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Grain legumes are the main source of protein for subsistence farmers in the semi-arid tropical regions where production is limited by many biotic and abiotic stresses. Knowledge of resource capture by crop species can be one of the options in improving the productivity of crops in this environment. A study was conducted on three grain legume species, viz. common bean (Phaseolus vulgaris L.), chickpea (Cicer arietinum L.) and cowpea (Vigna unguiculata L.) to study their radiation interception, radiation use efficiency (RUE) and seed yield under three water regimes in a semi-arid region of Ethiopia in two seasons. The water regimes were well-watered control (C), mid-season/flowering and pod setting period water deficit (MS) and late season/pod filling period water deficit (LS). It was found that dry matter production was highly associated with the fraction of photosynthetically active radiation (PAR) intercepted (F, r = 0.83* to 0.96**), which in turn is positively correlated with LAI (r = 0.78* to 0.98*) across all species and water regimes. The lowest values of F, RUE, canopy extinction coefficient (K), seed yield and harvest index (HI) were recorded in the MS treatment. The reduction in RUE due to MS was 39, 30 and 29% and due to the LS was 18, 19 and 17% for beans, chickpea and cowpea, respectively. RUE was more sensitive to early stage than late stage reproductive water deficit in beans and chickpea while it was not significantly affected by any of the stress treatments in cowpea. RUE was positively significantly correlated with seed yield (r = 0.85** to 0.89**) and HI (r = 0.72* to 0.88**) over the two seasons. It is concluded that attainment of high LAI that reduces soil water evaporation, intercepts and converts radiation into dry matter efficiently, and partitioning of the dry matter to the seed is the major requirement of a high seed yield in grain legumes in semi-arid environments. (c) 2006 Elsevier B.V. All rights reserved.