Leaf Gas Exchange, Water Status, Spatial Dispersion, and Gender in Gynodioecious Bidens sandvicensis (Asteraceae)

Abstract

How the cost of reproduction is paid in dimorphic populations is a fundamental question in plant reproductive biology, and females of dioecious species in arid environments often show lowered drought tolerance and survivorship attributable to higher reproductive allocation. The cost of reproduction in gynodioecious populations is less understood but can provide information on the selective advantages of dimorphism. Leaf gas exchange rates, water status, and spatial dispersion were measured in the two sex morphs of gynodioecious Bidens sandvicensis from two neighboring populations during typical seasonal drought on Kauai, Hawaii, in which females produce on average 33% more seeds per flowering head. Both females and hermaphrodites showed instantaneous and integrated gas exchange behavior typical of mesophytic, herbaceous, heliophytic dicots, with values for Rubisco carboxylation capacity, electron transport capacity, stomatal limitation, maximum net carbon uptake, maximum water use efficiency, and carbon isotopic fractionation within the midrange of values reported for these species. Midday water potentials during the period of flowering and fruiting approached the limit of drought tolerance reported for mesophytic species. Overall, females had marginally significantly lower photosynthetic function and water use efficiency than hermaphrodites, consistent with a passive trade‐off between leaf function and reproductive allocation. The sex morphs were dispersed at random in relation to each other, with no evidence of niche partitioning on soil moisture or elevation between the sex morphs within the two populations, and thus no evidence of a fitness difference attributable to physiological costs. The absence of large physiological differences or niche partitioning in B. sandvicensis and other gynodioecious species may be due to less pronounced sex differences in reproductive allocation as well as the less derived status of gynodioecy relative to dioecy. The correlation between gynodioecy and drought in Hawaiian Bidens may result from higher self‐fertilization in larger floral displays, leading to an outcrossing advantage for unisexual females in arid habitats.