Dynamic extrafloral nectar production: The timing of leaf damage affects the defensive response in Senna mexicana var. chapmanii (Fabaceae)

• Premise of the study: Extrafloral nectar (EFN) mediates food for protection mutualisms between plants and defensive insects. Understanding sources of variation in EFN production is important because such variations may affect the number and identity of visitors and the effectiveness of plant defense. We investigated the influence of plant developmental stage, time of day, leaf age, and leaf damage on EFN production in Senna mexicana var. chapmanii . The observed patterns of variation in EFN production were compared with those predicted by optimal defense theory. • Methods: Greenhouse experiments with potted plants were conducted to determine how plant age, time of day, and leaf damage affected EFN production. A subsequent field study was conducted to determine how leaf damage, and the resulting increase in EFN production, affected ant visitation in S. chapmanii . • Key results: More nectar was produced at night and by older plants. Leaf damage resulted in increased EFN production, and the magnitude of the response was greater in plants damaged in the morning than those damaged at night. Damage to young leaves elicited a stronger defensive response than damage to older leaves, in line with optimal defense theory. Damage to the leaves of S. chapmanii also resulted in significantly higher ant visitation in the field. • Conclusions: Extrafloral nectar is an inducible defense in S. chapmanii . Developmental variations in its production support the growth differentiation balance hypothesis, while within‐plant variations and damage responses support optimal defense theory.