Leaf growth, gas exchange and assimilation performance of cowpea varieties in response to Bradyrhizobium inoculation

Supplying nitrogen to crops through selecting high N fixing legumes and effective inoculant is one of the key strategies to improve crop productivity. However, studies related to the effect of Bradyrhizobial inoculation on leaf growth, its functioning in relation to photosynthesis, and transpiration efficiency (WUE) of cowpea [Vigna unguiculata (L.) Walp] varieties in the tropics were inadequate. A two-year field experiment was conducted at three sites to evaluate the effect of inoculation on leaf growth, gas exchanges and photosynthetic efficiency of cowpea varieties. The study treatments were composed of four varieties, Keti (IT99K-1122), TVU, Black eye bean, and White wonderer trailing and three levels of inoculation (non-inoculated or inoculated with Bradyrhizobium strains CP-24 or CP-37). Gas exchange was measured on live plants at 67–77 days after sowing, between 8:00 to 11:00 a.m. and 14:00 to 16:00 p.m. Leaf growth parameters (leaf number and leaf area) were measured by destructive sampling, and the yield data was determined by harvesting plants in the three central rows at physiological maturity. Variety TVU performed best in terms of leaf number, photosynthesis rate, and WUE. Whereas, Black eye bean revealed superior performances for leaf area, leaf area index, and stomatal conductance compared with the rest two varieties. The effect of inoculation was significant with 14.0, 23.8, 13.7, and 11.0% advantage in leaf area, leaf area index, net photosynthesis, and WUE, respectively. Moreover, the performance of cowpea of the 2018 cropping season showed a relative advantage over 2019 in terms of leaf number, leaf area, leaf area index, net photosynthesis, and stomatal conductance. Therefore, inoculating cowpea varieties with effective Bradyrhizobium strain can be a viable alternative to enhance growth, gas exchange, photosynthetic efficiency, and grain yield.