The properties and interaction of auxins and cytokinins influence rooting of shoot cultures of Eucalyptus

Abstract

Success in Eucalyptus micropropagation varies with genotype. Although some protocols have proven suitable for suites of clones, many genotypes are recalcitrant to rooting. Their micropropagation is addressed empirically through the manipulation of auxins and cytokinins, which work antagonistically to produce roots and shoots, respectively. Rooting success of three genotypes with 0.1 mg/l indole-3-butyric acid (IBA) was initially recorded as 87, 45 and 41% for clones 1, 2 and 3, respectively. Further studies using the auxin signal transduction inhibitor ñ-chlorophenoxyisobutyric acid (PCIB) or the auxin conjugation inhibitor dihydroxyacetophenone (DHAP) indicated that the poor rooting response of clone 2 was not due to deficient auxin signal perception or auxin conjugation. Omitting kinetin during elongation, followed by auxin-free rooting, significantly increased root production in clone 2 (from 45 to 80.3%), but had no effect on clone 1. Gas chromatography-mass spectrometry (GC-MS) analysis of auxins and kinetin of shoots, prior to rooting, revealed a strong relationship (R2 = 0.943) between rootability and the shoot kinetin:auxin. Replacing kinetin with the less stable trans-zeatin significantly increased rooting of clone 2 (from 19 to 45%) and clone 3 (31 to 52%). It is suggested that root induction in poor-rooters is dependent on exogenous cytokinin depletion from in vitro shoots.

Key words: Eucalyptus, phytohormone interaction, root quality.