Fig. 1.4. Accession means of the number of days from seed germination to the first male and female flowers on plants of 30 cultivars and eight wild populations of Cucurbita pepo. (Data from Decker, 1986.)

      In their search for early staminate flower production in cucumber, Walters and Wehner (1994) examined 866 cultivars, breeding lines and plant introduction accessions. Earliness was normally distributed for the cultigens and ranged from 26 to over 45 days from planting to first staminate flower.

      Sex expression in the Cucurbitaceae is controlled by environmental as well as genetic factors. Unfavourable growing conditions (e.g. lack of water) can reduce flower production. In general, female sex expression is promoted by low temperature, low nitrogen supply, short photoperiod and high moisture availability, i.e. conditions that encourage the build-up of carbohydrates. These environmental factors influence the levels of endogenous hormones (especially ethylene, auxin and gibberellic acid), which in turn influence sex expression. Most studies examining this role of hormones have been conducted on cucumber, followed by squash, melon and watermelon. See Chapter 7 for details on the use of exogenous hormones to control sex expression.

      Temperature affects anthesis and, in squash, the length of time that a flower is open. Squash pollen is released at temperatures as low as 10°C, whereas cucumber, watermelon and melon flowers require higher temperatures for anther dehiscence. Warmer temperatures cause anthesis in squash to occur earlier in the morning. However, high temperatures (ca 30°C) accelerate squash flower closing, causing the corollas to close by mid- to late morning.

      Fruit development

      Pollen tube growth and ovule fertilization stimulate ovary enlargement. Subsequent fruit set depends on the quality of pollination (i.e. having enough ovules fertilized) and is affected by the presence of already developing fruit, leaf area, daylength and other environmental factors. Fruit on a plant may inhibit the production of additional pistillate flowers and the development of subsequent fruit. In cucurbit crops such as melon, watermelon and squash, fruit-thinning will allow fruit that remain on the vine to grow larger.

      If fruit are not developing on a plant by the end of the growing season, then the last group of ovaries may develop parthenocarpically. Parthenocarpy in cucumber and squash is promoted by low temperature, short daylength, old plant age and genetics. Some cucumber cultivars have genes for parthenocarpy and will set fruit without pollination. Parthenocarpic cultivars are common in the greenhouse trellis type, the Middle Eastern greenhouse (Beit Alpha) type and, more recently, field slicers and field pickles. Parthenocarpic pickling type has become popular in northern Europe and most of the USA, due to seedless fruit and higher yield. Field production of parthenocarpic cultivars depends on isolation from conventional cultivars having staminate flowers, as well as the exclusion of beehives from the area.

      Anderson (1894) determined that a developing squash fruit gained weight at an average rate of 1 g per minute. The greatest weight increase was at night. The growth rate for cucurbit fruit is influenced by exogenous conditions (e.g. higher temperatures and greater light exposure increase the growth rate) as well as by endogenous plant conditions, such as the presence of other developing fruit, which retards growth.

      Several studies on the inheritance and development of fruit shape in squash (C. pepo) were conducted by Sinnott (1932). He reported that fruit shape is evident in the shape of the immature ovary, with ultimate shape affected by both genetic and environmental factors. Also, fruit that are set first may be shaped differently than those set later, the difference being evident in the shape of the ovaries.

      Immature ovaries are usually green, although those of squash cultivars with gene B may be yellow. Fruit of various cucurbits, including luffa and bottle gourd, remain green until fruit senescence, at which time they turn tan or brown. Other cucurbits develop rind coloration changes during maturation. In these, chlorophyll depletion reveals the presence of additional pigments after pollination. For example, the green fruit of many squash cultivars become yellow or orange as they age. Colour changes usually begin at the blossom end of the fruit. In most wild and cultivated Cucurbita, rind patterns, such as stripes or mottling, are lightly visible on the ovary, becoming more distinct soon after pollination. However, the white fruit of ‘Mandan’ (C. pepo) reach almost full size before the dark green markings appear. Fruit markings persist at maturity for some squash cultivars, but fade away in senescent fruit of others.

      In young melon fruit of the Cantalupensis Group of cultivars, rapidly dividing cork cells develop below the epicarp. Near maturity, this growth breaks through to form a network of grey corky tissue covering the rind, as is evident in the netting of muskmelon cultivars.

      A unique case of adaptation to a particular ecological niche is exemplified by Cucumis humifructus Stent. This African species has a geocarpic fruit, similar to that of peanut. After flowering and setting fruit above ground, similar to other Cucumis species, the developing fruit is thrust downward and completes its development several inches below the soil surface. In its native land, the African anteater or aardvark (Orycteropus afer) consumes the subterranean fruit. It is a symbiotic relationship, with the aardvark using the fruit as a source of food and water in the arid area where the plant often grows, and providing a means of seed dispersal for the plant. C. humifructus is called aardvark cucumber because of this relationship, but it is more closely related to melon than to cucumber.

2

       CROP EVOLUTION AND DIVERSITY

EVOLUTIONARY HISTORY

      ‘Cucurbits are weeds waiting to become crops.’ While this anthropomorphic statement may be an over-simplification of crop evolution in the Cucurbitaceae, supporting evidence can be seen in the vast array of valuable plant products found within the family. Aboriginal plant gatherers were probably attracted to some of these products, particularly the relatively large, long-keeping and sometimes showy fruit. After fruit were taken back to camp, seeds that were purposely discarded, accidentally dropped or partially digested found new life on rubbish heaps, settlement edges or other disturbed areas near camp. Eventual recognition of the value of the resident cucurbits led to their appreciation, horticultural care and further exploitation. Finally, seeds and, more rarely, vegetative propagules were carried by and exchanged among migrating bands of these incipient cultivators, gradually turning the earliest cultivated cucurbits into domesticated crops.

      It was not only because cucurbits were wanted by human gatherers that they became domesticated. Certain physiological and genetic characteristics generally associated with weeds, or colonizing species, allowed cucurbits to adapt to human habitats. Fast, indeterminate growth, developmental plasticity in response to environmental conditions (especially regarding sex expression) and genetic diversity at the genomic, chromosomal and gene levels enabled these cucurbits to continue their survival through the coevolutionary relationship that humans call domestication. In turn, this relationship brought many changes to