Effects of Girdling and Defoliation on Root Activity and Survival of Eucalyptus regnans and E. viminalis Seedlings

Abstract

Fast-growing seedlings of E. regnans F.Muell. survive when bark-girdled, but die when defoliated or detopped or when the stem at a bark girdle is steam-killed. These differences are due to photosynthate passing bark birdles in large quantities, probably in internal primary phloem. Following defoliation or detopping, root elongation stops within 4 days, the root sap turns brown within 8-13 days, and the plant dies in 12-30 days. Total sugars and amino acids in the root sap changed only slightly following girdling or up to 8 days after detopping, but both increased tenfold 8-13 days after detopping. This increase is accompanied by qualitative changes in both sugars and amino acids, and by the appearance of inhibitor(s). Gibberellin and cytokinin activities in the sap are not affected by girdling or detopping. In contrast, fast-growing seedlings of E. viminalis Labill. survive detopping and, although root growth stops by 4 days, root sap stays colourless and without inhibitors.