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One-year-old `Concord' vines were fertigated with 0, 5, 10, 15, or 20 mm N in a modified Hoagland's solution for 8 weeks during summer. Half of the vines fertigated at each N concentration were sprayed with 3% foliar urea twice in late September while the rest served as controls. Four vines from each treatment combination were destructively sampled during dormancy to determine the levels and forms of N and carbohydrates. Nitrogen fertigation during the summer only slightly increased vine N concentration whereas foliar urea application in the fall significantly increased vine N concentration. In response to foliar urea application, concentrations of both free amino acid-N and protein-N increased, but the ratio of protein N to amino acid N decreased. Arginine was the most abundant amino acid in free amino acids and proteins, and its concentration was linearly correlated with vine N concentration. Concentrations of total non-structural carbohydrates (TNC) decreased slightly in response to N supply from fertigation. Foliar urea application in the fall significantly decreased TNC concentration at each N fertigation level. Starch, glucose and fructose decreased in response to foliar urea applications, but sucrose concentration remained unaffected. Approximately 60% of the carbon decrease in TNC caused by foliar urea application was recovered in proteins and free amino acids. We conclude that free amino acids account for a larger proportion of the N in vines sprayed with foliar urea, but proteins remain as the main form of N storage. In response to foliar urea application, part of the carbon from TNC is incorporated into proteins and free amino acids, leading to a decrease in the carbon stored in TNC and an increase in the carbon stored in proteins and free amino acids.

Four-year-old `Gala'/M.26 trees were grown under low (2.5 mm), medium (12.5 mm), or high (25 mm) N supply with balanced nutrients in sand culture and the cropload was adjusted to 5 fruit/cm² trunk cross-sectional area at 10 mm king fruit. After harvesting, half of the trees in each N treatment were sprayed twice with 3% urea a week apart in late September. Before budbreak the following spring, four trees from each treatment combination were destructively sampled for reserve nitrogen and carbohydrate analysis. Foliar urea application significantly increased tree N concentration and concentrations of both free amino acids and proteins, but decreased the concentration of total nonstructural carbohydrates (TNC) at each soil N supply level. When the carbon in free amino acids and proteins are taken into account, trees sprayed with foliar urea had similar levels of total sum of carbon in TNC, free amino acids and proteins. On a whole tree basis, trees sprayed with foliar urea had more N and less TNC. During the second year of the experiment, all the trees received normal N supply. Trees sprayed with foliar urea the previous fall had a significantly larger total leaf area and higher fruit set, fruit number, and total yield than those unsprayed. We conclude that fruit set and early fruit development as well as vegetative growth in spring is mainly determined by reserve nitrogen, not by reserve carbohydrates. Conversion of a portion of TNC to amino acids and proteins leads to better growth and fruiting of apple trees.

Four-year-old `Gala'/M.26 trees were grown under low (2.5 mm), medium (12.5 mm), or high (25 mm) N supply with balanced nutrients in sand culture and the cropload was adjusted to 5 fruit/cm² trunk cross-sectional area at 10 mm king fruit. At about 100 days after bloom, exposed fruit on the south side of the canopy were chosen for monitoring chlorophyll fluorescence and fruit peel samples were taken for measuring xanthophyll cycle pigments, antioxidant enzymes, and metabolites. At noon, the efficiency of excitation transfer (Fv'/Fm') of the sun-exposed peel was higher in the low N treatment than in the medium or high N treatments. Photochemical quenching coefficient did not differ between fruits in different N treatments. The Photosystem II operating efficiency was higher in the peel of low N fruit compared with medium N or high N fruit. However, maximum quantum efficiency (Fv/Fm) of fruit peel after overnight dark adaptation was similar across the N treatments. The xanthophyll cycle pool size expressed on peel area basis was larger in the high N fruit than in the low N fruit. This corresponds well with the thermal dissipation capacity, as indicated by efficiency of excitation transfer. Over 95% of the xanthophyll cycle pool in the sun-exposed side was present in the form of zeaxanthin and antheraxanthin at noon regardless of N treatments. Activities of superoxide dismutase and all the antioxidant enzymes and metabolites in the ascorbate-glutathione cycle were higher in the high N fruit than in low N fruit. The results indicate that apple fruit with a good N status have a higher photoprotective capacity in terms of xanthophyll cycle-dependent thermal dissipation and detoxification of reactive oxygen species compared with low N fruit.