Floral Water Costs and Size Variation in the Highly SelfingLeptosiphon bicolor(Polemoniaceae)

Abstract

Premise of research. Abiotic conditions play a significant, but poorly understood, role in shaping floral size variation within species. In particular, the water costs associated with producing and maintaining flowers may be limiting for plants in dry environments. Under such conditions, plants may be physiologically constrained to produce small flowers. In this study, I examined how floral traits of a highly autogamous annual, Leptosiphon bicolor (Polemoniaceae), varied with moisture availability in order to evaluate whether floral size variation is a result of physiological constraints associated with floral water costs. Methodology. I measured flower size, leaf size, leaf physiology, and fitness from 2005 to 2008 in 10 populations of L. bicolor that crossed a moisture gradient and in a greenhouse study with plants grown under uniform conditions. I used the δ13C of leaf tissues to infer water-use efficiency. Structural equation models were used to analyze the direct and indirect relationships among the measured traits. Pivotal results. Floral size increased with site moisture. Differences in floral size between populations were maintained in the greenhouse, suggesting some heritable differentiation along the moisture gradient. Floral transpiration rates per unit surface area were comparable to those of leaves. However, in the driest populations, floral surface area exceeded that of leaves. Therefore, during flowering, these plants lost more water from flowers than from leaves. Water-use efficiency increased with flower size in the drier populations, while this relationship was not observed in the wetter populations. Therefore, in order to produce large flowers under dry conditions, leaves had to close their stomata to compensate for floral water loss and maintain overall plant water balance. Conclusions. Floral water costs may be substantial, especially in dry environments. Under such conditions, moisture may indirectly constrain flower size because large flowers are physiologically difficult to support.