Survival and rooting of selected vegetatively propagated Eucalyptus clones in relation to supplied auxin.

Abstract

Eucalyptus spp. and hybrids dominate the global plantation forestry industry, and vegetative propagation through cuttings is the preferred method for their commercial use. However, the cuttings of some species and hybrids show recalcitrance to rooting. The first aim of this study was to improve percentage rooting of three clones of E. grandis x E. nitens (Clones 1, 2 and 3) identified by a commercial nursery as having variable rooting abilities. The second was to relate their rooting responses as cuttings to their rooting responses in vitro. Minicuttings (3.5 – 4 cm in length) (hereafter referred to as cuttings) were subjected to commercial nursery propagation practices. Initial results revealed that in the absence of exogenous plant growth regulators (PGRs), soft (juvenile, thin diameter) cuttings survived (87 – 95%) and rooted (29 – 32%) significantly better than hard (mature, thick diameter) ones (62 – 71% survival and 2 – 8% rooting). This validated the use of soft cuttings by the nursery and all subsequent studies were conducted with soft cuttings. The other nursery practice of applying the commercial rooting powder Seradix 2 (3 g kgˉ¹ indole-3-butyric acid [IBA]) adversely affected the survival and subsequent rooting of cuttings of Clones 1 and 2. Ensuing studies investigated: 1) the effect of mode of IBA application (powder vs. liquid); 2) concentrations of Seradix (0, 0.5, 1, 2 and 3 g kgˉ¹ IBA), applied at initial placement of cuttings and two weeks later; and 3) the influence of season on the survival and subsequent rooting of cuttings. Results showed that regardless of the mode of application, IBA significantly reduced percentage survival and rooting in cuttings of Clones 1 and 2. The delayed application of Seradix, two weeks after cuttings were initially set, resulted in a higher percentage survival and rooting than when cuttings were supplied with Seradix at initial placement. Nevertheless, the best survival for Clones 1, 2 and 3 (95%, 99% and 71%, respectively) and rooting (83%, 64% and 47%, respectively) occurred in the absence of Seradix. In addition, the survival and rooting of cuttings were seasonally variable, with particularly low rooting during winter (e.g. for Clone 1, 32%) when compared with summer (e.g. for Clone 1, 83%). Shoots from all the clones were multiplied in vitro, followed by elongation on either of two media (E1= kinetin, α-naphthalene acetic acid [NAA] and IBA; E2 = kinetin and indole-3-acetic acid [IAA]), and then rooting on 0, 0.1 or 1.0 mg 1ˉ¹ IBA. The latter were selected to typify the range of Seradix concentrations used for the cuttings (i.e. no IBA, low and high IBA concentrations). For all three clones, shoots elongated on E1 or E2 displayed high survival (> 80%) but failed to root without IBA in the rooting medium. For Clones 1, 2 and 3 the best in vitro survival (80%, 100% and 100%, respectively) and rooting (40%, 75% and 40%, respectively) occurred when shoots were elongated on E2 and rooted on 0.1 mg 1ˉ¹ IBA. However, 1.0 mg 1ˉ¹ IBA in the rooting medium severely inhibited survival (0 – 50%), irrespective of the clone or the elongation treatment used. Overall, cuttings demonstrated the best survival and rooting in the absence of exogenous IBA, which suggested that sufficient endogenous auxin was present within the shoots for successful root induction. The application of exogenous IBA may have disrupted the cuttings’ endogenous PGR balance resulting in an inhibition of survival and rooting. In vitro shoots required a low concentration of IBA (0.1 mg 1ˉ¹) in order to counteract the antagonistic effect of cytokinins that were supplied during the multiplication and elongation culture stages, and promote rhizogenesis. Essentially, both cuttings and in vitro shoots demonstrated adverse survival and rooting responses when subjected to excessively high IBA concentrations.