Water relations of white clover (Trifolium repens L.) in a drying soil, as a function of phosphorus supply and defoliation frequency

A glasshouse pot experiment was carried out to determine how white clover plants responded to repeated drying cycles, under conditions of varying P supply and defoliation frequency. Measurements included leaf water potential (Ψ(leaf)), evapotranspiration rate, soil mattic potential (Ψ(soil)), an assessment of visual water stress symptoms, and the rate of plant recovery on rewatering. The rate of decline in Ψ(leaf) per unit decline in Ψ(soil) was greater in frequently defoliated plants. High-P plants extracted more water per unit time, were able to maintain a higher Ψ(leaf) in dry soil than low-P plants, displayed minimal water stress symptoms, and recovered completely on rewatering. High-P, frequently defoliated clover plants displayed minimal water stress symptoms in dry soil, whereas frequently defoliated low-P plants showed extreme symptoms; these plants were unable to recover from severe water stress and most plants died during the final drying cycle. Infrequently defoliated, low-P plants partially recovered from the water stress condition, but the recovery was slower than that for infrequently defoliated high-P plants. The findings suggest that the combination of dry soil, low P supply, and frequent defoliation that would result from heavy grazing would threaten the survival of white clover plants in the field.