196 Percival and Barnes: Calcium-Induced Freezing and Salinity Tolerance folium) as measured by leaf chlorophyll fluorescence, electrolyte leakagen and chlorophyll content after fall fertilization with cal- cium nitrate and calcium nitrate borate. Such an increase in tolerance was not shown by autumn application of nitrogen alone indicating a positive role for calcium ions in enhancing plant freezing and salt tolerance. In addition, improvements in the freezing and chilling tolerance of landscape trees such as horn- beam and white poplar; crops such as beans, potatoes, avocados, mangoes, cherries;, and various grass species have been reported after spray applications of calcium fertilizers (Akhavankharazian et al. 1991; Anderson and Campbell 1995; Palta 1996; Percival et al. 1999). The practical implications of a hardiness gain of approximately 4°C (7.2°F) will depend heavily on geographic location. For example, within the United Kingdom, winters are in general characterized by a cool, wet growing season with temperatures rarely rising above 0°C (32°F). However, 5% of plants can be lost during production as a result of direct and indirect late frost injury in which temperatures as low as –4°C (25°F) have been recorded in late April and early May. Conse- quently, a hardiness gain of 4°C (7.2°F) would prove of value in relation to practical needs of the United Kingdom soft and top fruit, hardy ornamental stock, and bedding plant horticultural sectors (Cameron and Dixon 2000). Likewise, work by Raese (1996) concluded calcium spray-induced hardiness gains of 4°C (7.2°F) improved yield and bud recovery of apple and pear trees located throughout the Pacific Northwest and may be sufficient to save a orchard crop of apple or pears. The influence of calcium fertilization on improving the freez- ing tolerance of plant root systems has received little attention. In general, freezing damage to root systems of established trees is an uncommon phenomenon because root systems are, to a large degree, buffeted from major temperature fluctuation by the soil and mulch layer (Pellett 1971; Smit-Spinks et al. 1985). How- ever, on occasion, arborists may be confronted with tree roots coming into direct contact with cold-conducting concrete struc- tures such as sidewalks and roads, which add an additional ele- ment of predisposition concern. A similar situation may arise when the ground is surface-scraped under trees, removing the insulating soil layers (Randrup et al. 2001). Likewise, growing trees in planters, containers, and on rooftops exposes tree roots to low temperatures (Santamour 1979). Chilling damage to root systems is also a major problem of bare-rooted nursery, land- scape, and forestry transplants during cold storage before plant- ing out (McKay 1992, 1994). Freezing damage to roots is attrib- uted to mechanical disruption of cell membranes caused by de- hydration because extracellular ice crystals draw water from plant cells. Damage in turn is expressed aboveground as discol- oration, bleaching, shrinkage, and dieback of foliar tissue (Koz- lowski et al. 1991). Results of this study demonstrate a signifi- cant improvement in root freezing tolerance of both test species after calcium application as manifest by reduced electrolyte leak- age values, as measures of membrane structural stability and cell wall strength, after a –5°C (23°F) and –6.5°C (20°F) treatment. Results of this study may, therefore, also be of pertinence in relation to needs of the forestry industry and producers of con- tainer-grown ornamental stock for the establishment and produc- tion of plants able to withstand greater freezing temperatures during the production and outplanting process. A high correlation between twig and leaf tissue calcium con- tent and the freezing temperature required to cause 50% electro- lyte leakage was recorded in this study indicating the higher the ©2008 International Society of Arboriculture internal tissue calcium content, the greater the tolerance to freez- ing. Improvements in the freezing tolerance of calcium-treated plants are achieved through alterations to the structural stability of cell walls and plasma membranes resulting from calcium links between phosphate and plasma lipids and a concomitant increase in physical cell wall strength (Legge et al. 1982). Calcium has also been implicated in controlling enzyme activity involved with freezing resistance. For example, cold temperatures in- creased levels of a calcium-dependent NAD kinase responsible for activating enzymes causing the synthesis of proteins neces- sary to influence the transcription of genes specific to cold ac- climation. The progress of freezing injury can be also be halted by bathing or washing freeze–thaw-injured tissue in calcium- based solutions (Monroy et al. 1993; Berbezy et al. 1996). Elec- trolyte leakage is widely used to measure freezing damage through alterations in cell membrane structural integrity (McKay 1992; Percival and Galloway 1999). Reduced electrolyte leakage values in calcium-treated trees after freezing damage therefore indicate increased membrane structural stability and cell wall strength caused by calcium fertilization in both test species. Im- portantly, improvements in freezing and salinity tolerance were recorded approximately 3 to 5 months after the last calcium spray treatment. Such a result indicates at least 5 months en- hanced freezing and salinity tolerance is conferred postcalcium treatment in both evergreen oak and apple. Differences in the degree of freezing and salinity tolerance gained were noticeable between the calcium products used in this study. In general, calcium hydroxide, calcium nitrate borate, and calcium metalosate improved twig, leaf, and root freezing and salt tolerance of apple and evergreen oak to a greater degree than calcium chloride, calcium sulphate, calcium nitrate, and a calcium–magnesium complex. Because the total amount of cal- cium applied per tree was calibrated at 8 g (0.3 oz; 2 g [0.07 oz] per spray, four sprays per tree), then this would indicate an important role for the anion, i.e., hydroxide, nitrate borate, meta- losate, attached to the calcium ion. The positive influence of these anions may result from 1) a direct influence of the anion on freezing and salinity tolerance, 2) a positive synergistic interac- tion between calcium and the anion; 3) improved uptake of the calcium ion facilitated by the anion present; or 4) a combination of all three. Metalosate is an amino acid-chelated calcium that is rapidly translocated into the phloem cells. Although a positive influence of select amino acids such as proline (Delauney and Verma 1993; Sieg et al. 1996; Thomashow 1999) and polyamines (pu- trescine, serotonin, spermidine) on the stress tolerance of plants is well documented (Shen et al. 2000), the influence of amino acids applied as a spray either singly or in combination with calcium is limited. Likewise, no positive role for hydroxide, sulphate, chloride, and magnesium alone on improving freezing tolerance of plants can be found. Field observations have sug- gested a link between enhanced leaf tissue damage in oilseed rape grown in low boron soils (Ye 2005). Likewise, chill- induced bleaching of young Eucalyptus urophylla leaves has been frequently observed in boron-deficient soils (Dell and Malajczuk 1994). In controlled experiments, exposure to 5°C (41°F) significantly increased the electrolyte leakage in young leaves of Eucalyptus urophylla at a low boron supply (5 m), but not at higher (15 m) rates, indicating a positive role for boron in reducing chilling damage (Lu and Huang 2003). Nitrogen- based fertilizers have been shown to reduce plant tolerance to
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