SAFETY ZONE AS A BARRIER TO ROOT-SHOOT ICE PROPAGATION

Winter wheat, rye and triticale were sown at a depth of 12 cm to promote internode elongation. The root-shoot junction was separated by an internode about 4.5 cm long. The rate of ice propagation was measured separately across the root-internode and crown-internode at -4 degrees C. Ice nucleation in seminal roots was experimentally induced by ice nucleating 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 root-internode section was slower than in the internode alone, but differences exist in the various genotypes. The rate of ice propagation between the seminal roots and the internode did not correlate with the number of roots with unsafe vessels, as detected by the pressurized air method. The rate of ice propagation between the internode and the crown was approximately one third of the rate in the internode. The discontinuity in internode-crown section reached nearly 100% as was observed by the pressurized air method. Anatomical studies supported the discontinuity of vessels evaluated by the pressurized air method. Experimental data support the view that the barriers in root-internode and in internode-crown section reduced the rate of ice propagation but these barriers are not correlated with either cold hardiness or number of safe roots.