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Physiological and morphological responses of Populus trichocarpa Torr. & Gray and P. trichocarpa x P. deltoides Bartr. ex. Marsh were compared between plants grown in the field under near-ambient solar ultraviolet-B (UV-B; 280-320 nm) radiation (cellulose diacetate film) or grown under subambient UV-B radiation (polyester film). Measurements were made on plants of plastochron index (PI) 30 to 35. Net photosynthesis, transpiration, stomatal conductance, dark respiration, leaf chlorophyll, light harvesting efficiency of photosystem II, and foliar UV-B radiation-absorbing compounds (i.e., flavonoid derivatives) were measured in expanding (leaf plastochron index [LPI] 5), nearly expanded (LPI 10), and fully expanded leaves (LPI 15) on intact plants. Plants were then harvested and height, diameter, biomass allocation and leaf anatomical attributes determined.

Midday biologically effective UV-B radiation (UV-BBE) was 120.6 and 1.6 mJ m-2 s-1 under the cellulose diacetate and polyester films, respectively. Net photosynthesis, transpiration, and stomatal conductance were significantly greater in mature leaves (LPI 15) under subambient UV-B radiation than under near-ambient UV-B radiation. Concentrations of UV-B radiation-absorbing compounds (measured as absorbance of methanol-extracts at 300 nm) were significantly greater in mature leaves (LPI 15) grown under near-ambient UV-B radiation than subambient UV-B radiation. Neither chlorophyll content nor the intrinsic light harvesting efficiency of photosystem II differed with UV-B radiation treatment.

Height, diameter, and biomass were not significantly affected by UV-B radiation regime in either clone. Leaf anatomical development was unaffected by UV-B radiation treatment in P. trichocarpa x P. deltoides. For P. trichocarpa, leaf anatomical development was complete by LPI 10 under near-ambient UV-B radiation, but continued through LPI 15 under subambient levels. Fully expanded leaves (LPI 15) were thicker under subambient UV-B as a result of greater development of palisade parenchyma tissue.

Near-ambient levels of UV-B radiation apparently cause shifts in allocation of carbon from leaf development to other pools, probably including but not limited to, UV-B absorbing compounds. Such reallocation curtails leaf development and reduces photosynthetic capacity compared to subambient levels of UV-B radiation and may effect growth over longer time periods.

Keywords: Populus trichocarpa, Populus trichocarpa x P. deltoides, deciduous trees, ultraviolet-B radiation, net photosynthesis, transpiration, stomatal conductance, dark respiration, light harvesting efficiency of photosystem II, chlorophyll content, UV-B radiation-absorbing compounds, height, diameter, biomass accumulation, leaf anatomical development.

This project was funded by the Cooperative State Research Research Service, United States Department of Agriculture (grant number 94-37100-0312), the Agriculture Research Center, College of Agriculture and Home Economics, Washington State University (project number 0113), and the Northwest Scientific Association.