GROWTH, LEAF ANATOMY, AND PHYSIOLOGY OF POPULUS CLONES IN RESPONSE TOSOLAR ULTRAVIOLET-B RADIATION

We compared the physiological and morphological responses of rooted cu ttings of Populus trichocarpa Torr. & Gray and P. trichocarpa x P. del toides Bartr. ex Marsh. grown in either near-ambient solar ultraviolet -B (UV-B; 280-320 nm) radiation (cellulose diacetate film) or subambie nt UV-B radiation (polyester film) for one growing season. Midday biol ogically effective UV-B radiation was 120.6 and 1.6 mJ m(-2) s(-1) und er the cellulose diacetate and polyester films, respectively. Gas exch ange, leaf chlorophyll, light harvesting efficiency of photosystem II, and foliar UV-B radiation-absorbing compounds (i.e., flavonoid deriva tives) were measured in expanding (leaf plastochron index (LPI) 5), ne arly expanded (LPI 10), and fully expanded mature (LPI 15) leaves of i ntact plants of plastochron index 30 to 35. Plants were then harvested and height, diameter, biomass allocation and leaf anatomical attribut es determined. Net photosynthesis, transpiration, and stomatal conduct ance were significantly greater in mature leaves exposed to subambient UV-B radiation than in mature leaves exposed to near-ambient UV-B rad iation. Concentrations of UV-B radiation-absorbing compounds (measured as absorbance of methanol-extracts at 300 nm) were significantly grea ter in mature leaves exposed to near-ambient UV-B radiation than in ma ture leaves exposed to subambient UV-B radiation. The UV-B radiation t reatments had no effects on chlorophyll content or intrinsic light har vesting efficiency of photosystem II. Height, diameter, and biomass we re not significantly affected by UV-B radiation regime in either clone . Leaf anatomical development was unaffected by UV-B radiation treatme nt in P. trichocarpa x P. deltoides. For P. trichocarpa, leaf anatomic al development was complete by LPI 10 in the near-ambient UV-B radiati on treatment, but continued through to LPI 15 in the subambient UV-B r adiation treatment. Mature leaves of P. trichocarpa were thicker in th e subambient UV-B radiation treatment than in the near-ambient UV-B ra diation treament as a result of greater development of palisade parenc hyma tissue. We conclude that exposure to near-ambient UV-B radiation for one growing season caused shifts in carbon allocation from leaf de velopment to other pools, probably including but not limited to, UV-B absorbing compounds. This reallocation curtailed leaf development and reduced photosynthetic capacity of the plants compared with those in t he subambient UV-B radiation treatment and may affect growth over long er periods of exposure.