Phosphorus nutrition and water stress effects on proline accumulation in sorghum and bean

Phosphorus deficiency and drought can reduce plant growth and alter metabolic processes such as N metabolism. The amino acid proline often accumulates in leaves of many plants grown under water stress, but information about effects of P nutrition on proline accumulation is not available. Two sorghum [Sorghum bicolor (L.) Moench] and two bean (Phaseolus vulgaris L. and P. acutifolius A. Gray) cultivars with different tolerances to drought were grown in a greenhouse in soil mixes with different levels of P with and without water stress imposed by withholding water to determine the effects of P nutrition and water stress on proline accumulation. Shoot dry matter of sorghum and bean increased with increasing P level regardless of water stress. Leaves of water-stressed sorghum had lower P concentrations than non water-stressed plants, while leaves of water-stressed and non water-stressed bean had similar P concentrations. Leaf diffusive resistance (r(s)) values became greater and leaf water potential (Psi(L)) values became more negative (greater tension) as severity of water stress increased compared to non water-stressed plants. These traits became lower and less negative (less tension), respectively, and nearly comparable to non water-stressed plants 5d after water stress was relieved. Both plant species had higher proline accumulation during water-stressed than under non water-stressed conditions, but differed in their response to added P. Sorghum leaves had highest proline when grown with high P, while bean leaves had highest proline when grown with low P. Five days after plants were relieved of water stress, proline accumulation in sorghum leaves decreased extensively yet remained at relatively high levels, while proline accumulation in bean leaves decreased to the level for non water-stressed plants. High proline accumulation in leaves of water-stressed plants grown at high P levels might be an adaptive response to drought for sorghum, but did not appear to be so for bean.