Water deficit stress affects on N-2 fixation in cowpea inoculated with different Bradyrhizobium strains

The objectives of this experiment were to select strains of Bradyrhizobium sp. resistant to water stress, envisaging an increase in N-2 fixation in co wpea (Vigna unguiculata [L.] Walp.), and to verify the plant's adaptive phy siological responses to water stress. The experiment was carried out in gre enhouse conditions using random complete blocks subdivided into plots adjus ted to soil water potential levels of -6.0, -75.0, and -85.0 kPa, and subpl ots containing strains of Bradyrhizobium sp. (SEMIA 6145, 6086, 6002 and NF B 700), with four blocks. The soil was a Yellow Latosol with pH 6.3. The cr op used was cowpea cv. IPA 204. Stress was applied continuously beginning 1 5 d after planting, by the control of water potential through a porous cup. Various parameters were evaluated every 7 days, until final harvest at 45 d. There was significant interaction between Bradyrhizobium strains and wat er stress. At the more negative psi(m), plants inoculated with the SEMIA 61 45 had higher LHb concentration, ureide-N, psi(w) and root dry matter, form ing associations of greater symbiotic efficiency, while plants inoculated w ith SEMIA 6086 were not resistant to stress. LHb concentration apparently w as not inhibited at psi(w)-1.0 MPa in cowpea. The Bradyrhizobium strains ma y have affected the metabolism of N assimilation and/or transport.