288 Effects of Foliar Urea on Reserve Nitrogen and Carbohydrates in Young Apple Trees with Different Nitrogen Background

Abstract

Bench-grafted Fuji/M26 plants were fertigated with seven nitrogen concentrations (0, 2.5, 5.0, 7.5, 10, 15, and 20 mM) by using a modified Hoagland solution from 30 June to 1 Sept. In mid-October, half of the fertigated trees were sprayed with 3% urea twice at weekly intervals, while the other half were left as controls. The plants were harvested after natural leaf fall, stored at 2 °C, and then destructively sampled in January for reserve N and carbohydrate analysis. As N concentration used in fertigation increased, whole-plant reserve N content increased progressively with a corresponding decrease in reserve carbohydrate concentration. Foliar urea application increased whole-plant N content and decreased reserve carbohydrate concentration. The effect of foliar urea on whole-plant reserve N content and carbohydrate concentration was dependent on the N status of the plant, with low-N plants being more responsive than high-N plants. There was a linear relationship between the increase in N content and decrease in carbohydrate concentration caused by foliar urea, suggesting that part of the reserve carbohydrates was used to assimilate N from foliar urea. Regardless of the difference in tree size caused by N fertigation, the increase in the total amount of reserve N by foliar urea application was the same on a whole-tree basis, indicating that plants with low-N background were more effective in using N from urea spray than plants with high-N background.