Alternate partial root-zone irrigation reduces bundle-sheath cell leakage to CO2 and enhances photosynthetic capacity in maize leaves

The physiological basis for the advantage of alternate partial root-zone irrigation (PRI) over common deficit irrigation (DI) in improving crop water use efficiency (WUE) remains largely elusive. Here leaf gas exchange characteristics and photosynthetic CO2-response and light-response curves for maize (Zea mays L.) leaves exposed to PRI and DI were analysed under three N-fertilization rates, namely 75, 150, and 300 mg N kg(-1) soil. Measurements of net photosynthetic rate (A(n)) and stomatal conductance (g(s)) showed that, across the three N-fertilization rates, the intrinsic WUE was significantly higher in PRI than in DI leaves. Analysis of the CO2-response curve revealed that both carboxylation efficiency (CE) and the CO2-saturated photosynthetic rate (A(sat)) were significantly higher in PRI than in DI leaves across the three N-fertilization rates; whereas the N-fertilization rates did not influence the shape of the curves. The enhanced CE and A(sat) in the PRI leaves was accompanied by significant decreases in carbon isotope discrimination (delta C-13) and bundle-sheath cell leakiness to CO2 (Phi). Analysis of the light-response curve indicated that, across the three N-fertilization rates, the quantum yield (alpha) and light-saturated gross photosynthetic rate (A(max)) were identical for the two irrigation treatments; whilst the convexity (kappa) of the curve was significantly greater in PRI than in DI leaves, which coincided with the greater CE and A(sat) derived from the CO2-response curve at a photosynthetic photon flux density of 1500 mu mol m(-2) s(-1). Collectively, the results suggest that, in comparison with the DI treatment, PRI improves photosynthetic capacity parameters CE, A(sat), and kappa of maize leaves and that contributes to the greater intrinsic WUE in those plants.