G.M. Darrow, The Strawberry: History, Breeding and Physiology
AT AN ELEVATION of 9,500 feet to 10,500 feet in the volcanic soil of the mountain tops at Guachi, which is only a few miles south of the equator in Ecuador, the Indians have long raised one of the original strawberry varieties, the Ambato. The hundreds of acres planted to Ambato are on mountain tops in a very cold, dry climate, mostly above the areas where any other crops are grown, and even above where the Indians make their homes. Strawberries are grown also in many areas where semi-tropical crops are raised. They are raised extensively with citrus fruits and avocadoes in the arid areas of southern California, and they are raised in other humid semi-tropical areas, such as Japan, India, Colombia, Australia and Florida, where mangoes and pineapples (Plate 20-1), as well as oranges and avocadoes, are grown. For many years several hundred acres of strawberries have been grown under glass in the Netherlands, either in ground beds of strawberries covered by movable greenhouses from about January to June, or in beds which during the winter have cold frames and sash placed over them. On the coast of Japan, fancy strawberries are grown for the early market by terracing the very steep hillsides facing south and setting the plants between concrete blocks which cover the slopes. Instead of glass, mats protect the plants from winter cold and unseasonable frosts. In much of Japan, strawberries are a part-of-the-year-crop alternating with rice.
Because the cultivated strawberry is a hybrid of two highly variable octoploid species, it is possible to raise strawberries profitably under extremely different conditions: from irrigated desert to areas receiving 100 inches of rainfall; from sea level to elevations of 10,500 feet; from cold areas with -50' F. to the semi-tropics; under the continuous light of summer in the Arctic to the twelve-hour day under the Equator (Fig. 20-1); in glass houses and under glass or plastic covers, with concrete blocks, and with plastic ground covers as mulches; as a six-month crop, and as a crop occupying the same soil for hundreds of years. The same variety may be continuous-fruiting in one location and fruit for two or three weeks only in another. Other varieties may be everbearing under most conditions, so long as temperatures permit growth.
In part this remarkable range of adaptation is due to genetic variability, but it is also due to the high adaptability and plasticity of the strawberry plant itself. In the preceding chapter the structures of the strawberry plant have been described, thus providing a background for the subject-matter that follows: essentially, a description of the strawberry plant's relationship to its climatic environment. The most important and obvious climatic factors, as far as they affect the strawberry plant, are those of temperature and of daily light period, and it is these which are studied most extensively in this chapter. Other factors of varying importance are also treated. These are moisture (drought, heavy rainfall, high and low humidity) and the quality and intensity of light.
Maps showing the areas of the world where the strawberry species are native indicate the geographically wide adaptation of the strawberry. Fragaria vesca, assumed to be the basic diploid species, or similar to the basic species from which the octoploid strawberries were derived, has the widest distribution (see Map 8-1), while the hexaploid moschata has one of the most limited distributions. The parents of the cultivated strawberry display an extremely wide adaptation. F. virginiana and F. ovalls are found from the tundras of Canada and Alaska south to the very mild climate of southern Louisiana and New Mexico. Low temperatures in central Alaska where the strawberry is native reach -60' F. in winter and high temperatures are up to 100' F. In south central Louisiana the range is from O' F. to 110' F. For F. chiloensis temperatures range from -30' F. in winter to 90' F. in summer, along the coast of Alaska, and from about 230 F. to 1151 F. in south central coastal California. In Chile, where the Chilean is native, temperatures range from the no-frost areas near Concepcion to very low temperatures (-301 F. and lower) in the Andes Mountains. Cultivated strawberry varieties have not been developed for all these temperature extremes in which the wild kinds are found, but they are grown, even extensively, where the wild is not native-in southern California, in Florida, in parts of India, and in parts of Japan.
Low Temperature Effects
Effects of low temperatures on crowns are discussed on page 323 (Figs. 19-15 and 19-16). Relatively low temperatures may occur at any time from fall to spring. Angelo (1939) showed that the varieties tested completed their hardening at 32' F. within seven days, but that twelve hours at 68' F. (relatively warm temperature), then 12 hours at 32' F. produced greater hardiness than continuous exposure at 32' F. Gibson and Mastodon hardened the most and Aroma the least. Droughts seemed to lessen the rate of hardening. As protection from low temperatures, mulches could be applied after a week of severe frosts.
The ability of plants to harden is also important in winter and spring. In the North, the plants usually remain dormant all winter. They are still living, but at a very slow rate. If the temperature reaches 161 F. inside the plant of cultivated varieties, injury may occur, with the killing point about 101 F. Tests in Minnesota indicated that Burgundy was the hardiest; Beaver, Dunlap, and Howard 17 slightly less; and Gem and Catskill considerably less hardy to low temperatures. With a rise in temperatures above 32' F., plant functions increase rapidly, even though top growth may not be evident. However, after some growth has occurred in early spring, successive night frosts still can harden the plant. Severe frosts after such hardening do not injure the plants, but such frosts without previous hardening can cause serious injury, when the flower buds within the crowns may even be killed. Slight differences in the rate and extent of hardening of varieties in the fall, winter, or spring may be all important for a variety's success.
By using selections of the native western strawberry in crosses with cultivated varieties, the Cheyenne, Wyoming, Station has originated varieties that withstand temperatures of -40' F. without snow cover or mulch. Several varieties have been introduced for Manitoba, Alberta, and Saskatchewan that withstand the low temperatures of these areas, but with covering of mulch or snow. Likewise, similar crosses have resulted in varieties for central Alaska, which survive six weeks of continuous light in summer and winter temperatures to -50' F. Flowers of selections of the native strawberry of North Dakota have been found to withstand temperatures of 181' F. without injury, while flowers of ordinary varieties are killed at 28' F. (Darrow and Scott, 1947). Flowers and berries of the wild chiloensis of the Andes Mountains of Chile are uninjured by freezing temperatures in midsummer, and the berries can be harvested the day following such temperatures. As has been discussed elsewhere, older cultivated varieties such as Keens Seedling and Hovey and even the Albritton of today have the non-hardy Chilean in their ancestry. It is probable that, with two highly heterozygous octoploid parental species, hardiness will be an important factor to be considered in strawberry breeding for a long time to come.
Relatively little is known of the photoperiods for different species of strawberries. Different clones of all species may be expected to react differently (Figs. 20-2 and 20-3). At least some clones of the diploid F. vesca grow at all day lengths. In a greenhouse in Washington, D.C., they made greater growth under the longer daily light periods, but made a fairly vigorous growth under the shortest days of midwinter (Darrow and Waldo, 1934). F. moschata showed more response to long light periods than vesca, but less than cultivated varieties. The clones of F. virginiana (Fig. 20-4), which were studied, showed greater response to longer periods in winter than did moschata and vesca, while those of chiloensis showed the greatest differences, some growing well under the short days of winter and others showing response to longer light periods. Undoubtedly a greater range of selection of virginiana clones, as from Louisiana, Maryland, and northern Canada, would have shown equal increase in range of response.
Though the length of the daily photoperiod in general has the greatest effect on the growth and annual cycle of plant development, light intensity seems of special importance to the everbearing varieties, which grow less vigorously than ordinary June-bearing varieties after being given a low temperature rest period and extended light periods with artificial light. Everbearing varieties have been bred mostly in northern areas with longer days and relatively intense light. They grow poorly in winter (Fig. 20-5). The cultivated strawberry is so variable that even everbearers should show great variation in response to intensity.
Response of Varieties to Light and Temperature
The result of hybridizing two species with highly variable responses to temperature and photoperiods is the modern strawberry whose immense reservoir of genes makes possible the selection of varieties extraordinarily adaptable to local conditions. Consequently, the variety best adapted to a region is usually one selected in that region, with its particular complex of day length and temperature. Everbearing varieties are "long-day" plants forming flower buds under the long days of summer in northern states, more flower buds at seventeen-hour-days than at fifteen hours (Downs and Piringer, 1955) and relatively few at eleven and thirteen hours. Ordinary varieties are "short-day" plants forming flower buds when the days become short and the temperatures are low, in late summer and fall.
Runners are initiated only when the day length is twelve hours or longer and when temperatures are above 50' F. Downs and Piringer (1955) found runner production to increase with increasing day length, up to 15 hours. A temperature of at least 73' F. and a fifteen-hour day is effective for rapid runner production. Branch crowns in ordinary varieties tend to form most freely when the days are too short for runners and yet too long for flower buds. If plants are brought into the greenhouse as early as September 1 in Maryland and given artificial light to lengthen the daily light period, normal summer growth is induced and no flower clusters are produced, in most varieties at least. If plants are brought into the greenhouse after January 1 and exposure to cold weather, normal summer growth is induced, even without additional light (Fig. 20-3).
Varieties have characteristic temperature-day length responses which determine their regional adaptation. Southern varieties grow under short days at relatively low growing temperatures and need little or no rest period. Northern varieties grow very little under short days and, if first exposed to short daily light periods, require a low-temperature dormant period to break their rest period. The rest period is caused by one or more plant hormones produced by a short-day low-temperature complex and is broken either by long photoperiods at growing temperatures, or by temperatures near the freezing point, either in light or darkness. Plants of over 80 varieties and species when not given a full rest period in Maryland continued to flower and fruit until July and some until August, but plants of the same varieties given a low temperature rest period formed no more fruit buds at normal temperatures.
Response of varieties to light conditions during October and November in the field is considered somewhat indicative of their regional adaptation. During this period varieties adapted to southern states produce relatively large leaves with long petioles while northern varieties grow little or not at all. In southern states fruit clusters are large and branch basally when developed from buds formed under short daily light periods of fall and early winter, but late-winter and spring-formed flower buds usually develop into high-branching clusters (Fig. 20-6). Late falls with long periods of sunny days and cool nights are considered favorable to extensive fruit-bud development and to large crops in the following spring, but it also induces fasciation in varieties subject to this trouble.
At various latitudes with their differences in photoperiod, light intensity, and temperature, the plant responses are different also. At Ambato, Ecuador (see Fig. 9-1), almost directly under the Equator with days of just over twelve hours of light and where temperatures are below 60' F. much of the time, flower buds and fruit are produced by the Ambato variety the entire twelve months of the year. In Maryland it is barely possible to obtain a single flower on this variety. The Missionary is everbearing in Colombia at 4' north latitude and in Guatemala at 14' north latitude. In Mexico at 20' latitude at 4,000 feet elevation the strawberry is still everbearing, with a day length of about eleven hours in December and thirteen and one-half hours in June. Fruit production there begins in January and continues to June, when it is stopped, in part by the rainy season. In Florida, at 26' to 28' latitude, near sea level fruit production is heaviest from January through March, but continues to some extent to April and May, when it ceases with high temperatures. In California, from 31' to 37' latitude, along the coast with its cool climate, some varieties have a fruiting season of about two months but others initiate flower buds and produce fruit all summer and fall, for six to eight months, until stopped by low temperatures. In northern states from 40' to 45' and northward, where no flower buds are normally produced in the spring, the fruit production period is about three weeks.
Because the daily light period and the amount and quality of light received varies so widely for early, mid, and late winter, and because varieties respond so characteristically to additional light, the regional adaptation of varieties may be determined by their response to additional light and temperature in the greenhouse in Maryland (Darrow, 1929). New selections and varieties may be taken into the greenhouse September 1 and be given artificial light to make a fourteen-hour day; their growth can then be compared with that of standard varieties of each area. For still more specific tests for local adaptation, the light periods may be varied from thirteen to sixteen hours and the average temperatures from 55' to 65' F. at night and 70' F. during the day.
Studying the field growth of nine varieties-two southern (Missionary and Klondike), two middle latitude (Aroma and Gandy), and five northern ones (Howard 17, Dunlap, New York, Parsons, and Sample)-during the summer, Darrow (1930) concluded that daylight temperatures of about 73' F. were optimum and that high growth rates were made from about 68' to 79' F., while at daylight temperatures above or below this range, growth .9
rates were generally much lower. Went (1957) obtained the best leaf development of the Marshall at 57' to 63' F. He found that Marshall flowered and fruited throughout the year at 42' to 50' F. with continuous light, but with temperatures of 57' F. at a sixteen-hour day no flowers were initiated. Flowers were initiated,at all temperatures tried with an eight-hour day.
Experimentally lowering the temperature to a constant 60' F. under fif teen-hour days induces flower-bud initiation in June-bearing varieties. Likewise, shortening the daylight period to ten hours with a constant 70' F. temperature induced flower-bud initiation in all the varieties tested except Fairfax. Tests of many species and varieties indicate that both photoperiods and low temperatures are all-important in inducing flower-bud initiation in June varieties. Short days in the fall cause the strawberry to become dormant. In a test of 51 varieties none became dormant when placed in the greenhouse September 1 and exposed to electric lights, in addition to day light, until 10 P.m.
Fruit Development Period
The period from first blossoms to first ripe berries varies greatly with temperature, so that in areas of very low temperature, 60' F. or below, berries may take weeks longer to mature than in areas where much of the daylight temperatures is above 60' F. For most of the United States and Canada, the period from first blossom to first ripe berry is approximately one month and lessens as the season advances and the weather becomes warmer. Clarke (1931) in New Jersey found the five-year average for Howard 17 to be 31.2 days from first bloom to first picking. The period varied from twenty-nine to thirty-five days for different years, and for 20 varieties for three years the range for all varieties for all years was twenty-five to thirty-eight days, with an average of 32.1 days. Though the two varieties with the shortest periods were late kinds, late varieties in general were among those with the longest periods.
Wilson and Giamaloa (1954) in Louisiana tagged flowers as they opened and obtained the days to ripe fruit. Their total range for four varieties for three years was eighteen to forty-one days with most of the berries ripening in twenty-three to twenty-eight days. Temperatures were not given.
Though the fruit development period of different varieties seems to vary slightly, in most strawberry-growing regions the difference is probably not often important and certainly far less important than the time of first and full bloom in determining the season of ripening of varieties. In areas where temperatures after flowering are low, the period of fruit development would be exaggerated and might be important.
Vitamin C Content
The vitamin C content of strawberries is relatively high and varies with temperature, light, and variety. In general, conditions that favor the production of a large crop of large and high-flavored berries also favor a high vitamin C content. Highest flavor is obtained with sunny days and cool nights. Production of vitamin C on sunny days in general parallels the production of high sugar content; and vitamin C (as in the case of sugar) is retained when the nights are cool and respiration is low. Long days of sunlight increase the vitamin C content. In general, therefore, vitamin C may be some what increased in the longer days toward the end of the strawberry season, but it may actually be higher at the beginning of a season with cool nights and sunny days than at the end of a season with cloudy weather and warm nights.
The rate at which sugar accumulates in berries depends on the rate of transport of sugar from the leaves (governed by temperature and sunlight) and on the rate of loss of sugar by respiration (governed largely by temperature). Flavor is affected very little by soil type and that little only because soil type affects moisture supply, temperature, and plant growth. Flavor is also affected relatively little by fertilizers. However, in one test on sandy coastal plain soils in North Carolina, berries from plots receiving muriate and sulfate of potash, or potash plus nitrogen, seemed sour and lacking in flavor, while those from superphosphate plots were sweeter and better in flavor. The berries from the superphosphate + nitrogen fertilized plots seemed much the sweetest and finest flavored. Preserves made from the berries from phosphorus + nitrogen plots scored higher in flavor than those from control plots and still higher than those from the potash plots.
Went (1957) studied flavor of the Marshall at three different daylight temperatures, 62.6', 73.4', and 86' F., at three different night temperatures, 50', 57.2', and 68' F., and at three different light intensities of supplemental light added to eight hours of daylight. He concluded that sugar content was entirely a function of light intensity during the day and was independent of the day or night temperature, or photoperiod; and that acidity was a function of ripeness, the riper the berry, the less its acidity. At eight hours of daylight high aroma was produced, but the light intensity had to be fairly high-above 600 foot-candles. Most important, the eight hours of daylight had to be at a temperature of less than 59' F. for good aroma; even 50' F. for the eight hours was sufficient. The rest of the light period could be at a higher temperature. Finally, in a further test, two hours of 1,500 foot-candles of light at 50' F., or eight hours at 700 foot-candles was sufficient for high flavor.
Just as wine vintages of some years, due to favorable light and temperature, are much superior to those of other years, the flavor and character of strawberries of some years are much superior to those of others. The frozen and preserved berries reflect these differences. Because of the effect of climate in rating varieties for flavor, it is important to indicate the region where they were grown. Thus, Darrow (1947) rated Marshall 9 in flavor (10 = highest, 1 = lowest) in Oregon but only 3-5 in flavor in Maryland, while Howard 17 was rated eight in Massachusetts, but only six in Maryland.