Response of tomato (Lycopersicon esculentum L.) cultivars to foliar application of zinc when grown in sand culture at low zinc
Plants of two tomato cvs., 'Blizzard' and 'Liberto' were grown in sand culture in a glasshouse at Zn concentrations of 0.15 and 7.70 μmol 1 in the nutrient solution. Foliar treatments entailed applying zinc as either 0, 0.35 or 3.5 mmol 1 ZnSO·7HO to the tops of plants grown at low zinc (0.15 μmol 1) in nutrient solution twice a week during the course of the experiment. Plants treated with 0.15 μmol 1 Zn in the nutrient solution and high levels of zinc (3.5 mmol 1) applied as a foliar spray showed a significant decrease in the production of dry matter, chlorophyll and green fruit yield as compared with those grown both at 7.70 μmol 1 zinc in the nutrient solution and at 0.15 μmol 1 zinc in nutrient solution with 3.5 mmol 1 zinc applied as a foliar spray. There were differences between the cultivars but no consistent link between these differences and nutrient concentrations within the plant. Concentrations of K, Mg and Zn were lower in the leaves and fruit of both the cultivars in 0.15 μmol 1 zinc in the nutrient solution treatment as compared with both the 7.70 μmol 1 in the nutrient solution treatment and with supplementary foliar applications of zinc at 0.35 mmol 1. Potassium and Mg were also lower in the leaves and fruit of both the cultivars receiving foliar applications of zinc at 3.5 mmol 1. The concentrations of Fe and P were significantly higher in the leaves of plants grown in low (0.15 μmol 1) root zone zinc treatment and P was also higher in both the leaves and fruit of plants receiving foliar applications of zinc at 3.5 mmol 1. In the roots, concentrations of Zn, Fe, P and K increased with increasing zinc concentration in the nutrient solution and also as a foliar spray. Mg was lower in the roots of plants in the 7.70 μmol 1 zinc in the nutrient solution as compared with all other zinc treatments. These results clearly indicate that foliar application of zinc can overcome the negative effects of zinc deficiency on plant growth when it is applied at optimal range.