1079 HORTSCIENCE, VOL. 34(6), OCTOBER 1999 HORTSCIENCE 34(6):1079–1081. 1999. Received for publication 21 Sep. 1998. Accepted for publication 22 Dec. 1998. Oregon Agricultural Ex- periment Station technical paper no. 11505. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regula- tions, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1 To whom reprint requests should be addressed. Current address: 1100 N. Western Ave., Wenatchee, WA 98801-1299. E-mail: habib@wsu.edu 2 Professor. Distribution of Urea-derived Nitrogen Supplied to Apple Leaves Habib Khemira 1 , Timothy L. Righetti 2 , and Anita N. Azarenko 2 Department of Horticulture, Oregon State University, Corvallis, OR 97331-7304 Additional index words. Malus ×domestica, urea, 15 N, fertilization Abstract. Fruit tree responses to foliar urea sprays are variable. We hypothesized that such variability is a function of leaf age–related changes in urea-N mobility after urea is absorbed. Two experiments were conducted to study the distribution of urea-derived N in shoots and branches of apple (Malus ×domestica Borkh.) trees. Urea labeled with 15 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 15 N was found in tissues other than the treated leaf. Fall-applied urea- 15 N, however, was detected in high concentrations in dormant buds and bark of the spurs to which the treated leaves were attached. Almost no N was exported to neighboring tissues. 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; less urea-N was exported to the buds following later fall applications. The effectiveness of spring-applied foliar urea sprays for fruit trees is controversial. Some researchers have claimed that foliar urea applied in the spring is equally or more effec- tive than soil N applications in improving fruit set and subsequent fruit size and yield (Blasberg, 1953; Fisher and Cook, 1950). Oth- ers have found that the effects of this practice are largely confined to the sprayed leaves and do not affect fruiting or the N status of the entire tree (Forshey, 1963). We believe that one of the main factors behind this inconsis- tency is lack of within-plant mobility of urea- derived N when urea is applied at low concen- trations, so as not to injure tender tissues early in the season. Proebsting (1957) found no yield response of pear trees (Pyrus communis L.) to spring foliar-sprays of urea at concentra- tions ranging from 0.2% to 0.5% (w/v). In contrast, Rejeb et al. (1991) reported increases in yield and shoot growth of ‘Bartlett’ pear trees after one or two postbloom sprays of a 1.5% (w/v) urea solution. Sanchez et al. (1990) argued that N moving into expanding pear leaves is used to build cell structure rather than being incorporated into enzymes or storage compounds. Structural N may be more diffi- cult to remobilize than N that accumulates in leaves later in the season (Khemira et al., 1998). We hypothesized that, following urea application at low concentrations to young expanding leaves, N would remain in those leaves and exhibit little, if any, export to other lected from one of the two shoots with a treated leaf: 1) the treated leaf, 2) the older leaf imme- diately below, 3) a younger leaf from the shoot-tip (≈8 cm away), 4) a 2-cm-long stem section 1 cm below the treated leaf, and 5) a similar section 1 cm above the treated leaf. These two stem sections were combined into one sample. The section of the shoot bearing the treated leaf was discarded to preclude any direct contamination with 15 N from runoff during application. This sampling was repeated 6 weeks later. After sampling, tissues were carefully washed with 0.1% Alconox detergent (Alconox, New York) and thoroughly rinsed once with warm tap water and once with deionized water to remove residual urea from external surfaces. The samples were then oven- dried at 60 ° C for at least 3 d and ground to pass a 40-mesh screen before the isotopic composi- tion was determined by mass spectrometry at Isotope Services (Los Alamos, N.M.). Atom percent 15 N values were converted to nitrogen derived from the fertilizer (NDFF) using the following formula (adapted from Hauck and Bremner, 1976): NDFF = N atom% N N atom% N 15 natural abundance 15 tissue 15 natural abundance 15 urea ( 29 ( 29 ( 29 ( 29 – – where 15 N natural abundance was considered equal to 0.37 atom percent. Data were analyzed as a completely randomized design using PROC GLM (SAS 6.12; SAS Institute, Cary, N.C.). Expt. 2. Mobility of urea-derived N from fall foliar applications. In 1992, shortly after harvest (7 Oct.), 15 spurs of comparable size and one branch were selected on each of three, single replicate, 8-year-old ‘Golden Delicious’ apple trees on M.7A rootstock. The leaves were still green. The spurs and branches were sprayed individually with a 7.5% (w/v) aque- ous solution of urea enriched in 15 N (1.346 atom % 15 N). The solution contained 0.1% (v/ v) Triton X100. Branches and spurs surround- ing the treated tissues and the ground under the tree were covered with plastic sheets to pre- clude any contamination with labeled urea. The treated spurs and branches were then covered with plastic bags and the whole trees sprayed to runoff with a 7.5% (w/v) solution of nonlabeled urea. Fallen leaves were removed from under the trees to prevent ground con- tamination with 15 N. On 23 Dec., five buds and 2-cm-long strips of surrounding bark from the individually treated spurs and five others from the treated branches were collected from each tree. Additionally, buds and bark 10 cm below (proximal 1) or above (distal 1) and those 20 cm below (proximal 2) or above (distal 2) a treated spur were sampled. Buds and bark were also sampled from an adjacent and a distant branch. This sampling was repeated at full bloom (12 Apr. 1993) and the following winter (4 Jan. 1994). The buds and bark strips were washed, processed, and analyzed as in Expt. 1. Data from each sampling date were analyzed separately as a randomized com- plete-block design (each tree was a block) using PROC GLM (SAS 6.12; SAS Institute). organs. To investigate this hypothesis, the distribution of N derived from urea sprayed onto apple leaves in spring and autumn was studied. Since Oland (1960) first introduced the use of autumn urea sprays to increase the N re- serves of apple trees, a considerable amount of literature has accumulated on the subject (O’Kennedy et al., 1975; Oland, 1960, 1963; Sanchez et al., 1990; Shim et al., 1972, 1973; Williams, 1965). However, the relationship between time of urea application in the fall and translocation efficiency of the resulting N has received little attention. Since leaves may be injured and photosynthetic capacity of the tree decreased after spraying, a late application of urea may be more favorable than an early one. However, following a late fall application of urea, maturation of the abscission zone and severance of the vascular connections between the leaf petiole and the stem or spur may limit movement of N into the tree (Hill-Cottingham, 1968). We hypothesized that urea sprays ap- plied during late stages of leaf senescence would be least efficient in providing N to the bud. We therefore initiated an experiment to study the relationship between time of urea application in the fall and movement of urea- N from leaves to buds. Materials and Methods Expt. 1. Mobility of urea-derived N from young leaves. Four weeks after bloom (28 Apr. 1994), aqueous solutions of 15 N-depleted urea (0.01 atom % 15 N) (Isotec, Miamisburg, Ohio) at 0.5%, 1%, or 2% (w/v) concentration were painted with a brush onto both sides of two young expanding shoot leaves (≈2 cm long and 30% of final size) on each of nine, single replicate, 4-year-old ‘Golden Delicious’ apple trees on M.7A rootstock. The solutions contained 0.1% (v/v) nonionic surfactant Tri- ton X100. Each treatment was applied to three randomly selected trees in the plot. Four weeks later, the following plant material was col-