EFFECT OF DROUGHT STRESS ON GROWTH, SUGAR CONCENTRATIONS AND AMINO-ACID ACCUMULATION IN N-2-FIXING ALFALFA (MEDICAGO-SATIVA)

Drought stress was induced in symbiotically N-2-fixing alfalfa (Medicago sativa L. cv. Franken neu) by adding PEG 6000 to the nutrient solution, thus decreasing the water potential to -0.5 MPa. Relative water content and leaf water potential were significantly reduced after a 3- and 5-day stress period. This resulted in inhibited dry weight production relative to control plants after 5 days. Analyses of glucose, fructose, and sucrose revealed no assimilate shortage in nodules. In contrast, decreased plant growth resulted in glucose and sucrose accumulation in shoots. Plant growth reduction due to drought stress was accompanied by increased amino acid concentrations in shoots, roots, and nodules. It is concluded that the generally observed inhibition of nitrogenase activity during drought stress can be best explained in terms of product inhibition of the enzyme as an efficient feedback regulation mechanism triggered by reduced nitrogen demand of the host plant.