Freezing temperatures are one of the major constraints for palm cultivation in temperate regions. Severe cold damage can destroy plant tissues and may severely reduce water conduction in the stem (Larcher and Winter, 1981; Meerow, 2005). When warmer weather returns, plant pathogens often attack weakened plants through damaged tissue (Meerow, 2005). Despite difficulties of successfully growing palms in freeze-prone regions, their unique landscape value makes them a major horticultural asset. Therefore, there is a continuous search for suitable species and cultivars (Broschat and Meerow, 2000; Francko, 2003). A rapid and accurate quantitative assessment of freezing injury would facilitate the screening of putatively cold-hardy new cultivars and varieties. Comparative data on freezing tolerance of diverse palm species obtained under controlled laboratory conditions are scarce with the report of Larcher and Winter (1981) appearing to be the only one available.
Several methods have been developed to quantify freezing damage in plants. Visual assessment of injury, e.g., percentage of leaf decay, water-soaked patches, and/or necrotic areas, is somewhat subjective and symptoms might take time to appear (Francko and Wilson, 2004; Percival and Henderson, 2003). Vital staining such as triphenyl tetrazolium chloride (TTC) has not been effective in all species (Steponkus and Lanphear, 1967). We found that TTC was unreliable for assessment of freezing damage in several palm species as a result of inadequate infiltration of the TTC solution into the leaf tissue (Equiza and Francko, unpublished data). This problem might be related to the particular anatomical characteristics of palm leaves, e.g., thick-walled fibers and mesophyll cells with silica incrustations (Larcher et al., 1991; Martens et al., 1980).
The electrolyte leakage method (Dexter et al., 1932) has been extensively used to quantify plant tissue damage by frost. For quantitative comparisons among species, the “Index of Injury” developed by Flint et al. (1967) has been considered to be an accurate indicator of freezing tolerance in many species, including palms (Larcher and Winter, 1981). However, it also has some limitations because it is destructive as well as labor-intensive and time-consuming.
More recently, chlorophyll fluorescence analysis has been used to evaluate plant responses to different environmental stresses (Baker and Rosenqvist, 2004; Maxwell and Johnson, 2000). Chlorophyll fluorescence assesses the fate of excitation energy in the photosynthetic apparatus in a nondestructive manner. The technique has been shown to provide a rapid, sensitive, and reliable diagnosis of plant tolerance to freezing temperatures in several species (Binder and Fielder, 1996; Percival and Henderson, 2003; Rizza et al., 2001). Furthermore, screening of excised leaf material using the chlorophyll fluorescence technique has provided a useful system with which to rank freeze tolerance in whole plants (Brennan and Jefferies, 1990; Percival and Henderson, 2003). Despite the growing use of this technique, its feasibility to evaluate freezing injury in palm species has not yet been examined. The objectives of the present study are to evaluate whether the chlorophyll fluorescence technique can be used to characterize freezing sensitivity in different palm species and to compare its advantages/limitations in relation to the electrolyte leakage method.
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