Cucurbits. James R. Myers
two pairs of stamens has resulted in some genera (e.g. Cucumis) having one small unilocular and two large bilocular stamens. The three stamens are usually attached to each other to some degree by their anthers, as in squash; filaments are short and often united also.
In the Cucurbitaceae, the subfamily Zanonioideae has relatively homogeneous pollen morphology. In the subfamily Cucurbitoideae, pollen is quite variable, but consistent within genera. For example: in squash, pollen grains are very large, spherical and spiny, but in cucumber the pollen grains are more globular and smooth.
Pistillate flowers have an inferior ovary below the hypanthium. The pistil, which often has a fused style but separate stigma lobes, generally has three or five carpels. The fleshy placentae bear numerous ovules in most species, but only one in chayote.
Floral nectaries attract pollinating insects. These structures are borne inside and at the base of the hypanthium in both staminate and pistillate flowers. The nectary forms a continuous ring surrounding the base of the style(s) in the female flower, whereas the nectary and its associated pistil rudiment form a button-shaped mound at the centre of the male flower.
Fruit
Fruit of the Cucurbitaceae are extremely diverse in many characteristics, including size, shape, colour and ornamentation. Those of bryony are small (ca 5 mm), spherical, and green, red or black in rind colour. Angled luffas are club-shaped, about 60 cm long, and prominently ribbed. Some of the many shapes of bottle gourds are described in Chapter 4. The striped, mottled, bicoloured or solid-coloured fruit of squash are smooth, wrinkled, warted, furrowed or ridged. Sicyos has stiff, dry spines, whereas those on the teasel gourd are soft and fleshy.
Cucurbita maxima is well named, for it is this species that is the giant of the plant kingdom. Every year, there is a contest to grow the world’s largest pumpkin. In this contest any squash fruit with an orange skin is considered a pumpkin, and the winners are invariably C. maxima. Weights crossed the 500 kg threshold in 1999 and the 1000 kg threshold in 2014. The winning fruit in 2016 weighed an astounding 1193 kg. Giant watermelon contests are also run, with the winning fruit in 2015 weighing 137 kg.
Cucurbit fruit are generally indehiscent ‘pepos’, usually with one or three ovary sections or locules (Fig. 1.2). A pepo is a fleshy fruit with a leathery, non-septate rind derived from an inferior ovary. However, fruit of some cucurbit genera, e.g. Momordica and Cyclanthera, split at maturity. Fruit of squirting cucumber forcefully eject their seeds through a blossom-end pore. Fruit may be dry when mature, as in luffa, where seeds fall out through a hole at the bottom of the pendulous fruit. Mature fruit of many cucurbits have a hard, lignified rind, but various squash cultivars have been bred to have a tender rind.
Fig. 1.2. Frontal and cross section of female flower from Cucurbita pepo (spaghetti squash).
Cucurbit fruit flesh is generally white to pale yellow and moist, but cultivated cucurbits such as melon, squash and watermelon have been bred to have a range of flesh colours. Melon and squash have also been bred to include green or orange flesh, and watermelon flesh can be white, salmon yellow, canary yellow, orange, coral red or scarlet red. In melon, watermelon and squash, the flesh is derived from the fruit wall. In other species, including cucumber, the edible flesh may be mostly placental in origin.
Many cultivated cucurbits produce a fleshy fruit at maturity. Others, such as gourd and luffa, dry at maturation. When a luffa fruit matures and dries, what is left is a papery outer skin and a fibrous mass surrounding the seeds. This tangled mass of modified vascular bundles, consisting mostly of fibre cells, makes up a luffa sponge.
Seeds
There 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