Relatedness of Neighboring Plants Alters the Expression of Indirect Defense Traits in an Extrafloral Nectary-Bearing Plant

Abstract

Different degrees of genetic relatedness among plants living in close proximity create differences in resource allocation and change the expression of defense traits. I investigated the plasticity of defense traits in young Mallotus japonicus growing with half-siblings or non-siblings, and examined direct defense traits (trichome density, pellucid dot density, and total phenolic content) and indirect defense traits (number of extrafloral nectaries, volume of extrafloral nectar, and number of food bodies) on the leaves. The plants growing with non-siblings invested proportionally more in shoots at high root biomass. Most defense traits did not differ between the two conditions, but the volume of extrafloral nectar was significantly larger with non-siblings. Although trichome density was not correlated with total plant biomass, some relationships between defense and growth differed significantly between the conditions, indicating a trade-off between plant growth and the expression of each defense trait (representing defense allocation costs). The chemical defense traits (pellucid dot density, total phenolic compounds) were negatively correlated with total biomass in plants growing with half-siblings, but were significantly positively correlated with total biomass in plants growing with non-siblings. The indirect defense traits were significantly positively correlated with total biomass in plants growing with non-siblings. These results suggest that the relatedness of neighboring plants alters their defense strategy and defense allocation costs, and that the local genetic relatedness of plants may play an important role in plant–animal interactions.