Induction of freezing tolerance in jack pine seedlings: Changes in lipids, oxidation-reduction and antioxidant enzymes during cold acclimation

The freezing tolerance of jack pine seedlings (Pinus bankisana Lamb.) was acquired following conditioning at 5°C and a short photoperiod. A 1-week conditioning period significantly increased survival after exposure to --5°C, but was less effective on seedlings exposed to --10°C. Conditioning periods of 2 and 4 weeks resulted in higher levels of survival following exposure to both --5 and --10°C. Changes in two root plasma-membrane-bound activities, H+-ATPase and NADH-dependent ferricyanide reductase, were studied. in plasma membrane enriched fractions during conditioning and after freezing. Cold conditioning inhibited these two enzyme activities. However, post-freezing activities of both enzymes were enhanced by conditioning.; The changes in major polar lipids and their fatty acid compositions in jack pine roots and needles during cold conditioning were studied. After 4 weeks of cold conditioning, the proportion of phosphatidylchohne (PC) increased in root and needle tissues, while, in roots, monogalactosyldiacylglycerol (MGDG) was decreased during cold conditioning. The proportion of oleic acid (18:1) in roots and linolenic acid (18:3) in needles increased during cold conditioning. Following 4-weeks of cold conditioning, the resistance of root membranes to freeze-thaw stress and free radical attack increased as assessed by determining lipid peroxidation. Lower levels of lipid degradation were found in 4-week conditioned seedlings following a freeze-thaw event.; Cold conditioning led to gradual increases in sulfhydryl (SH) concentration in soluble proteins and the GSH/GSSG ratios in jack pine roots. In vivo and in vitro freeze-thaw caused a decrease in SH content in microsomal proteins; a lower level of SH oxidation was found in the microsomes of 4-week conditioned seedlings. In this study, we show that GSH has protective effects on freeze-thaw induced inhibition of H +-ATPase activity. The specific activity of ascorbate regenerating enzymes gradually increased. The activities of the enzymes involved in the ascorbate-glutathione cycle in jack pine roots were regulated by cold conditioning. Our results suggest that these enzymes play a protective role following the exposure of the seedlings to freezing temperatures.