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 (Medica go sativa L. cv. Franken neu) by adding PEG 6000 to the nutrient solut ion, thus decreasing the water potential to -0.5 MPa. Relative water c ontent and leaf water potential were significantly reduced after a 3- and 5-day stress period. This resulted in inhibited dry weight product ion relative to control plants after 5 days. Analyses of glucose, fruc tose, and sucrose revealed no assimilate shortage in nodules. In contr ast, decreased plant growth resulted in glucose and sucrose accumulati on in shoots. Plant growth reduction due to drought stress was accompa nied by increased amino acid concentrations in shoots, roots, and nodu les. It is concluded that the generally observed inhibition of nitroge nase activity during drought stress can be best explained in terms of product inhibition of the enzyme as an efficient feedback regulation m echanism triggered by reduced nitrogen demand of the host plant.