Application of Uniconazole Improves Photosynthetic Efficiency of Maize by Enhancing the Antioxidant Defense Mechanism and Delaying Leaf Senescence in Semiarid Regions

The degradation of photosynthetic pigments leads to leaf senescence and thus grain yield losses. We determined whether the application of uniconazole to maize in semiarid regions could reduce the degradation of photosynthetic pigments by enhancing the antioxidant defense system. We conducted a field study in the summers of 2015 and 2016 where seeds were soaked in uniconazole at concentrations of 0 (CK1), 25 (S1), 50 (S2), and 75 (S3) mg kg−1 and foliar sprayed at concentrations of 0 (CK2), 25 (F1), 50 (F2), and 75 (F3) mg L−1 at the eight-leaf stage. The application of uniconazole significantly improves the chlorophyll content, soluble protein content, and net photosynthetic rate, where the maximum values were obtained with the S1 and F1 treatments. Uniconazole significantly improved the activities of antioxidant enzymes comprising superoxide dismutase, peroxidase, and catalase, but reduced that of malondialdehyde (MDA) and the accumulation of reactive oxygen species (ROS) during the leaf senescence process. Treatments S1 and F1 had higher antioxidant enzyme activities but reduced MDA, superoxide radical, and hydrogen peroxide contents. Uniconazole significantly reduced leaf senescence in leaves in the bottom layer, while also increasing the middle layer leaf area and decreasing the top layer leaf area. The degradation of photosynthetic pigments was reduced by uniconazole because the enhanced antioxidant activities of enzymes protected plants from harmful ROS. Uniconazole significantly improved the photosynthetic traits, antioxidant defense system, and grain yield in maize in semiarid regions, where the most effective treatment was S1.