Acclimation of Pacific yew (Taxus brevifolia) foliage to sun and shade.

The success in clinical trials of the anti-cancer drug, Taxol(R), obtained from the bark of Pacific yew (Taxus brevifolia Nutt.), has raised interest in cultivation and regeneration of this little-known species. Pacific yew is shade-tolerant and it is not known whether the foliage can tolerate the high solar irradiances found on an open forest regeneration site or a nursery. Acclimation of Pacific yew to sun and shade was studied by comparing foliar physiology and morphology of male and female trees growing in full sun or shade. Interspecific foliar acclimation to sun was studied by comparing sun-grown English yew (Taxus baccata L.) with Pacific yew. No sex-specific acclimation was found in foliar physiology or morphology in either species. Sun-grown foliage of Pacific yew and English yew differed with respect to light harvesting, transpiration, stomatal conductance, leaf structure, stomatal distribution and foliar N concentrations and contents. Chlorophyll a fluorescence measurements indicated that shade-grown foliage of Pacific yew had larger and more efficient light harvesting systems than sun-grown foliage. Rates of CO(2) uptake and transpiration were similar in sun- and shade-grown foliage indicating acclimation of photosynthesis to the growth irradiance. Specific leaf area was significantly higher in shade-grown foliage of Pacific yew than in sun-grown foliage and was diagnostic of the light environment in which the foliage grew. Foliar N concentrations were not significantly different between sun- and shade-grown leaves of Pacific yew but sun-grown foliage had a higher N content. Physiological and morphological adjustments of Pacific yew foliage conferred tolerance to both high light and shade, enabling the trees to survive in a variety of light environments and indicating that Pacific yew is suited to nursery cultivation and regeneration of open sites.