Gender-related differences in gas exchange rates in the gender-switching species Arisaema triphyllum (Araceae)

Females in dioecious species are generally expected to have higher rates of photosynthesis than are males of the same species. This sexual dimorphism is believed to evolve to compensate for higher costs of reproduction in females relative to males. However, some studies have shown that males in dioecious species have higher rates of carbon assimilation than females. The current study measures photosynthetic rates in males and females of the same genotype in the gender-switching species, Arisaema triphyllum. Males were found to have higher photosynthetic rates on a per unit area basis. However, females had greater overall carbon assimilation rates because of their significantly greater leaf area. Photosynthesis in the current study was measured at flowering; presumably developing embryos were not acting as carbon sinks at this time. Arisaema triphyllum has a corm (an underground storage organ) that may be the primary sink for carbon. A regression including only females reveals a significant inverse relationship between pseudostem diameter (a proxy for corm size) and photosynthetic rate; as corm size increased, the photosynthetic rate decreased. This suggests that in very large females having greater stored resources, the corm is not as strong a sink as it is for smaller females, at least during the flowering phase of the growing season. For smaller females however, photosynthetic rates do appear to be sink-limited. There was no relationship between corm size and photosynthetic rates among males. Overall, males and females appear to have different patterns of assimilation, at least early in the growing season.