10 Percival and Barnes: Influence of Ca and N Fertilization on Freezing and Salinity Tolerance THE INFLUENCE OF CALCIUM AND NITROGEN FERTILIZATION ON THE FREEZING AND SALINITY TOLERANCE OF TWO URBAN TREE SPECIES By Glynn C. Percival1 and S. Barnes2 Abstract. Two field trials were undertaken to determine the influence of fall fertilization using two commercially available, calcium-based fertilizers (calcium nitrate, calcium nitrate borate) and a high-nitrogen fertilizer (N:P:K = 24:7:7), at a range of concentrations, on the freezing and salinity tolerance of two urban tree species, evergreen oak (Quercus ilex) and holly (Ilex aquifolium). In both the 2001 and 2002 field trials, fertilization with calcium nitrate and calcium nitrate borate at a concentration of 40 g/m2 (0.12 oz/ft2 ) increased the freezing and salinity tolerance of both species as measured by leaf chlorophyll fluorescence, electrolyte leakage, and chlorophyll content. In addition, calcium fertilization at this concentration significantly increased total plant dry weight recorded at the cessation of the experiment. Application of both calcium fertilizers at concentrations of less than 40 g/m2 provided no significant protectant properties. Applications of more than 40 g/m2 proved phytotoxic to the two test species. Irrespec- tive of concentration, applications of N:P:K (24:7:7) fertilizer did not enhance or increase susceptibility to freezing and salinity damage compared to nonfertilized controls. However, N:P:K (24:7:7) fertilization significantly increased leaf chlorophyll content and total plant dry weight. Results indicate that fall applications of calcium nitrate and calcium nitrate borate at 40 g/m2 can increase the freezing and salinity tolerance of evergreen oak and holly. Key Words. Evergreen oak; holly; chlorophyll fluorescence; electrolyte leakage; SPAD values; chlorophyll content; fertilizer. Attempts to improve hardiness, through plant breeding, are limited by the nature of freezing and salt tolerance, which are quantitatively inherited traits controlled by many genes. Consequently, there is a demand for protectant compounds that are both inexpensive and can be applied at relatively short notice. It is now well recognized that applications of calcium (Ca) can reduce stress injury in plants by increasing cell wall strength (Ca is a major nutrient responsible for cell wall rigidity), maintaining plasma membrane integrity and transport function (a primary site of tissue injury during stress), and inducing formation of new proteins with stress-protective characteristics (Palta 1996). Previous research has shown that applications of calcium sprays can improve winter hardiness of fruit trees and increase time spent in cold store of fruit and vegetables such as avocados, mangoes, cherries, and apples (Anderson and Campbell 1995; Raese 1996). Little investigation exists into whether applications of calcium can improve the freezing and salt tolerance of urban trees. Application of road de-icing salts in combination with direct freezing damage to urban trees following subzero tempera- tures during winter can result in significant tree mortalities (Dobson 1991; Percival and Fraser 2001). Likewise, late spring frosts can be devastating to newly emerged leaf tissue, resulting in reduced plant quality and often death (Cannel and Smith 1986; Cameron and Dixon 1997). Indeed, it has been estimated that freezing damage is indirectly and directly responsible for the deaths of more than 1 million urban trees annually on a global basis (Sakai and Larcher 1987). Protec- tion against freezing and salt damage will become more important as climatic change may increase the unpredictability of weather patterns, resulting in progressively later frosts on an annual basis (Biggs 1996), and increased traffic volume and road network expansion may increase the quantity of salt used for de-icing operations (Percival and Fraser 2001). ©2005 International Society of Arboriculture Objectives of this investigation were to (1) determine the effectiveness and feasibility of two commercially available Ca fertilizers (calcium nitrate and calcium nitrate borate) to improve the freezing and salt tolerance of two tree species commonly planted in urban environments; (2) determine the concentration conferring maximal resistance to freezing and salt damage; and (3) comparatively evaluate both Ca fertilizers against a conventionally used, high-nitrogen fertilizer (N:P:K = 24:7:7). METHODS Plant Material and Experimental Design Cell grown, 3-year-old, 45 cm (18 in.) high stock of ever- green oak (Quercus ilex L.) and holly (Ilex aquifolium L.) were obtained from a commercial supplier on 13 September 2001 and stored outdoors on a free-draining gravel surface. To ensure uniformity of stock for experimental purpose, trees were graded, and only those confirming to the characteristics specified in Table 1 were used. The experi- mental design consisted of three randomized, complete block designs at the University of Reading, Shinfield Field Experimental Station, Reading, United Kingdom. Within each block were eighteen 3 × 2 m (9.9 × 6.6 ft) plots
January 2005
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