Effects of Excess NH₄-N or NO₃-N Fertilizer Applications on Leaf Injury, Vegetative Growth, and Leaf Mineral Contents in Passion Fruit

Abstract

Sufficient application of nitrogen (N) is necessary to increase passion fruit production, although excess N is believed to be toxic. However, the impact of toxicity by excess use of N fertilizer is unknown. Three different solutions containing 100 mM NH₄-N, 100 mM NO₃-N, or intermixed 50 mM NH₄-N and 50 mM NO₃-N (Mix-N) were applied for 4 weeks to passion fruit grown in sand to understand the toxicity of excess N. Visible damage, vegetative growth, and leaf mineral contents were examined. Severe necrosis and defoliation were observed in the NH₄-N plants because of high accumulation of leaf NH₄-N. The necrotic leaves contained 1.2-2.5 mg·g^-1 dry wt NH₄-N, whereas healthy leaves contained only 0.1-0.4 mg·g^-1 dry wt. Passion fruit recovered quickly from this toxicity. Two weeks after fertilizer application was stopped, newly emerged leaves showed no symptom of necrosis or irregular accumulation of NH₄-N. Chlorosis and defoliation observed in the NO₃-N plants were attributed to a nutrient unbalance. No visible symptom appeared in the Mix-N plants even under equally high N concentrations of 100 mM.