Читать книгу Cucurbits - James R. Myers - Страница 24

‘Cucurbits are weeds waiting to become crops.’ While this anthropomorphic statement may be an over-simplification of crop evolution in the Cucurbitaceae, supporting evidence can be seen in the vast array of valuable plant products found within the family. Aboriginal plant gatherers were probably attracted to some of these products, particularly the relatively large, long-keeping and sometimes showy fruit. After fruit were taken back to camp, seeds that were purposely discarded, accidentally dropped or partially digested found new life on rubbish heaps, settlement edges or other disturbed areas near camp. Eventual recognition of the value of the resident cucurbits led to their appreciation, horticultural care and further exploitation. Finally, seeds and, more rarely, vegetative propagules were carried by and exchanged among migrating bands of these incipient cultivators, gradually turning the earliest cultivated cucurbits into domesticated crops.

It was not only because cucurbits were wanted by human gatherers that they became domesticated. Certain physiological and genetic characteristics generally associated with weeds, or colonizing species, allowed cucurbits to adapt to human habitats. Fast, indeterminate growth, developmental plasticity in response to environmental conditions (especially regarding sex expression) and genetic diversity at the genomic, chromosomal and gene levels enabled these cucurbits to continue their survival through the coevolutionary relationship that humans call domestication. In turn, this relationship brought many changes to domesticated cucurbits: the size of various plant parts increased; early female flower production was selected; seed dormancy was reduced; photoperiod response was eliminated; fruit bitterness was eliminated; and fruit flavour and appearance were improved.

Given the nature of cucurbits, it is not surprising that they are among the most ancient of cultivated plants. Archaeological evidence suggests a pantropical distribution for bottle gourd going back more than 10,000 years. Bottle gourd may have been cultivated in Asia, Africa and the New World at that time, or later if the oldest remains are of wild plants. Using molecular taxonomy, even older times can be seen, although humans did not exist then. Evidence of human use of cucurbits in the form of seeds or fruit rinds of Cucurbita pepo has been found at sites dating ca 10,000 bp in Mexico and ca 5000 bp in central USA (Smith, 1997a, 2006). Phytoliths are particles of silicon dioxide found in the cells of Cucurbita spp. These particles remain in soil samples and can be used as a diagnostic tool in archaeological studies of this genus. This technique has been used to document the presence of domesticated Cucurbita spp. in South America as early as 12,000 bp (Piperno and Stothert, 2003). Enlarged fruit parts indicate that other squash species were being cultivated for food in the New World by 7000 bp, if not earlier.

Cucumber (Cucumis sativus) is of Asiatic origin, with its primary centre of diversity in India and secondary in China. Although cucumber evolved from melon, the two species cannot be crossed. Cucumber can be crossed with Cucumis sativus var. hardwickii, C. s. var. sikkimensis and C. s. xishuangbannanensis. Also, Cucumis hystrix, a wild species with traits from both melon and cucumber, has been crossed successfully with cucumber. However, Cucumis setosus is a melon relative that looks like cucumber, but will not cross with it. Cucumber has been grown in Asia for several thousand years. The remains of Cucumis crops (melon or cucumber) in eastern Iran have been dated to the third millennium bc.

Melon (Cucumis melo) likely originated in India from C. trigonus or C. callosus (Sebastian et al., 2010). The Australian species C. picrocarpus has the highest DNA sequence similarity to C. melo. However, the oldest wild relatives of melon, such as C. hirsutus, C. humifructus and C. sagittatus, originated in Africa. Melon was grown in Egypt in the second millennium bc and in Iran during the third millennium bc. Humans moved the melon throughout the Middle East and Asia, making it an important vegetable in India, Egypt, Iran and China. Iran, Afghanistan and China are secondary centres of melon diversification. Spain is a tertiary centre of diversity.

Melon was one of the most important vegetables in ancient China, with archaeological remains there dating back to 5000 bp. The origin and domestication of melon is still debated, though evidence for two lineages have been described, with an Indian relative (C. trigonus) being the most closely related living relative to C. melo (Endl et al., 2018).

Watermelon (Citrullus lanatus) is an African native, and became important in northern Africa and southwestern Asia by 6000 bp. It is recorded in the Bible (Numbers 11:5) that the Jews missed the watermelons and cucumbers of Egypt during the Exodus in the 12th century bc. Probably, that passage refers not to cucumber but to snake melon, also known as Armenian cucumber. A watermelon relative, colocynth (Citrullus colocynthis), is probably the toxic fruit mentioned in the Bible in II Kings.