Mesophyll conductance (gm) limits the diffusion of CO2 from sub-stomatal cavities to the carboxylation site and is a significant limitation to photosynthesis. However, there is a lack of complete understanding of gm variability and its regulation under different environmental conditions, and relevant studies in grain legumes are scarce. My research projects aimed to characterize genetic variability in gm, to quantify the response of gm to short- and long-term environmental changes and to assess the relationship of gm with leaf water-use efficiency (WUE) in grain legumes. gm and leaf hydraulic conductance (Kleaf) were examined simultaneously under growth conditions to see if they show coordinated response. gm varied significantly among genotypes for most of the legumes studied, but the genotypes did not vary in cell wall and plasma membrane conductance (measured from the oxygen isotope method). gm responded to growth or measurement environments, increasing rapidly with increasing light intensity but decreasing under blue light. However, genotypes differed in their interactive response to water stress, light intensity and light quality. Rapid response of gm to changes in light intensity was affected by N source (N2-fixing or inorganic-N fed). Short-term temperature response of gm varied between species. Chloroplast membrane conductance varied among species and genotypes and with growth environment. Environmentally driven leaf anatomical traits were not the major factors determining gm, but genotypes differed in the degree to which leaf anatomy influenced gm. Our results did not show coordination between gm and Kleaf. gm was strongly associated with photosynthetic rate but not with stomatal conductance. The results of this project provide useful information for crop genetic improvement through gm in legumes under climate change scenarios. Increasing gm in legumes will increase photosynthetic rate and possibly WUE, when there is no increase in stomatal conductance.