Alleviation of water stress effects in cowpea by Bradyrhizobium spp. inoculation

Experiments were carried out to investigate the effects of different degree s of water stress on cowpea in the presence and absence of Bradyrhizobium s pp. inoculation and to evaluate physiological responses to stress. The soil used was Yellow Latosol, pH 6.3 and the crop used was cowpea (Vigna unguic ulata (L.) Walp.) cv. 'IPA 204'. Stress was applied continuously by the con trol of matric potential (psi(m)) through a porous cup. The lowered soil ps i(m) had a direct effect on the N-2 fixation, but the strains Bradyrhizobiu m introduced by inoculation in the cowpea plants were superior to the indig enous strain demonstrating the importance of inoculation in the stressed pl ants. At the more negative psi(m) plants inoculated with the strains EI 6 f ormed associations of greater symbiotic efficiency which helped the cowpea plants to withstand drought stress better than the strain BR 2001 and the u ninoculated control. The leghaemoglobin concentration was not inhibited in the drought-stressed plants at psi(m) -70 kPa when inoculated with the stra in EI 6, which confered a differential degree of drought resistance in plan ts. The psi(w) declined in the stressed plants reaching values of -1.0 MPa which was sufficient to cause disturbance in nodulation and biomass product ion.