Variations in direct and indirect defenses against herbivores on young plants of Mallotus japonicus in relation to soil moisture conditions.

Some plant species develop multiple defense traits. To test the hypothesis that plants with both direct and indirect defense traits mainly develop the direct and indirect defense traits under the low and high soil moisture conditions, respectively, the development of multiple defense traits on the young plants of Mallotus japonicus (Thunb.) Muell. Arg., and the number of ants on the plants were experimentally examined under different soil moisture conditions. Under the low soil moisture condition, the plant growth declined, and the trichomes and pellucid dots developed well. The pearl bodies developed on the plants under the high soil moisture condition. The volume of extrafloral nectar secreted and the number of ants on the plants significantly increased under the high soil moisture condition. These results clearly show that the young plants of M. japonicus firmly develop sound direct defense traits under the low soil moisture condition, and they develop indirect defense traits that are less reliable but have relatively low costs under the high soil moisture condition.