COLD-ACCLIMATION INCREASES TOLERANCE OF ACTIVATED OXYGEN IN WINTER CEREALS

Injury from freezing stress may be caused by degradative reactions initiated by activated oxygen. The relationship between tolerance of freezing and oxidative stresses was investigated in winter-cereal seedlings during cold acclimation. Freezing tolerance was evaluated as an LT(50) (lethal temperature causing 50 % mortality) based on regrowth from crown tissue frozen between -4 and -22 degrees C. Oxidative stress was imposed by two xenobiotic chemicals, paraquat, which generates superoxide, and acifluorfen, which generates singlet oxygen. Injury due to paraquat was evaluated by measuring the length of the necrotic region on leaves. Injury due to acifluorfen was evaluated as regrowth after dipping the excised crown into the herbicide solution. Tolerance to freezing stress increased as the seedlings were acclimated at 2 degrees C for periods up to 5 weeks. Paraquat injury of leaves was dependent on the concentration, duration after treatment, and stage of acclimation. A 10-mu L drop of 10(-3) M paraquat applied to the leaf and evaluated 96 h after treatment gave a differential response among stages of acclimation. Paraquat tolerance increased in proportion to freezing tolerance in all cereal cultivars evaluated, but other factors modulated the response in a genotype-specific manner. Similarly, the crown's tolerance of 10(-3) M acifluorfen increased with acclimation in proportion to the seedling's ability to acclimate to freezing stress. The correlation between the development of freezing and oxidative stress tolerances suggests that freezing tolerance at the subcellular level may be influenced in winter cereals by their ability to scavenge and detoxify activated forms of oxygen.