Relative Effects of Light, Soil Moisture Availability and Vegetative Size on Sex Ratio of Two Monoecious Woodland Annual Herbs: Acalypha rhomboidea (Euphorbiaceae) and Pilea pumila (Urticaceae)

Abstract

CID-BENEVENTO, C. R. (W. K. Kellogg Biol. Sta., Michigan State Univ., Hickory Corners, MI 49060). Relative effects of light, soil moisture availability and vegetative size on sex ratio of two monoecious woodland annual herbs: Acalypha rhomboidea (Euphorbiaceae) and Pilea pumila (Urticaceae). Bull. Torrey Bot. Club 114:293-306. 1987.-Biomass allocation to male and female reproductive structures, and total vegetative biomass, were determined for Acalypha rhomboidea and Pilea pumila plants growing in all treatment combinations of six light levels ( 1000 ,Einstein m-2 sec-') and three soil mixtures of low, medium and high waterholding capacity. Vegetative biomass and reproductive output increased with increasing light, peaking at the next to highest light level. Vegetative growth and reproduction were negatively affected by low soil moisture only at intermediate and next to highest light levels, where plants achieved greatest growth. In both species, reproductive biomass and also the number of staminate and pistillate structures increased with increasing vegetative size. With increasing light availability and increasing plant size, there was a greater increase in biomass investment into female reproductive function than in biomass allocation to male reproductive structures. Although sex allocation pattern varied over the resource gradients, both species allocated >83% of total reproductive biomass to female function in all environments. The results suggest that for these wind-pollinated woodland annuals with size-structured populations, an environment characterized by low wind velocity, the importance of rapid site preemption early in the growing season for reproductive success, and the time constraints of an annual life cycle may all interact to favor female-biased sex ratios for all individuals, regardless of plant size or resource availability.