Summary: Finally I tested for plant population variation in plant response to other key selective agents on the legume-rhizobium mutualism (light and nitrogen availability). I found that plants and rhizobia responded differently to changes in resource availability. Symbiosis was most beneficial for rhizobia under high light and low nitrogen conditions, as predicted by resource mutualisms theory. For plants, however, symbiosis was beneficial in low nitrogen treatments regardless of light conditions. These asymmetric effects of both traded resources are, in part, driven by plants' ability to control nodulation under unfavorable conditions.