Abstract
Apple (Malus domestica Borkh.) tree responses to foliar urea sprays are variable. Two separate projects were undertaken to study the influence of foliar urea application on tree growth and fruit quality of 'Fuji' apple and to examine distribution of urea-derived nitrogen (N) in shoots and branches of 'Golden Delicious' apple. In the first project, effects of monthly applications of three rates of foliar urea [(1.6-3.2 g urea/liter (S1), 4.8-9.6 g urea/liter (S2), and 8.01-16.0 g urea/liter (S3)] and one ground application of nitrogen on young 'BC-2 Fuji' tree growth, precocity, fruit quality at harvest and after storage, and leaf mineral concentrations were studied. 'BC-2 Fuji' apple trees that received a total of 197 g urea (90.6 g actual N) as a ground application over three growing seasons had greater trunk cross-sectional area (TCA), yield per tree, and yield efficiency than trees which received one of the three rates of foliar applications. 'Fuji' trees receiving ground-applied or foliarly-applied urea at the highest rate had the heaviest fruit. 'Fuji' fruit firmness was greatest in the lowest foliar (S1) treatment. In 'Fuji' apple, ground application resulted in greater than average leaf N in 1994 and the greatest leaf Ca and lowest percentage leaf dry weight, leaf K, Zn, and Cu in both 1993 and 1994. In the second project, three different experiments were conducted to study the distribution of urea-derived N-15 in shoots and branches of 'Golden Delicious' apple trees. Urea labeled with N was applied to young expanding leaves in spring and to senescing spur leaves in fall. At the low concentrations used [0.5%, 1%, and 2% (w/v)], very little spring-applied N-15 was found in tissues other than the treated leaf. Fall-applied urea-N-15, however, was detected in high concentrations in dormant buds and bark of the spurs to which the treated leaves were attached. The following spring, there was some redistribution of labeled N to adjacent buds. Foliar urea sprays applied immediately after harvest contributed most to bud N.