may be only one seed in the fruit, as in chayote, or more typically, tens to hundreds of seeds. Cucurbit seeds, which are rarely winged, are usually flat. The seed coat encloses a collapsed perisperm, an oily embryo and little or no endosperm. The tiny endosperm is consumed during seed development. Two cotyledons make up much of the contents of the seed. Seeds of some cucurbits are enveloped in a false aril of placental origin; in bitter gourd this sarcotesta is red and fleshy, attracting birds as seed dispersal agents.
Seed size, shape and colour vary greatly among the cultivated cucurbits (Fig. 1.3). The largest unwinged seed is that of Hodgsonia, measuring about 7 cm long. The nearly spherical seed of Bryonia is sometimes less than 3 mm in diameter. Cultivars within a crop, including watermelon and squash, may differ considerably in their seed sizes and other seed characteristics. Depending on the cultivar, watermelon seeds are white, tan, brown, black, red, or green. They can also have patterns on them, referred to as dotted, rimmed (dark seed margin), tipped (dark seed tip) or clump (dark seed centre).
Fig. 1.3. Seeds of the Cucurbitaceae. The largest seed (Fevillea cordifolia) shown here measures ca 53 mm across. Also shown are seeds of Cucurbita, Siraitia, Marah, Momordica, Sicana, Trichosanthes, Luffa, Lagenaria, Echinocystis and Melothria (the smallest seeds).
The complex seed coat anatomy of cucurbits has been well studied (Singh and Dathan, 1990). The testa develops from the outer integument of the anatropous, bitegmic, crassinucellate ovule. The inner integument degenerates in fertilized ovules. The mature seed coat in the subfamily Cucurbitoideae consists of an epidermis, hypodermis, main sclerenchymatous zone, aerenchymatous zone and inner parenchymatous or chlorenchymatous zone. In the Zanonioideae, the sclerenchymatous layer is poorly or not differentiated from the hypodermis, and the well-developed aerenchyma has distinctive lignified thickenings. Within each subfamily, there is further anatomical diversity among genera.
Seed germination
Seed maturation usually continues until the fruit starts to yellow with senescence. Some seed producers store mature cucurbit fruit after harvest to permit the seeds to develop further. However, if seeds are left too long in some fruit, they may germinate in situ. In a few cucurbits, such as chayote, germination is naturally viviparous.
Seed dormancy, which is common in various wild species, is not usually a serious problem in the major crops. Dormancy can occur in freshly harvested seeds of some cultivars, but this dormancy can be broken by a month or more of after-ripening, i.e. storing seeds in the fruit after harvest. Light and low temperature (< 15°C) are strong inhibitors of germination for many species. Under amenable conditions (e.g. low light levels, temperatures of 25–30°C and adequate but not soaking moisture), germination takes 2 days to 2 weeks if the seeds are not dormant. See Chapter 6 for experimental studies investigating seed germination physiology.
Plant growth and movement
Most cucurbits grow rapidly in warm weather, with stem extension growth outpacing leaf development in the tuberous perennials. In a single growing season, a wild buffalo gourd plant produced 360 shoots covering an area 12 m in diameter with a total vine length of over 2000 m (Dittmer and Talley, 1964). Among the annuals, bottle gourd stems can elongate up to 60 cm in 24 h, and wild cucumber (Echinocystis lobata), which is adapted to the short growing seasons of southern Canada, is considered one of the fastest growing vines. Holroyd (1914) reported that a single annual squash (C. pepo) plant produced 450 leaves on a vine measuring 43 m. Cucurbit root growth is also rapid, occurring at a rate of 6 cm per day for squash when conditions are favourable. Elite cultivars of pickling cucumber will go from seed planting to first harvest in 39 days if grown at the optimum growth temperature (32°C).
In large-fruited vining squash plants, total leaf area increases exponentially throughout the season until fruit set creates a large reproductive sink and vegetative growth is suppressed. During the period between flower primordia initiation and the start of fruit development in West Indian gherkin, differentiation of new vegetative organs decreases, the growth rate of existing vegetative organs increases, and water and nutrient intake drops; soon after fruit development begins, vegetative differentiation and water and nutrient intake resurge (Hall, 1949).
Biomass productivity differs among species and cultivars and is influenced by cultural practices (e.g. planting density, irrigation, fertilizer application) as well as local environmental conditions. Environmental factors most affecting growth rates are photoperiod and ambient temperature, either of which can affect the intake and effective utilization of water and nutrients. Many investigations of the relationship between growth and environmental factors have been carried out on species of Cucumis, particularly cucumber. The results of these studies, which were discussed in detail in Whitaker and Davis (1962) and Wien (1997), are summarized below.
1. The growth rate curve for a single leaf under continuous light is generally an S curve, but is affected by light intensity.
2. The rate of stem elongation is greater during 8 h days than during 16 h days, and plants grown under short-day conditions produce more nodes and leaves, but smaller total leaf and root areas.
3. Overall stem length may be greater under a long-day versus a short-day regime when nitrogen levels are high.
4. Under low-nitrogen conditions, plants grown during long days contain more carbohydrates at anthesis than plants grown during short days, but this carbohydrate relationship is reversed at fruit maturity.
5. Stem extension and leaf area growth rates are linearly dependent on mean ambient temperature during periods of optimum temperatures for growth (20–30°C, depending on other environmental conditions). However, in the range of 15–27°C, Grimstad and Frimanslund (1993) found that plant dry weight of cucumber had a sigmoidal response curve with inflections at about 17°C and 24°C.
6. When temperature rises above the optimum, leaf growth rate in young plants declines as material is redistributed to the stems, and cell division in developing leaves is reduced.
7. At below-optimum temperatures, relative leaf growth rate is independent of temperature and is controlled instead by light intensity.
8. Stem extension rates are lower than normal when night temperatures exceed day temperatures.
9. Low temperatures slow the development of apical buds.
In C. pepo, bushy plants with short internodes possess an allele that reduces biosynthesis of endogenous gibberellin. When these plants are treated with a high concentration (2.9–4.3 mmol l−1) of gibberellic acid, internode lengths become as long as those of naturally viny squash plants.
Breeders have also selected for bush cultivars in C. maxima. Research on bushy versus viny plants of this species indicates that
