The correlation of carbon isotope discrimination (△¹³C) with photosynthetic gas exchange and water use efficiency (WUE) in maize was investigated under low rainfall conditions with or without superabsorbent polymer (SAP). SAP (45 kg/ha) was mixed into the top 10 cm soil layer at sowing in lysimeters. Compared with the control plants not treated with SAP, the application of SAP increased net photosynthesis rate; stomatal conductance (g_s); transpiration rate; chlorophyll

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... ent (Chl) and intrinsic water use efficiency at leaf level (WUE_i), but decreased intercellular CO₂ concentration (C_i) and leaf △¹³C. In plants supplied with SAP, leaf △¹³C was positively correlated with C_i (r = 0.864, P < 0.01) and negatively correlated with g_s and WUE_i (r = –0.860 and –0.626, P < 0.01, respectively). Leaf △¹³C was not correlated with Chl with or without SAP. Grain △¹³C significantly decreased by 12.4% and showed a significant negative correlation with grain WUE under SAP treatments (r = –0.670, P < 0.05). These results suggest that in the presence of SAP, maize leaf and grain △¹³C could be good indicators for evaluating maize WUE during periods of low rainfall.