(334) Fruit Set and Yield in Relation to Reserve Nitrogen and Reserve Carbohydrates in `Gala' Apple Trees

Abstract

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.