Regulatory role of exogenous salicylic acid in the response of Zoysia japonica plants to freezing temperatures: a comparison with cold-acclimatisation

摘要

This study investigated the roles of exogenous salicylic acid (SA) in the response of Zoysia japonica plants to freezing stress, with cold-acclimatisation as a positive control. Sixty-day-old plants were sprayed with a 1.0 mM SA solution and, after 24 h, frozen at a series of sub-zero temperatures. For cold-acclimatisation, plants were exposed, in sequence, to 14 degrees C, 10 degrees C, 6 degrees C, and 2 degrees C for 24 h each, then stressed under the same sub-zero conditions as the SA pre-treated plants. Plant survival rates showed that SA pre-treated plants exhibited an increased tolerance to freezing, similar to that of cold-acclimatised plants, when compared to control plants (i.e., plants grown continuously under 28 degrees C/23 degrees C dayight) or to de-acclimatised plants (i.e., those first acclimatised at lower temperatures, then left at a warmer temperature of 28 degrees C/23 degrees C). Accordingly, both SA pre-treated and cold-acclimatised plants exhibited less chlorophyll damage than did control or de-acclimatised plants under all sub-zero temperatures tested, when compared to control values obtained from plants grown continuously under 28 degrees C/23 degrees C dayight. Under sub-zero freezing stress, the levels of H2O2 in both SA pre-treated and cold-acclimatised plants were lower than those in control or de-acclimatised plants, which correlated with decreased levels of malondialdehyde and electrolyte leakage in the SA pre-treated and cold-acclimatised plants. Furthermore, the application of SA, or a cold-acclimatisation, protected the activities of superoxide dismutase (SOD) and peroxidase (POD) against freezing stress. In comparison with the controls, cold-acclimatisation increased the SA contents of leaves to those levels found in SA pre-treated plants. Soluble protein contents increased in both SA pre-treated and cold-acclimatised plants, while increases in soluble sugar contents occurred in cold-acclimatised plants, but not in SA pre-treated plants, relative to the controls. Based on these data, pre-treatment with SA is proposed to improve tolerance to freezing stress in Z. japonica plants. This may be associated with increased levels of soluble proteins and/or the activities of anti-oxidative enzymes. Cold-acclimatisation and SA pre-treatment may involve, at least in part, common regulatory mechanisms to improve the freezing-tolerance of Z. japonica plants.