Читать книгу Cucurbits - James R. Myers - Страница 21

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 flowers. If squash plants are not pollinated, ultimately they may produce enlarged female flowers and parthenocarpic fruit.

Although staminate flowers usually appear a few days to a few weeks before pistillate flowers on squash vines, a few cultivars and some wild populations of C. pepo may produce female flowers first (Fig. 1.4), especially when spring temperatures are low. These wild populations also tend to have a higher ratio of female to male flowers than most cultivars (Decker, 1986).

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.