Effect of phosphorus on drought tolerance inChloris gayanaKunth andCoix lacryma-jobiL

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Abstract
In order to evaluate the role of phosphorus (P) nutrition on the drought tolerance of warm season forage crops, i.e. Rhodesgrass (Chloris gayana Kunth) and Job's tears (Coix lacryma-jobi L. var.ma-yuen Stapf.) which are considered to be the most and the least tolerant species to water stress, respectively, the crops were subjected to water stress at two P nutrition levels. Leaf water potential (ψ i), stomatal resistance (γ s), leaf area, shoot and root dry weight, root development, and photosynthetic rate (P o) were recorded. Shoot and root dry weights decreased at the lower P level and under water stress in both species. Total leaf area per plant decreased by stress and it was smaller at the lower P level than at the higher P level in both species under stress conditions. The root development of both species was markedly inhibited by water stress. In rhodesgrass, total root length and root surface area at the higher P level were 2.3 and 1.7 times larger than those at the lower P level, respectively. They were 2.3 and 2.7 times larger in Job's tears. Roots were distributed in deeper layers at the higher P level than the lower P level. Leaf water potential (ψ i) decreased and (γ s) increased in the water stress treatment. Stomatal sensitivity was increased and P o was decreased at the lower P level under water stress conditions in both species. However, at the higher P level, plants maintained a higher value of P o at low water potentials compared to the lower P level. The results indicated that a higher level of P nutrition increased the ability of plants to tolerate drought in both species through the increase of total leaf area, and development and penetration of roots in deeper soil layers. Furthermore, P o and γ s of the plants grown under low P nutritional regime were found to be more sensitive to reductions in ψ i, compared to the plants grown at a higher level of P nutrition.