Foliar applications of urea or ABA affect growth cessation, leaf senescence and abscission, cold acclimation and levels of reserve nitrogen and carbohydrates in nitrogen-treated apple nursery plants

Abstract

SummaryFoliar-applied urea and ABA, respectively, were evaluated as management tools for apple nursery plants to 1) increase reserve N for plants in low N and 2) enhance growth cessation, leaf senescence and abscission, and cold acclimation for those with high N. Container-grown apple plants (Malm domestica Borkh. ‘Gala’), grafted on M.26 rootstocks, were fertigated weekly with 150 mg l–1 N, from mid-May until 21 August, 18 September and 9 October, to represent relatively early-, mid- and late-N cut-off treatments, respectively. In early October, 3% urea was sprayed twice, at six-day intervals, on some of the early-N plants. In addition, 1000 mg –1 ABA was sprayed twice (at five-day intervals) on some of the mid- or late-N plants in early or mid-October, respectively. Leaf-N concentrations increased linearly with longer periods of N fertigation (2.5, 2.9, and 3.2% in the early-, mid-, and late-N plants, respectively). Growth cessation, leaf senescence and abscission, and stem cold acclimation were delayed when the N cut-off time was postponed. Foliar urea applications to the early-N plants effectively increase reserve N (expressed as total N or soluble protein concentrations) in dormant stem bark, wood, and roots. However, this treatment decreased the concentrations of total non-structural carbohydrates (soluble sugars and starch) in all dormant tissue types. Stem cold hardiness also was reduced by about 2 and 3 K in late fall and mid-winter, respectively. Foliar ABA applications to plants grown under mid- or late-N conditions were effective in enhancing leaf senescence and abscission. This led to improved efficiency of N withdrawal from the leaves into woody tissues, especially under late N cut-off conditions. ABA had no effect on growth cessation and cold acclimation. The increases in reserve N levels, as promoted by urea or ABA, resulted in improved regrowth the following spring.