The influence of fruit on the regulation of internal ethylene concentrations and sex expression in Cucurbita texana

Plants with labile sex expression often regulate the production of pistilla te and staminate flowers on the basis of developmental and environmental co nditions. In many monoecious plant species, developing fruits inhibit pisti llate flower production and promote staminate flower production. Because th e phytohormone ethylene promotes pistillate flower production in a variety of wild and cultivated plant species, we examined the influence of fruit de velopment on internal ethylene production and their combined effects on sex expression in field-grown Cucurbita texana (Cucurbitaceae). We compared th e sex expression of plants with fruit and without fruit, and we measured in ternal ethylene concentrations within the hollow internodes of their branch es. Plants without fruit had more than twice as many pistillate flowers tha n plants with fruit. Ethylene was greatest within internodes just below the tips of branches compared with more basal internodes (2-21 internodes from the tip) in both fruiting and fruitless treatments. Ethylene concentration s within branches with fruit were highest 18 d after pollination, just as f ruits reached their maximum volume. Branches with two or more developing fr uits had the lowest ethylene concentrations, whereas ethylene levels were s ignificantly higher on fruitless branches and one-fruited branches. Our res ults indicate that young developing fruits may send a signal that inhibits ethylene production, leading to the promotion of staminate flowers, and tha t, when fruits are no longer strong resource sinks, ethylene concentrations increase and promote pistillate flower production.