SAFETY ZONE AS A BARRIER TO ROOT-SHOOT ICE PROPAGATION

Winter wheat, rye and triticale were sown at a depth of 12 cm to promo te internode elongation. The root-shoot junction was separated by an i nternode about 4.5 cm long. The rate of ice propagation was measured s eparately across the root-internode and crown-internode at -4 degrees C. Ice nucleation in seminal roots was experimentally induced by ice n ucleating bacteria. The rate of movement of the ice front was measured by differential thermal analysis. The highest rate of ice propagation occurred in the internode. The rate of ice propagation across the roo t-internode section was slower than in the internode alone, but differ ences exist in the various genotypes. The rate of ice propagation betw een the seminal roots and the internode did not correlate with the num ber of roots with unsafe vessels, as detected by the pressurized air m ethod. The rate of ice propagation between the internode and the crown was approximately one third of the rate in the internode. The discont inuity in internode-crown section reached nearly 100% as was observed by the pressurized air method. Anatomical studies supported the discon tinuity of vessels evaluated by the pressurized air method. Experiment al data support the view that the barriers in root-internode and in in ternode-crown section reduced the rate of ice propagation but these ba rriers are not correlated with either cold hardiness or number of safe roots.