Calcium-Induced Freezing and Salinity Tolerance in Evergreen Oak and Apple cv. ‘Golden Crown’

Abstract

Greater variability in weather patterns and later spring frosts equate to poor winter hardiness, premature spring budbreak, and greater susceptibility to low-temperature damage and concomitant deicing salt application. A field trial was undertaken to determine the influence of a range of commercially available calcium fertilizers applied as foliar sprays on the freezing and salinity tolerance of two tree species, evergreen oak (Quercus ilex L.) and apple (Malus cv. ‘Golden Crown’). In all cases, application of calcium sprays increased twig, leaf, and root freezing and salt tolerance of both species as measured by leaf chlorophyll fluorescence and tissue electrolyte leakage bioassays. In the case of apple, a hardiness gain of 4.3°C (7.74°F) was recorded in twig tissue. In the case of evergreen oak, a hardiness gain of 2.1°C (3.78°F) was recorded in leaf tissue. After a –5°C (23°F) (apple) and –6.5°C (20°F) (evergreen oak) freezing stress, root electrolyte leakage values as a measure of cell membrane structural damage were 16% to 27% less in calcium-treated trees compared with noncalcium-treated controls. The salt concentration needed to cause 50% reductions in leaf chlorophyll fluorescence as a measure of photosynthetic efficiency rose by 0.2% to 1.2% in calcium-fertilized trees indicating a positive influence of calcium on enhancing leaf tissue tolerance to salt damage. Differences in the magnitude of freezing and salinity tolerance gained were noticeable between the calcium products used. In general, calcium hydroxide, calcium nitrate borate, and calcium metalosate improved twig, leaf, and root freezing and salt tolerance in both tree species to a greater degree than calcium chloride, calcium sulphate, calcium nitrate, and a calcium–magnesium complex. A significant correlation existed between increased freezing tolerance and internal tissue calcium content. Results of this study indicate that calcium sprays during late summer and fall can increase the freezing and salinity tolerance of evergreen oak and apple during the winter. This should be considered noteworthy for individuals involved in the management of trees in areas subject to subzero temperature fluctuations and/or concomitant applications of deicing salts in the form of sodium chloride.