Effects of environmental stress on leaf hair density and consequences for selection

We explored how five different environmental stresses and a benign environment affect the phenotypic expression of leaf hairs, and the potential for evolutionary response in this trait. To address these questions we planted full-sib families that had been selected for stress tolerance in a factorial design in which selection history was fully crossed with the current environment (eight families × six selection histories × six current environments × three blocks=864 plants). Our data suggest that leaf hair density is a complex character composed of two separable traits: leaf area and the number of hairs initiated per leaf. Leaf size is primarily controlled by the growth environment, whereas leaf hair initiation shows more genetically based variation. In the control and low water environments there was a negative linear relationship between leaf size and leaf hair density. However, within the low light and high boron treatments, leaf hair density remained constant across a range of leaf sizes, suggesting that these stresses disrupt normal leaf hair development. Selection gradient analysis showed that leaf hair density and leaf size were significantly positively associated with fitness in 4/6 of our environments. Our results suggest that environmental variation may diminish the ability of herbivores and pathogens to cause directional selection on leaf hair density.