Kinetin and Indole Acetic Acid Promote Antioxidant Defense System and Reduce Oxidative Stress in Maize (Zea mays L.) Plants Grown at Boron Toxicity

A study was conducted to assess the influence of boron (B) toxicity on functioning of antioxidant machinery to counteract oxidative stress in maize (Zea mays L.) plants as well as the mitigating effect of kinetin (KIN) and indole acetic acid (IAA) on these phenomena. Plants of maize cv. DK 647 F1 were exposed to 0.05 and 2 mM boron in nutrient solution 8 days after germination, and the plants were grown for a further 7 days in these conditions. After 15 days growth, deionized water (control), 1.0 or 2.0 mM of KIN, or IAA were applied to the leaves of maize plants once each 7 days. After 21 days of these treatments, the plants were harvested to evaluate growth, water relations, and oxidative and antioxidative systems. Boron toxicity significantly reduced dry matter, efficiency of photosystem II (Fv/Fm), and leaf relative water content in the maize plants when compared to those in non-stressed plants, but in contrast, it enhanced electrolyte leakage (EL), hydrogen peroxide (H2O2), free proline, malondialdehyde (MDA) and the activities of peroxidase, superoxide dismutase, and catalase in the maize plants. However, KIN or IAA applied as a foliar spray to maize plants grown at excess B caused a significant improvement in growth attributes, plant water status and the activities of various antioxidant enzymes as well as proline content, but they lowered EL, and H2O2 and MDA contents. Boron toxicity increased leaf B and reduced leaf K+, Ca2+, and P contents when compared to those in the control plants. Foliar applied KIN or IAA to the plant leaves lowered tissue B levels, but in contrast, it resulted in significant increases in Ca2+, K+ and P levels. The results of the study indicated that the spray of KIN and IAA, particularly at 2 mM, can mitigate to a significant extent the adverse effects of B toxicity on maize plants, which was found be associated with reduced content of B, H2O2, MDA as well as EL, and increased activities of key antioxidant enzymes in maize plants.