Metabolic changes associated to the unblocking of adventitious root formation in aged, rooting-recalcitrant cuttings of Eucalyptus gunnii Hook. f. (Myrtaceae)

In many tree species, the ability to produce adventitious roots is limited by specific changes related to the physiology of reproduction and ageing. In spite of its ecological significance, little is known about the physiological mechanisms determining the irreversibility of these non-rooting states in some species. In this study, we performed natural and hormone-induced rooting of stem cuttings from mature individuals of rooting-recalcitrant Eucalyptus gunnii in order to characterize the poorly known metabolic changes associated to the formation of adventitious roots in a rooting-recalcitrant tree species. Stem cuttings, either with or without an apical meristem, were put in a rooting greenhouse after having been treated with two different synthetic auxins, indole-3-butyric acid and 1-naphthaleneacetic acid. During the rooting processes changes in the secondary metabolism of the cutting stem tissue were observed and analyzed through different chromatographic techniques. Rooting of cuttings in E. gunnii proved to be hastened by the accumulation in tissues of methyl gallate. Its natural metabolization and disappearance during the long period of cutting stay in the rooting benches restored the adventive rooting ability. In the same period, a concomitant accumulation in cutting tissues of quercetin-like flavonoids was observed. Rooting was enhanced by the addition of indole-3-butyric acid and 1-naphthaleneacetic acid, although rooting of cuttings with apical meristem was not influenced by hormonal treatments. Seasonal conditions had no effect on the percentage of rooted cuttings, although rooting took 1 month longer in the autumn/winter trial. In general, auxinic treatments did not affect the production of cutting new leaves but did favor the initial differentiation of new root tissue. Rooting was blocked by the presence in the tissues of the inhibitor methyl gallate, the degradation of which allowed the rooting processes to take place. Metabolization and disappearance of this rooting inhibitor was associated with the production and further glycosylation of flavonols. Our results suggest that the switch between the metabolic pathways of gallic acid and shikimic acid and other flavonoid precursors is a crucial step in the process leading to the formation of adventitious roots in aged E. gunnii cuttings.