A technique is described in which cereal seedlings after germination and hardening under controlled conditions in a growth chamber were subjected to freezing in tubes immersed in a solution of ethylene glycol. Susceptibility to freezing was estimated by measuring electrical conductance on the linear 0-100 scale of a standard 'Avometer' using platinum probes inserted near the base of the lamina of the first leaf. The use of the technique is illustrated by two experiments with winter barley cultivars chosen to represent a wide range of winter hardiness. Although significant cultivar differences in mean conductance existed prior to freezing, adequate differentiation was obtained by measurements after freezing only. The differentiation of these cultivars on mean conductance values after freezing was well correlated with their established winter hardiness.It is concluded that the technique is sufficiently sensitive for the preliminary screening of early generation material but that, because of significant cultivar x hardeningduration interactions, advanced breeding lines should be tested over a range of hardening and freezing regimes.

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SummaryA winter barley variety, from the USSR, 13031, lacking vernalization requirement but sensitive to short days and more frost resistant than varieties now commercially grown in Britain, was hybridized with Shimabara, a Japanese variety which requires vernalization but is less sensitive to short days and more susceptible to frost than 13031. The resulting segregating population was then selected under short days and non-vernalizing conditions and in artificial freezing tests, in order to isolate recombinant lines.Five lines derived in this way, together with the parents and the standard winter barley variety, Maris Otter, were included in a detailed developmental study made in a glasshouse under cool, short days.Four of the lines had very similar developmental characters, namely few leaves and a high rate but short duration of spikelet initiation. Developmentally these selections resembled the parent Shimabara more closely than 13031 but the rate of spikelet initiation was faster than that of either parent and the duration of spikelet initiation was shorter. The fifth selection also resembled Shimabara more closely than 13031 but had more leaves and a lower rate and longer duration of spikelet initiation than the other selections. This selection was found to have a strong vernalization requirement.One selection closely resembled 13031 in its frost resistance but had low short-day sensitivity and no detectable vernalization requirement. The complementary characters of the two parents were therefore recombined in this line.Vernalization had little effect on the development of any of the genotypes under cool, short days in a glasshouse.