Plasticity in apical dominance and damage tolerance under variable resource availability in Medicago truncatula

The activation of dormant meristems following apical damage is an important mechanism for tolerance of herbivore damage, but its impact could vary with resource availability. Here, we examined central predictions of the limiting resource model (LRM), according to which high resource availability can support damage tolerance in plants with deterministic apical dominance, but will have limited or no effect in plants that are induced to increase branching by increased resource availability regardless of damage. We examined these predictions by studying the branching patterns of Medicago truncatula plants in response to both light and water availabilities and their effects on tolerance of apical damage. We used plants from environments that were predicted to select for different levels of apical dominance. Intact plants from the more productive and competitive population exhibited strong apical dominance and refrained from branching even under full light, whereas plants from the less productive and sparser population exhibited greater plasticity in apical dominance and readily branched under high water and light. In accordance with the LRM, these differences translated into differential responsiveness to apical damage: given abundant water, apical damage induced the activation of lateral meristems and increased pod and seed production in plants from the more productive environment, but not in plants from the less productive environment. These results suggest an adaptive association between deterministic inhibition of lateral meristems and compensatory ability, which supports the hypothesis that greater compensatory responsiveness to apical damage could be a derivative of adaptation to other environmental stresses, such as light competition.