NITROGENASE ACTIVITY OF COWPEA (Vigna unguiculata (L.) Walp.) DURING AND AFTER DROUGHT STRESS

Abstract

Greenhouse experiments were conducted with the objectives (1) to investigate the nitrogenase activity (NA) of cowpea (Vigna unguiculata (L.) Walp.) root nodules during the development of and subsequent recovery from drought stress and (2) to determine whether the changes in NA during and following drought stress are related to nodule water potential. Nitrogenase activity of root nodules decreased by more than 80% within 6–8 d of withholding water and recovered 1 or 2 d after watering. Nodule water potential declined significantly from approximately −0.2 MPa to −0.48 MPa with 8 d of stress and recovered to prestress levels within 24 h after watering. Midday abaxial stomatal conductance decreased significantly with stress but recovered within 24 h following watering. Midday leaf water potential did not change significantly during the experimental period. Nodule NA declined 2 d before that of nodule water potential in apparent response to declining soil water content. This response and the lag in the recovery of NA following drought stress after nodule water potential had returned to prestress levels support the hypothesis that nodule water potential per se is not the primary cause for the decline in NA of cowpea root nodules during drought stress.Key words: Vigna unguiculata (L.) Walp., nitrogenase activity, drought stress, recovery, cowpea