Role of proline and leaf expansion rate in the recovery of stressed white clover leaves with increased phosphorus concentration.

Osmotic adjustment (OA) and increased cell-wall extensibility required for expansive leaf growth are well defined components of adaptation to water stress in dry soil, which might interact with soil phosphorus (P) concentration and defoliation frequency for intensively grazed white clover in legume-based pastures. Experiments were conducted with frequently and infrequently defoliated mini-swards of white clover growing in dry soil with low and high P concentrations. The higher yielding high-P plants were able to dry the soil to greater soil water suctions; their leaves had lower water potential values, yet they showed fewer water stress symptoms and underwent a more complete recovery from the water stress symptoms on rewatering, than the low-P plants. High- P plants had greater OA, proline concentration and leaf expansion rate. On the other hand, low-P plants showed an increased osmotic concentration when there was no change in the total solute content per unit of leaf d. wt, indicating more loss of water from the leaf tissue. The key measures that appeared to be directly associated with plant recovery over a short period following water stress were increased proline concentration and leaf expansion rate, probably resulting from increased cell-wall extensibility rather than increased production of cells for the high-P plants.