Gender, light and water effects in carbon isotope discrimination, and growth rates in the dioecious tree Ilex aquifolium

1. Detailed understanding of the specific physiology of sexes in dioecious species is required to explain patterns in gender dimorphism. Under controlled-environment conditions we tested the hypothesis that sexes of the dioecious tree holly Ilex aquifolium L. (Aquifoliaceae) differed in growth and long-term potential water-use efficiency, as measured by carbon isotope discrimination (Delta C-13), and that these differences were dependent on the environmental context. 2. Patterns of response in Delta C-13 to the various combinations of light and water were gender-specific. Under more xeric conditions, females maintained significantly higher Delta C-13 than males. 3. Female plants exhibited significantly greater relative diameter growth rates than male plants. 4. As expected, Delta C-13 significantly increased with decreasing irradiance, and decreased with increasing limitation in water supply. Light and water effects were not independent, with a more pronounced drought effect in decreasing leaf Delta C-13 under unshaded than under shaded conditions. 5. Our results suggest that between-sex differences in physiology are context-dependent. Future studies attempting to assess gender dimorphism should take more account of gender-specific interactions with the environment. Gender-specific efficiency in water use could play a decisive role in explaining gender differences in growth and ecological interactions.