Andromonoecy and Variation in Phenotypic Gender of Passiflora Incarnata (Passifloraceae)

Abstract

We studied the perennial vine Passiflora incarnata Linnaeus (Passifloraceae) in Alachua County, Florida, from May to August 1983 to determine the breeding system and investigate seasonal changes in phenotypic gender of individual plants. Passiflora incarnata is an obligate outcrosser, incapable of self‐fertilization. The major pollinators were carpenter bees (Xylocopa sp.). The proportion of flowers setting fruit was not limited by pollinator service, but the weight of individual fruits and number of seeds/fruit was increased over naturally pollinated flowers by manually performing cross‐pollination. Morphological differences in style position among flowers caused some flowers to function primarily as males and others to function as hermaphrodites. Although some of the flowers that functioned as males set fruit when manually cross‐pollinated, the proportion of these male flowers capable of setting fruit was lower than the proportion of hermaphroditic flowers setting fruit when manually cross‐pollinated. Further, some male flowers had atrophied ovary and styles and were completely incapable of setting fruit. Passiflora incarnata is thus functionally andromonoecious. The relative production of male versus hermaphroditic flowers varied among individuals and over the course of the flowering season. Unmanipulated plants in the population became increasingly male‐biased in floral sex ratios as the reproductive season progressed. We attempted to modify phenotypic gender in experimental plants by limiting the ability of plants in some treatment groups to set fruit. Treatment groups significantly affected production of hermaphroditic flowers, but production of male flowers was not affected by treatment. Treatment did not significantly affect fruit weight, number of seeds per fruit or the percentage of hermaphroditic flowers that successfully matured fruit. These results suggest that andromonoecy in P. incarnata is a mechanism for adjusting allocation of reproductive effort to male and female function, and that maternal investment in this species is regulated primarily by varying production of hermaphroditic flowers.

Effect of Defoliation on Gender Expression and Fruit Set in Passiflora incarnata

“…The flowers of the genus Passiflora are highly specialized, with stamens, ovary, and tripartite style elevated onto an androgynophore (see May and Spears, 1988, for illustration). When the flower opens, usually between 1 1 15 and 1200 EST at our study site, the style is located well above the ring of five stamens, stigmas uppermost.…”

“…Maturing fruits significantly influenced gender expression of subsequent flowers on the same branch by decreasing the relative number of hermaphroditic flowers produced and increasing the relative number of male flowers produced. By removing maturing fruits, we were able to increase relative hermaphroditic flower production (May and Spears, 1988). Sexual expression in Passiflora appears to be labile and responsive to resource changes in the immediate floral environment.…”

Effects of floral herbivory, limited pollination, and intrinsic plant characteristics on phenotypic gender in Sanicula arctopoides

“…These results indicate that S. arctopoides does not actively change phenotypic gender expression in response to unsuccessful seed-set on early flowers, unlike other andromonoecious Apiaceae (Hendrix and Trapp 1981;Hendrix 1984) and other taxa Webb 1983, Stanton et al 1987;May and Spears 1988;Diggle 1993). However, herbivory of strictly vegetative tissue prior to flowering could still possibly cause changes in plant size or resource availability that would, in turn, induce gender changes.…”

“…Several studies of andromonoecious dicots have demonstrated that destruction of ovaries in hermaphroditic flowers leads to increased hermaphroditic flower production later in the same reproductive season (Hendrix and Trapp 1981;Hendrix 1984;May and Spears 1988). In one case at least (Hendrix and Trapp 1981), flowers at normally staminate positions developed as functional hermaphrodites.…”

Sex allocation in Carex (Cyperaceae): effects of light, water, and nutrients

“…Acer grandidentatum growing in semiarid shrublands also showed reduced femaleness under dry conditions (Barker et al 1982). No effect of water availability was seen in monoecious Ambrosia artemisiifolia L. (McKone and Tonkyn 1986) and in andromonoecious Passiflora incarnata L. (May and Spears 1988). The former authors interpreted the lack of effect of water on sex expression in the Ambrosia species as a reflection of this plant's adaptation to environments in which soil moisture fluctuates widely.…”