Prioritized contingencies: context-dependent regeneratory effects of grazer saliva

Naturally growing plants are able to plastically respond to myriad environmental challenges and opportunities. When confronted with multiple stresses, plants are expected to be able to prioritize their responses according to immediacy and predicted acuteness of these stresses. Here, we studied the interactive effects of competition and nutrient deprivation on growth responses of damaged Trifolium purpureum plants to salivary cues of a mammalian grazer. Salivary cues elicited marked growth responses in damaged but otherwise well-nourished and competition-free T. purpureum plants; however, this positive effect was annulled under Stipa capensis competition and was reversed under nutrient deficiency. The results suggest that the magnitude and direction of the effects of salivary cues on plant growth depend on an intricate prioritization of plant responses to prevailing and expected challenges and that T. purpureum plants perceive competition as a more acute stress than grazing. While herbivore saliva enables plants to reliably differentiate between herbivory and physical damage, the limited correlation between prevailing and future herbivory might reduce the informative value of salivary cues, rendering their effects weaker than those of prevalent competition and nutrient deficiency, whose continued detrimental effects are usually highly predictive. The results stress the importance of further studying the interactive effects of the acuteness and reliability of prevailing and anticipated stresses, and the informational content and adaptive value of environmental cues under various environmental circumstances.