Ant-Attendance in Extrafloral Nectar-Bearing Plants Promotes Growth and Decreases the Expression of Traits Related to Direct Defenses

In addition to direct defenses, some plant species provide extrafloral nectar (EF-nectar) and/or food bodies (lipid-rich particles) to attract ants for their own indirect defenses. To ascertain why such plants use indirect defenses, we investigated the respective costs of direct and indirect defenses of Mallotus japonicus seedlings grown with and without ants present. Mallotus japonicus plants growing with ants present (ant-present) secreted larger volumes of EF-nectar, containing greater amounts of sugars, as an indirect defense trait. These plants also showed chemical defensive traits, such as the number of pellucid dots and the amount of accumulated phenolics, to a lesser degree than plants without ants (ant-absent) did. Moreover, the ant-present plants grew faster. The estimated amounts of EF-nectar sugars and food bodies were small compared to the amount of phenolics. Plant biomass was correlated negatively with pellucid dot density and phenolic concentration. Plant height was correlated negatively with phenolic concentration. Moreover, leaf biomass was correlated negatively with trichome density. Taken together, these results suggest a tradeoff between the expression of direct defense traits and plant growth. Mallotus japonicus achieves more rapid growth with ants present. We propose that this occurs because these ants provide low-cost indirect defenses allowing plants to re-allocate their energy from direct defenses to growth instead. This mutual benefit apparently facilitates ant–plant defensive mutualism.