In cucurbits, hybrid seed production has been simplified by manipulation of sex mechanism and sex expression. Different techniques are developed to reduce the cost of hybrid seed production viz., use of male sterile line, gynoecious line, monoecious line and use of plant growth regulators for suppression of male flower and open pollination. Different methods employed for the production of hybrid seeds in cucurbits have been discussed below:
Bagging of female flowers and hand pollination:
A day prior to anthesis, the female flowers before they open on the plants of female parent are covered by butter paper bags or buds of female flowers are tied with rubber band or clip specially where flower is large like pumpkin. In the afternoon of the same day, unopened male flowers on the plants of male parent are protected from pollen contamination by bagging or tying the petals. In the next day morning (7-11 a.m.), pollens from bagged flowers of male parent are applied on the stigma of the protected female flowers. After hand pollination, the female flowers of female parent are again bagged or petals are tied. This practice is applicable in most of the cucurbits except muskmelon where andromonoecious sex is predominant. F1 seed is collected from the mature fruits harvested from the plants of female parent.
Pinching of male flowers:
Production of hybrid seeds by this method is the most simple and economical and can easily be adopted by the growers who can identify male or female flowers. F1 hybrid production in gourds can economically be done on large scale by pinching all the male flowers before opening from the female parent and the male parent is allowed to grow side by side of female parent for natural cross-pollination in isolation. One row of male parent can be sown after every three rows of female parent as suggested by Choudhary and Singh (1971) for producing F1 hybrid seeds on large scale in bottle gourd. All the fruit set in female parent would be necessarily through cross-pollination by insects. For this, as a precaution, there should not be a single male bud in female parent as it will promote self or sib-pollination within the female parent. Anthesis of the flowers of bottle gourd is in afternoon, the pinching operation should therefore be done in the forenoon. Isolation distance between different varieties should be kept about 400 metres. As the male flowers in bottle gourd, pumpkin and squash are quite big, showy, having long pedicels and less in number, the pinching operation can easily be performed. In Luffa, where male flowers are produced in racemes, pinching off male buds will not be complete and effective. For maximum fruit set and seed yield, availability of pollinator is pre-requisite. One medium sized bee colony per hectare would be enough in seed production block.
Chemical suppression of male flowers and open pollination:
With the advent of growth regulating substances, very significant results have been obtained with regard to sex modification in cucurbits. It has now been possible to prove that the two true-leaf stage is the most responsive stage for application of chemicals for sex modification. Specific chemicals are known to induce femaleness or maleness as desired. In cucurbits like bottle gourd, pumpkin and summer squash, female flowers can be increased by the application of ethrel (2-chloroethyl-phosphoric acid) at the rate of 200-300 ppm at two true-leaf and four true-leaf stage. Ethrel helps in suppressing the staminate flowers and initiating pistillate flowers successively in the first few flowering nodes on the female parent. The row of male parent is grown by the side of female parent and natural cross pollination is allowed. In the absence of insect pollinators, hand pollination is possible when two sexes are separate. Precaution should be taken that during natural cross pollination, there should not be any other variety except the parents of hybrid. Four to five fruits set at initial nodes containing hybrid seed would give sufficient seed yield. Complete suppression of male flowers in summer squash can be achieved at higher concentration of 400-500 ppm ethrel applied twice and it has made hybrid seed production comparatively easier in this crop.
Choudhury, B. and Singh, B. 1971. Pusa Meghdoot and Pusa Manjari, two high yielding bottle gourd hybrids. Indian Horticulture 16:15-16.
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Working as a Senior Scientist at ICAR-Indian Agricultural Research Institute, Pusa, New Delhi