Taxonomy, Morphology, and Physiology
by Todd C. Wehner
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609
Watermelon (Citrullus lanatus) has 22 chromosomes (2n=22, x=11). The genus Citrullus belongs to the subtribe Benincasinae. Similar genera in the Cucurbitaceae are Acanthosicyos and Eureiandra. Other members of the Cucurbitaceae with 22 chromosomes include Gymnopetalum, Lagenaria, Momordica, Trichosanthes, and Melothria. None appear to be closely related to watermelon. In 1924, four species (C. lanatus, C. colocynthis, C. ecirrhosus, and C. naudinianus) were listed based on their distribution in Africa.
In 1930, L.H. Bailey proposed dividing cultivated watermelon C. vulgaris, into botanical variety lanatus and botanical variety citroides. The variety citroides includes the citron or preserving melon, which produces fruit with hard, inedible flesh, and green or tan seeds. The species could be classified based on the cucurbitacin or bitter principle content. One group of closely related species (C. lanatus, C. colocynthis, and C. ecirrhosus) had cucurbitacin E as the bitter substance, while the other group (C. naudinianus) had cucurbitacin B and E (and their derivatives). Morphological and cytogenetic studies have revealed that the four species are cross compatible with each other. The maintenance of identity of the different species was attributed to geographical isolation, differences in flowering habit, genetic differences, and structural changes in chromosomes.
The genus Citrullus has now been revised to include C. lanatus (syn. C. vulgaris), C. ecirrhosus, C. colocynthis, and C. rehmii. C. ecirrhosus is more closely related to C. lanatus than either is to C. colocynthis. There are two other closely related species: Praecitrullus fistulosus from India and Pakistan, and Acanthosicyos naudinianus from southern Africa.
Morphology and Physiology
Watermelon is a warm-season crop. It is not chilling resistant and requires a long growing season. Flowering and fruit development are promoted by high light intensity and high temperature. Watermelon is the only economically important cucurbit with pinnatifid (lobed) leaves; all of the other species have whole (non-lobed) leaves. The leaves are pinnately divided into three or four pairs of lobes, except for an entire-leaf (non-lobed) gene mutant controlled by the nl (non-lobed) gene (Fig. 3.1). Watermelon growth habit is a trailing vine. The stems are thin, hairy, angular, grooved, and have branched tendrils at each node. The stems are highly branched and up to 30 feet long, although there are dwarf types (dw-1 and dw-2 genes) with shorter, less-branched stems. Roots are extensive but shallow, with a taproot and many lateral roots.
Watermelon has small flowers that are less showy than other cucurbits. Flowering begins about 8 weeks after seeding. Flowers of watermelon are staminate (male), perfect (hermaphroditic), or pistillate (female), usually borne in that order on the plant as it grows. Monoecious types are most common, but there are andromonoecious (staminate and perfect) types, mainly the older varieties or accessions collected from the wild. The pistillate flowers have an inferior ovary, and the size and shape of the ovary is correlated with final fruit size and shape. In many varieties, the pistillate or perfect flowers are borne at every seventh node, with staminate flowers at the intervening nodes. The flower ratio of typical watermelon varieties is 7:1 staminate:pistillate, but the ratio ranges from 4:1 to 15:1.
The fruit of watermelon are round to cylindrical, up to 24 inches long and have a rind 0.4 to 1.5 inches thick. The edible part of the fruit is the endocarp (placenta). That contrasts with melon (Cucumis melo), where the edible part of the fruit is the mesocarp. Fruit as large as 262 lb. have been recorded, but usually they weigh 8 to 35 lb. In Asia, even smaller watermelon fruit in the range of 2 to 8 lb. are popular. Fruit rind varies from thin to thick, and brittle to tough.
Seeds continue to mature as the fruit ripens and the rind lightens in color. Seeds will be easier to extract from the fruit if the fruit is held in storage (in the shade or in the seed processing room) for a few days after removing them from the vine. If seeds are left too long in the fruit they will germinate in situ. There is no dormancy in watermelon seeds, so they can be harvested on one day, cleaned, dried, and planted on the next day. Seeds germinate in 2 days to 2 weeks depending on temperature and moisture conditions. Seeds will not germinate below 60°F. The optimum germination temperature is 85 to 90°F, especially for triploid seeds. For germination of triploid hybrid seeds, temperature and moisture are more critical, and it is especially important to avoid excess moisture.