Use of cytoplasmic male sterility (CMS) in hybrid breeding requires effective male fertility-restoring lines. In sorghum, very few restoring lines that can restore fertility in A3 CMS have been reported. To identify the reasons for this deficiency, F1 and F2 hybrids of an A3 CMS line crossed with the line IS1112C, a donor of fertility-restoring (Rf) genes for A3 cytoplasm, and testcrosses of fertile plants to A3 CMS lines were grown under contrasting water availability regimes in dryland and irrigated field plots. In the irrigated plots the frequency of fertile plants in testcrosses was twice that in dryland plots (P < 0.05). Fertile plants from the F2 family grown in the irrigated plots showed significantly higher restoration ability than fertile plants from the same family grown in dryland plots. F3 plants from the F2 family grown in irrigated plots yielded on average a sixfold higher frequency of fertile plants in testcrosses than F3 plants derived from dryland plots (P < 0.01). Fertility of testcross hybrids correlated negatively with air vapor pressure deficit (VPD) at flowering (r = −0.96; P < 0.01) suggesting that VPD is a trigger for downregulation of Rf genes for A3 cytoplasm.