EFFECT OF PHOSPHORUS FERTILIZATION ON WATER-STRESS IN DOUGLAS-FIR SEEDLINGS DURING SOIL DRYING

A growth chamber experiment was conducted to determine if P fertilizat ion to enhance the P nutrition of otherwise N and P deficient Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings reduces water str ess in the seedlings during drought periods. Seedlings were grown in p asteurized mineral soil under well-watered conditions and fertilized p eriodically with a small amount of nutrient solution containing P at e ither of three levels: 0, 20, or 50 mg P L-1. By age 6 mo, leaf nutrie nt analysis indicated that N and P were deficient in control (0 mg P L -1) seedlings. The highest level of P fertilization, which doubled lea f P concentration, did not affect plant biomass, suggesting that N def iciency was limiting growth. When these seedlings were subjected to dr ought, there was no effect of P fertilization on leaf water potential or osmotic potential. Furthermore, P fertilized seedlings had lower st omatal conductance and net photosynthesis rate. These results indicate that enhanced P nutrition, in the presence of N deficiency, does not reduce water stress in Douglas fir seedlings during drought periods.