Stem diameter variations and cold hardiness in walnut trees

The effect of freezing temperatures on stem diameter was measured in the fi eld and in climatic chambers using linear variable differential transformer s (LVDT sensors). In acclimated stems, there was reversible stem shrinkage associated with freeze-thaw cycles. The maximum shrinkage correlated with s tem diameter (thickness of the bark). The wood was responsible for only 15% of the shrinkage associated with a freeze event, and experiments with isol ated bark showed that connection with the wood was not necessary for most o f the freeze-induced shrinkage to occur. Considering the amount of stem shr inkage associated with summer drought in walnut, the amount of contraction of the bark with freezing was actually much less than might be predicted by water relations theory. Reversible stem shrinkage occurred in living tissu es, but not in autoclaved tissues. For the latter, swelling was observed wi th freezing and this swelling could be explained by the bark alone. Similar swelling was observed during September and October for non-acclimated plan ts. Water was lost with each freeze-thaw cycle starting with the first, and freezing injury of the bark, with discoloration of tissues, was also obser ved in non-acclimated plants. Given that the diameter fluctuation patterns were dramatically different for acclimated versus non-acclimated plants, an d for living versus autoclaved tissues, LVDT sensors could represent a nove l, non-invasive approach to testing cold hardiness.