An artificial screening technique based on lethal temperature for 50% kill of plants (LT₅₀) was developed to evaluate the freezing hardiness in ltalian ryegrass (Lolium multiflorum Lam.) breeding programs. This technique was found to be suitable for evaluating the changes in freezing hardiness in natural and artificial environments, and the freezing hardiness of ltalian ryegrass was enhanced through this screening method. To elucidate the mechanisms of freezing hardiness, it was found that the N-terminal amino acid sequence of a protein induced by hardening was similar to the partial amino acid sequence of the abscisic acid (ABA)-inducible protein of alfalfa. ABA thus appeared to be involved in the induction of freezing hardiness in the hardening process. An artificial screening technique based on rooting on the flooded soil surface was also developed to evaluate the wet tolerance. The enhancement of the wet tolerance was achieved through selection using this artificial screening technique. To elucidate the mechanisms of wet tolerance, it was shown that aerenchyma formation in roots under flooding conditions was closely associated with the wet tolerance. lncrease in the number of roots with the development of the aerenchyma thus appeared to enhance the wet tolerance. Analysis of the alcohol dehydrogenase (ADH) isozyme and protein synthesis patterns indicated that anaerobic metabolism was possibly induced in roots under flooding conditions.