The effect of the growth retardant Paclobutrazol on the in vitro growth and development of Betula and Populus species

Abstract

The effects of the application of increasing concentrations of the growth retardant paclobutrazol (PBZ) to in vitro cultures of Populus and Betula species was studied, with a view to translating the effect into the field, and ultimately to control undesired tree growth commercially.
Plant growth and associated regulatory effects were studied under controlled conditions over an eight to twelve week culture period, after which explants were harvested.
At harvest, explants of both species were removed from their culture universals after growing on hard agarose gel media containing a range of PBZ concentrations. Explant fresh weight, height and dry weight was recorded. Spent culture medium was frozen, thawed and centrifuged for analysis, by High Performance Liquid Chromatography (HPLC), of PBZ uptake. Other
investigation carried out included the influence of PBZ on explants' response to directional light, and the phytosterol content of explant leaves.
It was shown that treatments of PBZ, irrespective of concentration, had a regulatory effect on the growth of explants of both species. Recorded height, fresh weight and dry weight of explants showed a reduced growth proportional with increased PBZ concentration. Explant response to the higher concentrations was different for each species, with Betula showing a higher degree of retardation at lower concentration than Populus. Uptake of PBZ also appeared to be species dependent, the average uptake of the available PBZ being 70% and 51% for Populus and Betula, respectively. Even though there was very little growth at high concentration, explants appeared reasonably healthy; no symptoms associated with tree decline, chlorosis, die back and or death, were observed on explants treated with PBZ, however a failure to leaf and severe stunting indicated potential over regulation at 5 pM in Betula explants.
This investigation has shown that a dose-dependant response can be obtained from tissuecultured tree species and that the response appears to be species specific.