University of Tasmania
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The effect of paclobutrazol on flowering activity and gibberellin levels in Eucalyptus nitens and Eucalyptus globulus

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posted on 2023-05-26, 03:26 authored by Hasan, O
Experiments prior to this project demonstrated the capacity of the plant growth retardant paclobutrazol to enhance flowering in commercially important Eucalyptus nitens and E. globulus trees and produced anecdotal evidence of this material reducing time to first flowering. Paclobutrazol is known to reduce the levels of endogenous gibberellins (GAs) in several species and hence it was hypothesized that the effects of paclobutrazol on flowering in these Eucalyptus species may be mediated by an effect on GA levels. The lack of previous identification of GAs in this genus necessitated the development of extraction and purification procedures to identify and quantify GAs. The compounds identified suggested that the early 13- hydroxylation pathway was the dominant mechanism for production of GAs, with GA 1 as the likely biologically active compound. Persistence of a paclobutrazol induced increase in flowering of grafted E. nitens material was related to the continued depression of endogenous GA concentrations. Additionally, higher concentrations of GA1 were found to be correlated with reduced flowering responses in this reproductively competent material. A lowering of endogenous GA levels, in combination with a co-requisite of a period of cold has been associated with the induction of first flowering in newly grafted E. nitens material. The effects of cold treatment were not mediated by an effect on levels of GA1 or GA2o· Soil applied paclobutrazol was shown to travel up stems and accumulate in leaf tissue. Breakdown in plant tissue was shown to be rapid, with a half-life likely to be substantially less than 21 d. Soil and foliar application methods were shown to produce different patterns of metabolism of labelled paclobutrazol, as demonstrated by HPLC separation of labelled metabolites extracted from growing apices. The rate of breakdown in the soil was observed to be variable, but slow in comparison to that within plant tissues and may be the source of the considerable persistence of effects of paclobutrazol application observed in some field trials. Application of paclobutrazol to 6 month old E. globulus seedlings resulted in the production of flower buds at less than 18 months of age despite the retention of juvenile foliage. One year later, following normal bud development, anthesis and pollination, capsules were produced, while maintenance of material in a range of growth conditions over the second winter again demonstrated the strong requirement for cold seen in grafted E. nitens, as well as revealing an apparent promotion of flowering associated with reduced pot size. The reduction in generation time achieved using commercial seedlings was ca. 50% (3 years) which should be of major benefit to tree breeders, given that the long generation time of eucalypts is a major determinant of the rate at which genetic gains can be made by conventional tree breeding methods. In reproductively competent seedlings and grafts, paclobutrazol application was confirmed to increase the average number of flower buds per seedling. This could be advantageous when seed requirements from an elite tree are high, or when yearly seed yield tends to be variable.


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