A seedling test for measuring cold hardiness in Lolium perenne cultivars is described. Seedling performances of the cultivars in this test were closely related to those of mature plants in an artificial freezing test and to reported survival under field conditions in U.K. winters.With two-leaf seedlings it was possible to analyse hardiness into leaf damage, regrowth and plant kill enabling an index of hardiness to be compounded. Using this index in conjunction with lethal dose 50% estimates, meaningful separation of cultivars was possible.The possible application of this technique for screening breeding material and for detailed physiological work is discussed.

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Assessments of freezing tolerance, estimated in terms of temperature at which 50% mortality occurs (= LT 50 ), were obtained for 86 Lolium perenne accessions using a glycol bath test on naturally hardened field plants . Significant correlations were obtained between LT50 and both heading dates of accessions and minimum temperature of the coldest month near their sites of origin . The effects of temperature and reproductive development on the hardening/dehardening of plants and their growth after freezing are discussed .

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