Some endogenous factors affecting root formation on hardwood cuttings of two clones of apple (Malus sylvestris Mill.) rootstocks : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University
Hardwood cuttings of easy-to-root (MM 106) and difficult-to-root (EM XII) apple (Malus sylvestris M.) rootstocks were sequentially harvested and planted according to the East Malling system, over a period from late autumn (April) until early spring (October). Tissue samples collected on harvest dates and from cuttings undergoing root initiation, were analysed for endogenous, plant growth regulators. Promotion of root formation by a concentrated, quick-dip treatment of Indole-butyric acid (IBA) was only observed in shoots with a high natural ability to initiate roots, when planted as cuttings. Root initiation potential appeared to be directly related to endogenous levels of an indole-acetic acid (IAA)-like growth promoter. No positive correlation between root initiation and bud dormancy, endogenous Abscisic Acid, Cytokinin or Rooting Cofactor levels was established. Reciprocal donor-grafting experiments showed that a root promoting stimulus was graft transferable from MM 106 into EM XII and that a promoter chromatographically similar to IAA accumulated in EM XII stocks grafted with MM 106 scions. No active rooting inhibitors were located from the seasonal or donor-grafted tissue samples. A separation of the modes of action of IAA and IBA in promoting root formation was shown. IAA appeared to be the fundamental physiological promoter of adventitious root formation; the number of roots increasing with an increased concentration of IAA. IBA seemed to have a supporting role in promoting root formation, and could not further promote rooting if applied at increasing concentrations above a threshold level. IBA appeared to be active in the root initiation process by protecting the endogenous IAA levels in the cutting base. Studies of the metabolism of 14C-IAA in the cuttings base indicated that IAA was protected from enzymic degradation and conjugation (both inactivation processes) by the application of IBA prior to planting. IBA was only active in promoting rooting if an optimum level of IAA was present. The results are discussed in terms of the integrated control of root formation on woody shoots and a hypothetical model of the physiological control of root initiation is proposed on the basis of the present work and evidence available in the literature.