EFFECT OF DEFOLIATION ON GENDER EXPRESSION AND FRUIT SET IN PASSIFLORA INCARNATA

We examined the effect of defoliation on gender expression and fruit set in a north-central Florida population ofthe andromonoecious vine, Passiflora incarnata, during the 1984 flowering season. At three times during the flowering season (May, June, July), the leaves adjacent to flowers at four stages of development were removed, and subsequent sex and fate of each flower were determined. Defoliation affected gender of the flower by significantly decreasing the probability that the styles of the flower would deflex and the flower would thus function only as a pollen donor. Flowers were sensitive to defoliation at any stage before anthesis, though the sensitivity appeared to decrease in the most mature category of flower buds. Fruit set of hermaphroditic flowers remained unaffected by defoliation, but the probability of fruit set and gender expression were significantly influenced by the time of the flowering season. We conclude that the local photosynthate environment determines flower gender in Passiflora, but branch or entire plant photosynthate resources can compensate for local resource fluctuations and play an important role in fruit set and flower bud abortion. The sexual lability of Passiflora incarnata appears to be an adaptation to uncertain resource levels at a fine scale, caused by nearby developing fruits and the possibility of herbivore defoliation. RESOURCE LIMITATION, naturally occurring or artifically induced, has been shown to affect the reproductive biology of several species of flowering plants (Stephenson, 1981, 1984; Bertin, 1982; LeeandBazzaz, 1982; Louda, 1984; May and Spears, 1988). The limiting resource may be mineral nutrients (Lee and Bazzaz, 1982; Stephenson, 1984), or water (Holtsford, 1985; Marshall, Fowler, and Levin, 1985; Smith-Huerta and Vasek 1987), but frequently energy limits female reproductive success (Stephenson, 1981, 1984; Hendrix, 1984; Pitelka, Ashmun, and Brown, 1985). Depending on the plant species and the time during the reproductive cycle when resource limitation occurs, limitation can result in reduced flower production, increased flower abortion rates, or increased fruit abortion rates (Stephenson, 1981). If necessary resources become scarce early in the growing season, the plant may reduce the number of flowers produced (Pitelka et al., 1985); resource limitation that occurs after the flowering season has begun can result in flower and fruit abortion (Stephenson, 1981). The frequency of flower and fruit abortion suggests that resources frequently limit the re'Received for publication 28 September 1987; revision accepted 18 March 1988. We are grateful to the staff of Payne's Prairie State Park for the use of the Park, and to the University of Florida Department of Zoology for logistic support. 2 Current address: Biology Dept., Stetson University, DeLand, Fl 32724. productive output of many plant species (Stephenson, 1984). If so, the production of "surplus" flowers might allow the plant to take advantage of a sudden increase in resource availability (the Resource Boom hypothesis, Holtsford, 1985), or the surplus flowers might be a means of "bet hedging" for the plant if flowers or fruits are likely to be damaged or eaten (the Reproductive Assurance hypothesis, Holtsford, 1985). Other explanations have also been offered to explain high flower and fruit abortion rates. "Male competition" for a limited number of ovules could lead to the production of more flowers than could successfully set fruit, although surplus flowers could contribute to the fitness of the plant by functioning as pollen donors (Wilson and Rathcke, 1974; Stephenson and Bertin, 1983; Willson and Burley, 1983; Bawa and Webb, 1984; Stephenson, 1984). Surplus flowers could also be a means of providing a plant with "female choice" among potential clutches of offspring, allowing the plant to mature the most fit fruit and aborting the remnant (Stephenson and Bertin, 1983; Willson and Burley, 1983). However, both the "male competition" and the "female choice" models assume that available resources limit the number of fruits and seeds that a plant can produce (Stephenson and Bertin, 1983). Resource limitation thus affects sexual expression in many hermaphroditic species by converting morphologically hermaphroditic plants into plants that are functionally andro-