Phosphorus reserves increase grass regrowth after defoliation

Accumulation of P above levels that promote growth, a common plant response called "luxury consumption", can be considered as a form of reserve to support future growth when the nutrient can subsequently be mobilized. However, the effect of P reserves on regrowth following defoliation has not been demonstrated. We tested the hypothesis that P luxury consumption increases plant tolerance to defoliation. We performed two experiments with four grass species from a continuously grazed temperate grassland in the Flooding Pampa (Argentina). The first experiment, aimed at generating P luxury consumption by fertilization, resulted in one species (Sporobolus indicus) showing luxury consumption. In this way, we were able to obtain plants of S. indicus with similar biomass but contrasting amounts of P reserves. The second experiment evaluated the subsequent regrowth following defoliation on a P-free medium of these plants differing in P reserves. Regrowth was larger for plants that had shown P luxury consumption during a previous period than for plants with lower levels of P reserves. During regrowth these plants showed a clear pattern of P remobilization from the stubble, crown, and root compartments to the regrowing tissue, in addition to a likely reutilization of P present in leaf-growth zones. This work is the first showing that high levels of P reserves can confer tolerance to defoliation by promoting compensatory growth under P deficiency.