Sensitivity and response of Eucalyptus grandis × E. nitens cuttings to auxin treatment for rooting is consistent with, and can be predicted by in vitro morphogenesis

Summary

The ability of Eucalyptus cuttings to root is paramount for any successful clonal forest establishment programme. However, the common practice of inducing rooting by dipping the shoots in Seradix^®, containing indole-3-butyric acid (IBA), before planting proved ineffective for many clones. The responses and sensitivities of three E. grandis × nitens clones to auxins, both as cuttings and in vitro, were investigated. Varying the IBA concentration in the dip revealed dissimilar sensitivities to exogenous auxin among the three Eucalyptus clones. Increasing the IBA concentration, from 0.5 g IBA kg^–1 dustable powder (Seradix^® 0.5) to 8.0 g IBA kg^–1 dustable powder (Seradix^® No. 3), decreased the percentage of cutting survival in clone 1 from 70% to 6% and in clone 2 from 83% to 33%, but had a slightly positive effect in clone 3 (from 75% to 80%), and generally did not affect rooting. In vitro, all shoots elongated on media containing 0.37 mg l^–1 indoleacetic acid (IAA; a weak auxin), while only 62% of clone 1, 28% of clone 2, and 23% of clone 3 shoots elongated on media containing 0.35 mg l^–1 αnaphthaleneacetic acid (NAA) plus 0.05 mg l^–1 IBA. Shoots that elongated on medium containing IAA had significantly higher survival percentages, and hence yields of rooted shoots. These results indicated that, while clones may vary in their sensitivity to the exogenous application of an auxin, a supra-optimal auxin concentration (whether applied in vitro or on cuttings) was detrimental to the yield of rooted shoots.