Effect of water stress on nodule physiology and biochemistry of a drought tolerant cultivar of common bean (Phaseolus vulgaris L.)

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 O-2, diffusion resistance and induced nodule senescence. A time-course analysis of the IO 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. (C) 1999 Annals of Botany Company.