Foliar Applications of Urea Affect Nitrogen Reserves and Cold Acclimation of Sweet Cherries (Prunus avium L.) on Dwarfing Rootstocks

Seasonal uptake, storage, and remobilization of nitrogen (N) are of critical importance for plant growth. The use of N reserves for new growth in the spring is especially important for sweet cherry (Prunus avium L.), for which new shoot and fruit growth is concomitant and fruit development occurs during a relatively short bloom-toripening period. Sweet cherries grafted on precocious, dwarfing rootstocks such as the interspecific (P. cerasus · P. canescens) hybrids Gisela 5 and 6 tend to produce large crops but smaller fruit when crop load is not balanced with adequate leaf area. Study objectives were to: 1) characterize natural N remobilization during fall and winter to canopy reproductive and vegetative meristems; 2) determine the effect of fall foliar urea applications on storage N levels in flowering spurs; 3) determine whether differential storage N levels influence spur leaf formation in spring; and 4) determine whether fall foliar urea applications affect the development of cold-hardiness. During fall, total N in leaves decreased by up to 51% 1⁄2dry weight (DW) and increased in canopy organs such as flower spurs by up to 27% (DW). The N concentration in flower spurs increased further in spring by up to 150% (DW). Fall foliar applications of urea increased storage N levels in flowering spurs (up to 40%), shoot tips (up to 20%), and bark (up to 29%). Premature defoliation decreased storage N in these tissues by up to 30%. Spur leaf size in the spring was associated with storage N levels; fall foliar urea treatments increased spur leaf area by up to 24%. Foliar urea applications increased flower spur N levels most when applied in late summer to early fall. Such applications also affected the development of cold acclimation in cherry shoots positively during fall; those treated with urea were up to 4.25 8C more cold-hardy than those on untreated trees. Nitrogen is the most important element for maintaining growth and high productivity in tree fruits (Titus and Kang, 1982). Sweet cherries (Prunus avium L.) on precocious, interspecific (P. cerasus · P. canescens) Gisela (Gi) rootstocks (e.g., Gi3, Gi5, Gi6, Gi12) are likely to produce large crops but small-sized fruit when total leaf area is not adequate to support such crop loads (Andersen et al., 1999; Lang, 2000; Whiting and Lang, 2004). When root uptake of N is limited in early spring by cold soils and low canopy transpiration rates (Zavalloni, 2004), the restricted availability of N for growth may contribute to unfavorable leaf area-to-fruit ratios. Nitrogen supply affects leaf, bud, and fruit development. Early-season growth in trees during spring is supported by remobilization of stored N, so they do not depend wholly on N supplied by current-year root uptake (Millard, 1996). Zavalloni (2004) found that sweet cherries absorbed little soil N before budbreak, even up to 3 to 4 weeks after budbreak; therefore, storage N is critical for supplying initial spring growth. Ayala and Lang (2004, 2008) have shown that spur leaves supply the majority of the photosynthetic carbon for cherry fruit growth, and their development and expansion are limited to the first 3 to 4 weeks after budbreak, coinciding with the supply of N from remobilized storage reserves. Sweet cherry flowering occurs before full expansion of leaves; consequently, the early stages of flowering and fruiting as well as initial vegetative growth depend on the storage reserves from the previous season (Keller and Loescher, 1989; McCammant, 1988). Other deciduous trees, like apples (Malus domestica Borkh.), also are dependent primarily on storage reserves for spur leaf formation; however, because apple spur leaves begin expanding before anthesis (thus establishing an evapotranspirational flow within the tree), N uptake by roots begins at approximately full bloom (Neilsen et al., 1997). Consequently, from bloom to fruit set or early fruit development, apple fruit draw on N sources from both reserves and root uptake, whereas cherries draw primarily on N reserves during this period. The fruit development period in apple also is longer than in cherry, meaning a greater proportion of fruit development occurs during active current-season carbon and N uptake. Foliar applications of urea in fall have had positive effects in deciduous fruit trees (e.g., Titus and Kang, 1982). Urea is the most widely used form of N for foliar applications because of its rapid absorption, low phytotoxicity, and high solubility (Bondada et al., 2001; Yamada et al., 1965). Johnson et al. (2001) showed that N for normal growth of peach (Prunus persica Batsch.) buds, shoots, and roots can be supplied adequately by foliar urea applications during the growing season. Because sweet cherries bloom early and have a short fruit development period, a better understanding of storage N, and its manipulation and remobilization, may lead to better orchard management strategies for improving fruit quality on high-yielding, vigor-reducing rootstocks such as Gisela 5. Little is know about the physiological relationship between nutrients and perennial plant cold tolerance mechanisms; practices that stimulate prolonged shoot growth in late summer such as soil-applied N can increase tissue susceptibility to damage by low temperatures in fall or winter. Some foliarapplied nutrients can help promote cold acclimation and stress resistance and/or enhance repair mechanisms after damage (Raese, 1996; Stover et al., 1999). In sweet cherry, the effect of fall foliar urea applications on plant susceptibility to freezing injury is unknown. Therefore, the objectives of this study were to: 1) characterize natural N remobilization from leaves to reproductive and vegetative meristems in sweet cherry during fall and winter; 2) determine the effect of fall foliar urea applications on storage N levels in flowering spurs; 3) determine whether differential storage N levels in spurs influence spur leaf formation in spring; and 4) determine whether fall foliar urea applications affect the development of cold-hardiness. Materials and Methods The following experiments were conducted at Michigan State University’s Clarksville Horticultural Experiment Station (lat. 42.842 N, long. 85.242 W). All experimental trees had been subjected to standard cultural practices (i.e., dormant pruning, spring soil fertilization, timely pesticide applications) in the years before imposition of treatments. Expt. 1. Nitrogen remobilization from leaves during fall senescence was characterized in 6-year-old ‘Sandra Rose’/Gi5 and ‘Rainier’/Gi5 sweet cherry trees. There were five single-tree replications per cultivar. In Fall 2005, 10 shoot leaves, 10 flowering spur leaves, and 10 flowering spurs with buds Received for publication 5 Jan. 2011. Accepted for publication 8 May 2011. We gratefully acknowledge the International Fruit Tree Association (IFTA) and the Michigan Agricultural Experiment Station for financial support of this work and a graduate research assistantship for T. Ouzounis, respectively. We also gratefully acknowledge intellectual input and consultative guidance from colleagues Eric Hanson, David Rothstein, Jim Flore, and Lynn Sage during the course of this project. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 46(7) JULY 2011 1015 | SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION