Size-dependent Sex Ratios in the Monoecious, Wind-pollinated Annual, Xanthium strumarium

Abstract

-I examined the numbers and mass of male and female inflorescences produced by the monoecious, wind-pollinated, self-compatible annual, Xanthium strumarium. Sex ratio (number and mass of male flowers number and mass of female flowers) increased with plant size and resource status for greenhouse plants that were experimentally induced to flower, and for naturally occurring plants. Similarly, plants in low-density sections of an experimental garden were larger and had higher sex ratios than those in high-density sections, presumably because of reduced competition. The observed direction of sex change with plant size was unusual, since most species become more female as plant size increases. Sex ratio in this species appeared to be related to branching pattern. Female inflorescences were produced at all nodes, while male inflorescences were absent from about the second to the fourth nodes of main and side shoots. Whole-plant sex ratio may be determined by these location templates for inflorescences, so conditions that alter branching patterns might have important effects on reproductive allocation and reproductive success.