Effect of Water Stress on Nodule Physiology and Biochemistry of a Drought Tolerant Cultivar of Common Bean (Phaseolus vulgarisL.)

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Plants of EMGOPA-201, a drought tolerant cultivar of common bean(Phaseolus vulgaris), were maintained either at 90% soil field capacity (SFC) or stressed by reducing SFC to 70, 50 or 30% over a 10 d period. Plant dry weight was not affected by any of these treatments although the number and weight of nodules was reduced at 50 and 30% SFC. Nitrogenase activity, determined by the acetylene reduction assay (ARA), was also reduced, on a plant basis, at 50% SFC and was almost stopped at 30% SFC. The latter treatment caused a marked increase in nodule O2diffusion resistance and induced nodule senescence. A time-course analysis of the 10 d 30% SFC treatment showed a decrease in leaf water potential from -0.5 to -0.87 MPa by 8 d, with a cessation of dry weight increase after 3 d, when leaf water potential was -0.65 MPa. Proteins in the host plant fraction of nodules decreased to 50% of control values by 10 d and leghaemoglobin (Lb) content was also lower at this stage. The activity of sucrose synthase (SS) showed a 76% reduction between 3 and 6 d, whilst glutamate synthase (GOGAT) activity showed a 40% reduction. The activity of other key enzymes of carbon metabolism was also reduced after 10 d. Nodule sucrose content increased to double that of control nodules by 6 d, before declining back to control levels at 10 d. Starch content fell by 3 d and continued to fall throughout the stress period. The results are discussed in terms of drought tolerance strategies in relation to growth and metabolism in whole plants and nodules.