Impact of soil water regimes on physiological responses and water use efficiency (WUE) for Vigna unguiculata L. Walp. (cowpea) inoculated with rhizobia still remains implicit. Therefore, the goal of the current study was to examine the leaf gas exchange, abscisic acid (ABA) and hydraulic signaling, WUE and carbon and oxygen isotopic compositions (δ¹³C and δ¹⁸O) of cowpea under different soil water levels. The treatments included soil water regimes at three levels (90%, 70%, and 50% of soil water holding capacity (SWHC)) and two inoculation forms (inoculated and non-inoculated with rhizobia). The results showed that across the inoculation treatments, reduced soil water regimes depressed both stomatal conductance (g_s) and photosynthesis (A_n) of the leaves, nonetheless, the decrease of g_s was more pronounced compared with the reduction in A_n. Consequently, the intrinsic water use efficiency (WUE_i) was improved in the treatments under decreased soil water conditions. Plant WUE was also improved when soil water contents decreased as exemplified by the increased leaf δ¹³C and δ¹⁸O, indicating the enhanced plant WUE was mainly attributed to the decrease of g_s. Significant interactions between soil water regimes and rhizobia treatments for root water potential (RWP), leaf water potential (LWP), and g_s were found due to the different responses of rhizobia to varied soil water regimes. Inoculation could improve plant water status and g_s under 70% and 90% SWHC compared to 50% SWHC with negative effect from rhizobia. A moderate soil water regime is suggested for cowpea production in terms of high WUE with a minor biomass reduction.

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