Cucurbits. James R. Myers
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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 bush cultivars have a more uniform growth pattern, respond better to high-density planting, and produce a greater percentage of fruit versus vegetative biomass during short growing seasons (Loy and Broderick, 1990). This last effect is partly due to the fact that fruiting begins sooner in bush cultivars, which in turn suppresses vegetative growth. However, photosynthetic rates in bush plants increase during fruit set in response to increasing sink demand, which may be possible because of a proportionally thicker palisade layer in bush plant leaves (Loy and Broderick, 1990).
In climbing cucurbits, the stems often revolve, twist and extend upwards. Darwin (1906) made several interesting observations on the revolving nature of cucurbit stems and tendrils. He noted that the average rotation rate was 100 min per revolution in wild cucumber. Light affects this movement, with stem tips, including the two uppermost internodes subtending the apical meristem, following the sun throughout the day.
Generally, the slightly curved tendril becomes sensitive to mechanical stimulus on its concave side when it is one-third grown. At that stage, it reacts quickly (in under 2 min), with coiling caused by the elongation of parenchymatous cells on the convex side of the tendril. The revolving movement of a tendril does not stop after it has coiled, but its ability to coil is limited after it stops revolving.
Sex expression
No cucurbit species is known to have only or primarily functionally hermaphrodite flowers. Instead, most cucurbits are monoecious; that is, they have separate male and female flowers on the same plant. Among its genera, the Cucurbitaceae also has a high rate of dioecy, where staminate and pistillate flowers occur on separate plants. Some genera have only dioecious species. Half of the 135 cucurbit species surveyed in India by Roy and Saran (1990) were dioecious, a much higher proportion than is typical for angiosperm families. Cultivated cucurbits that are dioecious include oyster nut, fluted pumpkin, ivy gourd, pointed gourd and monk fruit (luo-han-guo, Siraitia grosvenorii).
Primitive cucurbits are typically monoecious, as are the majority of domesticated cucurbits. Most squash and watermelon cultivars are monoecious, although genes for different forms of sex expression are known for these crops. For example, some watermelon cultivars may have three types of flowers on the same plant: staminate, pistillate and perfect (hermaphrodite). The wild-type watermelon is andromonoecious (staminate followed by perfect flowers). Many cucumber cultivars are monoecious, but others are gynoecious or predominantly gynoecious, and round-fruited cucumber such as ‘Lemon’ and ‘Crystal Apple’ are andromonoecious. All seven sex types have been identified in cucumber: androecious, gynoecious, hermaphroditic, monoecious, andromonoecious, gynomonoecious, and trimonoecious (staminate, perfect and pistillate flowers). Angled luffa is monoecious with some exceptions, such as the cultivar ‘Satputia’, which has only hermaphrodite flowers. Most round-fruited melon cultivars are andromonoecious, with the fruit formed from perfect flowers. The bisexual flowers of these plants are often borne on the first or second node of the lateral branches. There are exceptions, however. Monoecious cultivars such as ‘Athena’ have been selected to have short, almost round fruit developing from pistillate flowers. ‘Banana’ and other melon cultivars of the Flexuosus Group are monoecious. The group includes snake melon (Armenian cucumber) and pickling melon, with long fruit having crisp white bland flesh, similar to cucumber.
Monoecy is the ancestral condition in cucurbits. Dioecy and other forms of sex expression have arisen in various evolutionary lines in the family. Single genes can determine the occurrence of unisexual plants in normally monoecious species, as in the case of dioecy (all male or all female flowers on a plant) in C. pepo. In melon, cucumber and many other cucurbits, two or more genes are involved in sex expression, sometimes (as in Luffa) with each gene having three or more alleles. The development of heteromorphic sex chromosomes (e.g. ivy gourd) to determine dioecy is considered to represent the ultimate degree of evolution from monoecy in the Cucurbitaceae.
Monoecious cultivars may differ in degree of female sex expression, some having a higher proportion of female to male flowers. Generally, they produce many more staminate than pistillate flowers and go through a progression of floral development. Nitsch et al. (1952) determined that young squash plants are initially vegetative, then bear only underdeveloped male flowers. Later, they produce only normal male flowers, then bear normal female as well as male flowers. The proportion of female to male flowers increases as the plant grows older, and the plant eventually produces only female