THE ROLE OF ABA IN FREEZING TOLERANCE AND COLD-ACCLIMATION IN BARLEY

The role of ABA in freezing resistance in nonacclimated and cold-accli mated barley (Hordeum vulgare L.) was studied. Eleven nonacclimated cu ltivars differed in their LT50, ranging from -10.8 to -4.8 degrees C. Sugars, free proline, soluble proteins and ABA were analyzed in nonacc limated cultivars and during cold acclimation of one cultivar. There w as an inverse correlation between LT50 and both ABA and sucrose conten ts. Exogenous ABA caused a decrease in the freezing point of leaf tiss ue in the cultivar with the lowest level of endogenous ABA, but not in the cultivar with the highest level, suggesting that ABA in the latte r may be near the optimum endogenous level to induce freezing toleranc e. Plants of cv. Aramir treated with.ABA or allowed to acclimate to co ld temperature increased their soluble sugar content to a similar leve l. The LT50 of leaves of cold-acclimated cv. Aramir decreased from -5. 8 to -11.4 degrees C, with biphasic kinetics, accumulating proline and soluble sugars with similar kinetics. The biphasic profile observed d uring cold acclimation could be a direct consequence of cryoprotectant accumulation kinetics. ABA and soluble protein accumulation showed a single step profile, associated mainly with the second phase of the LT 50 decrease. Thus, a significant increase in endogenous ABA is part of the response of barley to low temperature and may be required as a si gnal for the second phase of cold acclimation. Endogenous ABA contents in the nonacclimated state may determine constitutive freezing tolera nce.