Zinc sulphate or zinc nanoparticle applications to leaves of green beans

The green bean (Phaseolus vulgaris L.) is a very widely grown food crop that contributes significantly to human dietary needs in many countries due to its high content of protein. This study evaluates foliar applications of ZnSO₄ versus that of zinc oxide nanoparticles (ZnO NPs) to leaves of the green bean cv. ‘Strike’ and records the plant responses in terms of Zn uptake and concentrations of photosynthetic pigments and bioactive compounds. The experiment was conducted under greenhouse conditions in Chihuahua, Mexico, with a completely randomised experimental design with 10 replicates. The two treatments were foliar applications of either an aqueous solution of ZnSO₄ or an aqueous suspension of ZnO NPs (both 150 mg · L⁻¹). The application of ZnO NPs significantly increased concentrations of Zn²⁺ in the leaflets, roots, stems and pods of chlorophylls a and b (values 15.40 μg · g⁻¹ and 11.64 μg · g⁻¹, respectively). Sucrose concentration was also increased by Zn²⁺ applications, but no differences were found in total phenols (TP), total flavonoids (TFl) or antioxidant capacity (AC). In the pods and seeds, Zn²⁺ application left sucrose and TFl concentrations unchanged, but the TP increase was significant. The AC was affected by both zinc sources and only in the pods. The applications of ZnSO₄ or ZnO NPs significantly increased the biomass accumulation (79.10 g · p⁻¹ and 84.70 g · p⁻¹ DW) and yield (55.64 g · p⁻¹ and 53.80 g · p⁻¹ FW). These results suggest that the application of ZnO NPs could represent a worthwhile biofortification strategy in the commercial production of green bean cv. ‘Strike’.