Source of nitrogen nutrition (nitrogen fixation or nitrate assimilation) is a major factor involved in pea response to moderate water stress

The effect of the source of nitrogen nutrition (nitrogen fixation or nitrat e assimilation) on the response of pea plants to a gradual and moderate wat er stress was studied. Growth declined under water deficit, but nodulated p lants were less sensitive to drought than nitrate-fed plants. Stomatal cond uctance and internal CO2 concentration also decreased, but both were higher in nitrogen-fixing plants throughout the drought period, leading to better maintenance of carbon assimilation rates under water deficit. Glycolate ox idase, a key enzyme in the photorespiratory cycle, declined by 50 % in nitr ogen-fixing plants under water deficit, although it was not affected in nit rate-fed plants. Nitrogen assimilation declined during the drought period a nd was independent of nitrogen source. Free amino acid content declined in leaves of plants grown under both nutrition regimes, reflecting the decreas e in nitrogen assimilation. Water stress led to carbohydrate accumulation i n pea plants grown with either nitrogen source, but it was higher in nitrog en-fixing plants. Roots showed the greatest carbohydrate and amino acid acc umulation in both nutritions regimes, with significantly greater increases in free amino acids in nitrate-fed plants. It is concluded that the nitroge n source is a major factor affecting pea responses to water stress, althoug h the difference in sensitivity seems to be related not to the nitrogen ass imilation process but to complex interactions with photorespiratory flux an d stomatal conductance.