Abstract:

Changes in the membrane protective system and Ca2+ homeostasis in young grape (Vitis vinifera) plants during the enhancement of chill resistance with heat acclimation and heat resistance with cold acclimation were examined to explore the mechanism of cross adaptation to temperature stress. MDA content in heat (cold) acclimated leaves by cold (heat) stress was less than that in the non-acclimated control plants which showed that heat (cold) acclimation could alleviate the damages of cell membrane caused by cold (heat) stress. Higher leaf levels of glutathione (GSH) and ascorbate acid (AsA) also were found with pre-cold (heat) acclimation under heat (cold) stress than in the non-acclimated control plants. Cold and heat stresses have been shown to cause transient elevated levels of cytosolic free Ca2+, but there is little evidence showing that Ca2+ homeostasis was maintained in cross adaptation, a crucial factor for confirmation of the existence of cross adaptation and the elucidation of the mechanisms of cross adaptation. Ca2+-homeostasis was demonstrated by examining Ca2+ distribution using the cytochemical method of the calcium antimonite precipitation. The results revealed calcium was distributed mainly within the vacuole and intercellular spaces at the optimum growth temperature. When the plants were treated by either heat or cold acclimation for 3 h, calcium levels in the cytoplasm increased dramatically. Meanwhile, deposits in the vacuole concentrated in the tonoplast and deposits in the intercellular space decreased after heat acclimation whereas, after cold acclimation, the deposits in vacuole decreased significantly but did not change in the intercellular space. These results indicate that heat acclimation mostly pumped Ca2+ out of intercellular spaces whereas cold acclimation pumped Ca2+ out of vacuoles. These results revealed two physiological phenomena: on one hand, heat (cold) acclimation increased Ca2+ in cytoplasm to increase the integrity of membranes under cold (heat) stress; on the other hand, increased Ca2+ in the cytoplasm induced the expression of GSH and AsA genes increasing GSH and AsA levels.When non-acclimated plants were stressed at 44 ℃ or -3 ℃ for 3 h, compared to control plants, more Ca2+ particles were located in the inner side of the plasma membrane, fewer Ca2+ particles were in the intercellular spaces and vacuole, the plasma membrane and chloroplast showed signs of damage, and chloroplast ultrastructure was destroyed. However, when heat acclimated plants were exposed to chilling stress at -3 ℃ for 3 h and cold-acclimated plants were exposed to heat stress at 44 ℃ for 3 h, more Ca2+ particles were found in the intercellular spaces and in vacuoles and few Ca2+ particles were on the inner side of the plasma membrane. Cell membranes and chloroplasts were not destroyed. These results indicate that too much intracelluar Ca2+ can be pumped off in heat-acclimated cells under cold stress or cold-acclimated cells under heat stress but cannot be pumped off in non-acclimated cells, suggesting that calcium homeostasis is very important in grape mesophyll cells during cross adaptation to temperature stress.