Open Access Repository

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


Downloads per month over past year

Hasan, O 1993 , 'The effect of paclobutrazol on flowering activity and gibberellin levels in Eucalyptus nitens and Eucalyptus globulus', PhD thesis, University of Tasmania.

PDF (Whole thesis)
whole-Hasan-the...pdf | Download (5MB)
Available under University of Tasmania Standard License.


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
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.

Item Type: Thesis - PhD
Authors/Creators:Hasan, O
Additional Information:

Copyright the Author-The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Item Statistics: View statistics for this item

Actions (login required)

Item Control Page Item Control Page