Temperature acclimation of root respiration in Douglas-fir and western red cedar seedlings

This study examined the ability of seedlings of coastal and interior provenances of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western red cedar (Thuja plicata Donn) to acclimate root respiration rates to growth temperature. A second objective was to determine whether acclimation involved an increase in activity and/or capacity of the alternative path of electron transport, a phenomenon observed in many crop species. Since western red cedar has less genetic variation than Douglas-fir, one might expect better acclimation in cedar. Seedlings were grown hydroponically in computer-controlled mist boxes placed inside an environmental chamber (18/6 hr photoperiod, day/night). This allowed for precise manipulation of root temperature (11, 18, 25°C), while shoot temperature was unchanged between treatments (25/18°C, day/night). Respiration of excised root segments was measured with an oxygen electrode. Potassium cyanide and salicylhydroxamic acid were used to estimate the activity and capacity of the alternative path. Oxygen response curves were constructed, seedling water potentials were determined and shoot samples were analyzed for carbon isotope discrimination. Total plant weight, root to shoot biomass and root density were also measured. The results showed that both the coastal and interior provenances of western red cedar and the coastal provenance of Douglas-fir were able to acclimate, while interior Douglas-fir was not. Acclimation did not seem to involve an increase in electron partitioning through the alternative path. It was suggested that the increase in alternative path respiration that is often seen at low temperatures is independent of compensatory respiration and sometimes occurs coincidentally with it to reduce the damage caused by active oxygen species, which can be a problem at those temperatures. There is evidence that the cold-grown roots were able to increase the supply of O₂ to the mitochondria, thus compensating for the increased O₂ demand that occurs upon acclimation, although the manner in which they did this remains unclear. Seedlings appeared to be able to adjust stomatal conductance in order to avoid experiencing water stress. The optimal root growth temperature was between 18 and 25°C for both species. It seems that root signalling could be involved in rhodoxanthin accumulation in cedar shoots.Forestry, Faculty ofGraduat