Phosphorus concentrations in the leaves of defoliated white clover affect abscisic acid formation and transpiration in drying soil

Increased leaf phosphorus (P) concentration improved the water-use efficien cy (WUE) and drought tolerance of regularly defoliated white clover plants by decreasing the rate of daily transpiration per unit leaf area in dry soi l. Night transpiration il;as around 17% of the total daily transpiration. T he improved control of transpiration in the high-P plants was associated wi th an increased individual leaf area and WUE that apparently resulted from net photosynthetic assimilation rate being reduced less than the reductions in the transpiration (27% vs 58%). On the other hand, greater transpiratio n from low-P plants was associated with poor stomatal control of transpirat ional loss of water, less ABA in the leaves when exposed to dry soil, and t hicker and smaller leaf size compared with high-P leaves. The leaf P concen tration was positively related with leaf ABA, and negatively with transpira tion rates, under dry conditions (P <0.001). However, leaf ABA was not clos ely related to the transpiration rate, suggesting that leaf P concentration has a greater influence than ABA on the transpiration rates.