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

Abstract

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 matric 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.