Growth, herbivory and disease in relation to gender in Salix viminalis L.

Many species of dioecious plants show sex-related differences in growth rate and rates of attack by various herbivores and diseases. The common pattern is for males to grow faster than females and to be less well defended against herbivores. In willows (Salix spp.), the predominance of female-biased sex ratios has been ascribed in part to differential feeding by herbivores. In this study of Salix viminalis, seven families grown on agricultural land showed no gender-related variation in shoot biomass or rates of herbivory by insects (lepidopterans and cecidomyiids). However, Melampsora rust disease was found to be more severe on females than on males when the plants were in a non-reproductive stage. After flowering and seed-set females tended to be more affected in some families but less affected in others. Although, on average, there was a female bias in the sex ratio of S. viminalis, sex ratios differed significantly between families. These ratios were not related to any of the recorded biotic agents, but rather to relationships between families. These results are interpreted in terms of resource allocation between reproduction, growth and defence, and causes for divergence from the expected patterns are discussed. The results may have implications for S. viminalis breeding strategies where the aim is to produce biofuel. For instance, these findings suggest that gender can be ignored when selecting for a high growth rate and resistance to Melampsora and certain insect pests.