are forming, but on the duration of cell division in each layer and on how much the cells enlarge and how much they pull apart. In F. vesca cell division ceases when the receptacle is the size of a pea, while in the garden strawberry cell division continues slightly longer. The cells of the cortex become 4 or 5 times as wide in the garden strawberry as those in vesca. Havis (1943) found little cell division from just before fertilization until maturity and concluded that only 15 to 20 percent of the growth after fertilization was due to cell division, and was not significantly different in the four varieties studied. The size of air space affects berry size also. The air spaces in vesca are so large that the berry is extremely light weight compared to large-fruited varieties. Even large-fruited varieties vary greatly in density, and a few varieties like Fairfax have so little intercellular air space that some berries may sink in water. Redheart also has almost exactly the specific gravity of water. Berries selected for canning have less air space and less oxygen inside and keep their color and flavor better than most varieties.
The length and diameter of the developing fruit was measured at Willard, North Carolina, to establish a measure of the size at different stages, from the time of pollen shedding to the stage of full ripeness, as shown in the table below:
FROST DAMACE. Frosts may either kill the flowers outright or injure them so as to cause nubbins or misshapen berries. When a flower is injured by cold the pistils, which are the tenderest part, are killed first. If most pistils are killed, a nubbin with a wisp of dead pistils may result, or, if killed after fertilization, the embryos do not develop and a seedy spot on the berry results
AUXIN CONTROL OF GROWTH. Nitsch (1950) removed all achenes from fruits four days after pollination and growth stopped. Growth also stopped when the achenes were removed seven, twelve, nineteen, and twenty-one days after pollination. Although growth was stopped when removed at twenty-one days, the berries turned red at the usual time, twenty-six to thirty days after pollination. When one ovule was fertilized the flesh around it developed (Fig. 19-42); when three ovules were fertilized three areas developed; when only a ring of ovules was fertilized only the ring of flesh developed. When seeds were removed and an auxin like beta-naphthoxyacetic acid (100 p.p.m. in lanolin paste) was applied nine days after pollination, the berry grew to almost full size and ripened like normal berries. Nitsch (1949) found the greatest auxin activity and the greatest amount of auxin in the achenes twelve days after pollination and separated seven different growth substances natural to the berry; the most prominent being indole-3-acetic acid. No auxin was found in the receptacle. Thompson (1963) found that when berries were eight days old, 0.1 percent of 2-naphthoxyacetic acid was necessary for effective growth in replacing achene effects; after ten days, 0.03 percent; and after twelve days, only 0.003 percent was necessary-less than 1/30 of that required only four days earlier. He noted also that the embryo sac divided slowly at first, but between seven to ten days, began to grow rapidly. Zielinski and Garren (1952) applied 50 p.p.m. beta-naphthoxyacetic acid to half-grown Marshall and obtained 32.7 percent increase in size, indicating that under some conditions additional auxin may be effective.
Berry Shape in Different Climates
BERRY SHAPE DIFFERS WITH CHANGES IN CLIMATE. Klondike is almost globose in eastern United States, but is long-conic with a neck in southern California. The shape of many varieties is more uniform in the Northeast and on the Pacific Coast than in the South. The Ambato is long-conic at Ambato, Ecuador, but short, globose-conic in Maryland (Fig. 9-1).
The tip of the berry may be meristematic under some conditions, especially when growth is checked in periods of cool weather. Leaves or even a plant may grow from its tip (Fig. 19-43). The development of bracts instead of seeds is usually due to the aster-yellows virus disease.
BERRY SHAPE. The common shapes of strawberry varieties are shown in Fig. 19-44. Most berries of Klondike are oblate, of Shasta globose conic, and of Florida Ninety long conic with a neck. The general shape of the berry is indicated to some extent by the shape of the receptacle in the small flower bud the previous fall. The shape of the berries on the cluster depends on their position (Figs. 19-45 and 19-34), the primary berry tending to be irregular and broad wedge-shaped, and the later berries much more uniform. However, the shape may be affected by conditions in the fall and during the growth of the fruit in the spring. The multiple-tipped shapes of Marshall shown in Fig. 19-46 were probably caused by cold, dry weather in the autumn affecting the flower buds. Irregular shapes of berries also result from conditions affecting the fall growth.
FASCIATION. Fasciation (Fig. 19-47) results from favorable growth conditions in late fall when days become too short for normal development of the particular variety and is most serious from Eastern Virginia southward. The flower bud broadens and in severe cases no fleshy fruit develops in the spring at all and the plants are barren. All gradations are found from a slightly flattened stem of the primary berry (Fig. 19-47A) to a coxcomb berry shape as in Figure 19-47B (see also Fig. 21-15). Fasciation is a varietal characteristic, some varieties, like Missionary and Blakemore, never fasciating. Many varieties succeeding in the North cannot be grown in the South because of this trouble and all Southern varieties must be quite resistant to fasciation. In the spring, insects, disease, lack of pollination, clrouth, humidity, and frost all affect the shape of the berry.
IRRIGATION AND BERRY SIZE. Irrigated berries are not as firm as non-irrigated ones, though the difference is not usually great. The average water content of several strawberry varieties in Maryland ranged from 92.6 percent to 89.7 percent in 1929; 91.3 percent to 88.1 percent in 1930; and 92.5 percent to 91:8 percent in 1931. The primary berries normally had the highest water content and the later berries slightly lower water content. In 1929, for four successive pickings the average water content of all varieties was 92.6, 92.2, 90.9, and 89.7 percent (Darrow, 1936). In 1925, in Maryland, in a period of low humidity and very high temperature the ripening berries even dried on the plants to resemble raisins, a condition seldom observed. Because most of the root system is in the top six inches of soil, and because the leaves develop rapidly in the spring and transpire water freely, they pull water from berries in very dry weather and cause smaller than maximum size. Irrigation to keep berries growing to maximum size may be needed as often as every three to seven days, if sufficient rain does not fall.
FIRMNESS OF BERRY. Though breeding has resulted in much firmer berries, little is known of the structure or chemistry of a firm berry. The cell walls of firm berries like those of Blakemore, Albritton, and Dixieland, do not pull apart as much when growing as do the cell walls of softer berries. As a consequence, such varieties do not become as hollow and thus have less air spaces inside. Their epidermis is tougher. Low water-soluble protein is usually associated with firm berries (Sistrunk, 1962). The larger berries of a variety are softer than the small ones, for they have a higher moisture content and probably have a different skin composition. High rates of application of nitrogen may produce larger berries. Temperature and humidity both affect firmness greatly. Sparkle is considered to be a firm berry in the northern United States, but it is too soft to be a market berry in the warmer climate of Maryland. In a very warm humid period berries of some varieties puff and are unmarketable (Fig. 19-48)Rains in cool weather, unless long continued, do not greatly soften berries, but in warm rainy weather berries may absorb water through the skin and become soft. Darrow (1932) reported on tests in North Carolina that where leaf growth is greatest the berries are softest, presumably because of the shading effect of the leaves. Berries soften as the temperature rises. Thus, when the air temperature at 8 A.M. was 74°F., inside a shaded berry it was 73°F. and inside berries in the sun it was 91°F. At ten o'clock with an air temperature of 77°F. berries in the shade were 84°F. and from 100°F. to 108°F. in the sun. Pressure tests indicate that for each rise of the temperature by 21.7°F. the toughness of the skin is doubled. Not only is the skin more resistant to injury at lower temperatures, but the respiration rate is lower, fungi that cause rots grow more slowly, and the berries keep much longer. Popenoe (1921) has told of the remarkable firmness of the Ambato variety (Fig. 9-1) which grows slowly-in the cold arid climate of highland Ecuador with a day length of just over twelve hours. The berries could be carried in large 26- to 33-pound boxes on mule back for many miles with no apparent injury. When grown in humid climates, the Ambato is no firmer than ordinary sorts. In Chile, with its cool, dry climate, the White Chilean is as firm as the Ambato. Various means have been devised to measure firmness but most breeders still judge firmness by feel-the friction of the thumb rubbing the surface of the berries in the field until the skin breaks, and by shipping tests. Firm-fruited wild plants have been found in both ovalis and chiloen is and relatively firm ones in virginiana. These have yet to be utilized by breeders. The Wilson of the United States and La Constante of Europe were relatively firm l9th-century varieties. Neunan, Hoffman, Missionary, and Klondike, of earlier varieties, were also relatively firm; Blakemore and Tennessee Shipper are much firmer and Massey, Albritton, and Dixieland are among the firmer large-fruited ones. Recently when grown under short days in the South, Klonmore, Headliner, Dabreak, and Florida Ninety have proved to be firm.
DESSERT QUALITY. The dessert quality or flavor of strawberries is dependent both on inherited characteristics and on how they are affected by conditions. Caldwell and Culpepper (1935) concluded that best flavor was dependent on the sugar-acid-tannin ratio combined with volatile esters that make up aroma. As the berry matures, the acids decrease and sugars increase. When unripe berries lose chlorophyll and turn whitish, they have maximum water content. From the white stage on, sugars increase rapidly and continue until the berries are fully ripe. Acidity declines rapidly and astringency slowly.
The tests of Went (1957) on Marshall also serve as a guide to the development of aromas. Different varieties have been originated that are best in different regions; that is, that develop high flavor under the average weather conditions of those regions. Although the development of flavor of some varieties may follow that of Marshall, some may not. Suwannee has high flavor under a very wide range of conditions. Fairfax has very high flavor under a much narrower range of conditions and probably follows more closely the general development of flavor in Marshall, although the latter has high flavor under a wider range of conditions.
DESSERT QUALITY OF RIPENING BERRIES. Smith and Heinze (1958) studied the development of berries from quarter-colored to fully colored. Berries of three kinds left to mature on the plants increased their size 23 percent to 57 percent, from quarter color to full color, depending on variety, and 12 percent to 23 percent from three-quarters color to full color. Quarter-colored berries of four varieties were harvested and stored until fully colored and rated 72 percent as good in flavor as those fully colored when picked. Also, the quarter-colored of two kinds stored until fully colored averaged 78 percent as much sugar and 33 percent more acid than the fully ripened ones.
Austin and others (1960) found that even greenish-white to 10 percent pink berries of the Sparkle developed full color at 85F. in forty-eight hours, 90 percent of full color at 65F. in ninety-six hours and did not develop good color at 55F. The flavor was considered as good as of those ripened on the plant.
VARIETIES FOR PROCESSING. The chief uses for strawberries have been freezing for later preserving and for dessert use. Varieties for preserving should be medium to light red to the center, and for packaging for dessert, varieties somewhat deeper red should be used. Varieties with white soft flesh and low in acid are unsatisfactory for processing. They should be firm, sub-acid, and aromatic. The varieties used are listed under Sources for Superior Quality.
CAPPING. Berries of the wood strawberry, vesca, separate from the cap or hull when ripe and are always picked without the cap. Many of the wild Virginian cap easily, although most do not. The native commercial chiloensis of Chile mostly cap with ease, and in picking the caps are left on the plant. Yet the cultivated Chilean of South America usually cap with some difficulty. Jucunda, an old variety of England, and still grown in Europe, is regularly picked without caps. Miss Kronenberg has used it successfully in breeding to obtain Juspa and Gorella, but they do not pick without caps as easily as Jucunda. In drought periods berries are more easily picked without caps than in moist weather, and under irrigation in the Pacific Coast berries of Marshall, Northwest, and others are harvested without caps because of this response (Fig. 19-49). The varieties having the best capping qualities are listed on page 394.
Vitamin C. Varieties differ greatly in their vitamin C content and, in one study, ranged from 39 to 89 units (mg) per 100 grams (Ezell, 1947). The average for strawberries has been estimated at about 60 units with Catskill having a high content, Midland about average, Blakemore slightly low, and Aberdeen very low, about half that of Catskill. Breeding for much higher vitamin content was shown to be possible by Darrow and associates (Ezell, 1947). Berries on the plant ripeuing in the sun have higher vitamin C content than those ripening in the shade. After picking, berries injured by bruising
tend to lose vitamin C rapidly. Uninjured berries lose no vitamin C for at least three days when stored at 40 to 75°F. Uninjured half-red berries increase in vitamin C but not so much as if they were ripened on the plant. Capped berries at about 75°F. lost between 10 and 15 percent of vitamin C in twenty-four hours and between 85 and 90 percent in forty-eight hours.
FLAVORS For the esters or volatile compounds, Winters (1964), states that about 35 odorous substances have been isolated so far, but that it still is impossible to reconstitute a really fresh flavor. Only the chief and most stable compounds have been isolated. When berries are crushed, the finest flavor is developed in one minute; in five minutes a noticeable change has occurred, and in ten minutes a marked change. By using low-temperature steam distillation a distillate (12 to 15 percent of fresh weight) is obtained which by dilution yields natural strawberry flavor.
Teranishi and associates (1963), of the U.S. Department of Agriculture at Albany, California, have recently reported on studies of volatile substances from strawberries. A direct chromatographic method of analysis of vapor from a single strawberry was developed to study the more stable components. The report lists 24 compounds of zone A of the chromatogram of strawberry oil which had over 150 components.