Abstract
In order to evaluate the role of phosphorus nutrition in drought tolerance of warm season forage crops, Chloris gayana and Coix lacryma-jobi var. mayuen which had been classified to the most tolerant and the weakest species to water stress, respectivily, grown under various levels of phosphorus were subjected to drought. The leaf water potential (Ψl), stomatal resistance (Υs), leaf area, shoot and root dry weights, root development, nutrient concentration of leaves, stems and roots were determined. Shoot and root dry weights of both species decreased with the increment of phosphorus deficiency and water stress. Water stress markedly decreased shoot and root dry weights in plants, but the decrease was lessened in phosphorus sufficient plants. The root development of phosphorus deficient plants was severely inhibited by the stress. The penetration and development of roots in phosphorus deficient plants was not depressed in deeper soil layers under the water stress condition. Phosphorus application enhanced leaf area expansion, leading to an increased total leaf area in both species under water stress. Leaf water potential Ψl decreased and Rs increased under water stress condition. Assuming the maximum stomatal resistance of 10 s/cm in Chloris gayana, 50% stomatal closure occurred at the Ψl of -0.8 〜 -1.0 MPa for phosphorus deficient plants and -1.4 〜 -1.6 MPa for phosphorus sufficient plants. The values for Coix larcryma-jobi var. mayuen was -0.7 〜 -0.9 MPa for phosphorus deficient plants and -1.2 〜 -1.4 MPa for phosphorus sufficient plants. Phosphorus deficiency increased the stomatal sensitivity and decreased the photosynthetic activity under the stress condition. However, phosphorus sufficient plants maintained higher photosynthetic rates than phosphorus deficient plants under low water potential condition.
