Auxin and gibberellins in Eucalyptus globulus

Auxin and gibberellin (GA) are both crucial regulators of growth in the vascular cambium, effecting elongation, differentiation and the rate of cell division in trees. In several herbaceous species auxin has been found to influence the biosynthesis of GAs. To investigate the possibility of a similar interaction in Eucalyptus globulus, auxin levels were manipulated by treatment and application and GA levels monitored in the intemodes of seedlings and the vascular cambium of trees. In seedlings, auxin reduction treatments typically reduced IAA by 10-fold or more and application of exogenous auxin restored auxin levels to levels similar to untreated plants. The effect of auxin reduction treatment on GAs was found to have its greatest impact on levels of GA\\(_{20}\\), whilst GA\\(_{19}\\) was unaffected. Application of exogenous auxin increased GA levels in seedlings, but did not restore them. Auxin levels were reduced below the girdle in trees but application of auxin had no effect on GA levels in the vascular cambium. Further, analysis of the metabolites of radio-labelled GAs fed to excised seedling stems in MS media showed no effect of auxin on GA metabolism. These results suggest that while auxin can affect GA content in primary tissues, it does not necessarily affect GA content in tree stems.