Effect of RhizobiumSp. Inoculation on N2-Fixing and Photosynthetic Activities of Two Cowpea [Vigna unguiculata(L.) Walp.] Genotypes

Time course of symbiotic N2-fixing and photosynthetic activities during vegetative growth from 30 d after plantation until pod set was measured in the CB5 and 7964 cowpea [Vigna unguiculata(L.) Walp.] genotypes of contrasting senescence traits. At emergence, seedlings were inoculated with a "non-cowpea miscellany" Rhizobiumstrain generally used to inoculate Cicer arietinum. Maximum N2-fixing activity occurred in inoculated CB5 and 7964 plants about 54 and 68 d after plantation, respectively. A similar temporal shift of maximum was found for net photosynthetic rate (PN), confirming a good coordination between the two processes. A higher PNwas found from the first measurements in inoculated plants of both genotypes as compared with uninoculated plants. Apparently, the maximum activity of both N2-fixation and PNwas timed to occur at a particular stage of plant ontogeny correlating the high N supply with the high N demand by the plant. Rhizobiuminoculation did not significantly affect partitioning coefficients of biomass to various plant organs but extended leaf longevity by about 10 d in the CB5 genotype, retarding thus the monocarpic senescence.