Effect of flower sex ratio on fruit set in pumpkin (Cucurbita maxima)

Flower sex expression and sex ratio are the two plant functional traits likely to explain pollination efficiency in monoecious animal-pollinated plants. Cucurbitaceae crop plants are staminate flower-biased, but, relatively little is known about the natural variation in flower sex expression and the effect of flower sex ratio on fruit set To address these, 325 plants of Cucurbita maxima were monitored for the entire flowering cycle and studied fruit set in 1515 pistillate flowers under open conditions for two years. The flower sex ratio of the fields and the plants and the flowering age (=phase) of the plants were used as the predictors of fruit set. The blooming in C. maxima started with the staminate flowers; 67% of the plants produced a staminate flower first, which continued for about ten days. The number of staminate flowers produced in the plants strongly predicted the number of pistillate flowers bloomed later. The sex ratio of the fields, rather than that of an individual plant, predicted the fruit set. Although pollinators are vital for pollination in animal-pollinated monoecious plants, our results suggest that flower sex ratio is also a crucial predictor of pollination efficiency. Monitoring flower sex expression in fields may be required to predict realistic fruit set and to take necessary measures, including facilitated pollination to improve fruit set.