Quantitative effects of leaf area removal on indirect defense of lima bean (Phaseolus lunatus) in nature.

Plants employ a diverse array of defensive traits against multiple enemies. While many plant defenses are well-studied, quantitative feedback effects of leaf area loss on the expression of defensive traits remain little understood. Extrafloral nectar (EFN; an indirect defense acting via the attraction of carnivorous arthropods) is generally considered 'cheap' as it is composed mainly of photosynthates. However, to what extent EFN secretion is related to the amount of intact photosynthetic leaf area is unknown. In this study, we measured the production of EFN, ant attraction, and herbivore damage in response to a gradient of leaf area removal in wild lima bean (Phaseolus lunatus) under natural conditions in southern Mexico. EFN production and ant recruitment were significantly decreased with increasing leaf area removal. Consequently, EFN production was inversely correlated with leaf area loss, which suggests that EFN is metabolically more expensive than previously thought. Further, we found increased herbivory in plants with reduced EFN secretion indicating additive negative feedback effects of leaf area loss. Our study is one of the first showing a quantitative negative impact of leaf damage on EFN secretion-one of the most widely distributed defensive traits in the plant kingdom.