Effects of enhanced UV-B radiation on growth and gas exchange in Populus deltoides Bartr. ex Marsh

The effects of enhanced UV-B radiation were evaluated in Populus deltoides Bartr. ex Marsh (eastern cottonwood), a fast-growing and shade-intolerant a ngiosperm with an indeterminate shoot-growth habit. Plants were exposed to three different UV-B radiation regimes under glasshouse conditions during a 95-d treatment period: ambient (7.5 kJ m(-2) d(-1) = 1x), twice-ambient (2 x), or triple-ambient (3x) biologically effective UV-B radiation (UV-B-BE) as calculated for Pullman, Washington, at the summer solstice. Net photosyn thesis increased in developing leaves (+ 10%, +36%), was unaffected in rece ntly mature leaves, and decreased in older leaves (-20%, -20%) in plants un der 2x and 3x treatments, respectively, compared to 1x plants. Similar effe cts were observed for stomatal conductance, transpiration rates, and chloro phyll concentrations. Dark respiration was unaffected by UV-B irradiation. Height, diameter, and biomass were reduced to the same degree by both 2x an d 3x treatments. The 3x treatment stimulated production of lateral shoots a nd resulted in a significant shift in the shoot : root ratio in favor of sh oots. Changes in growth and biomass allocation resulting from enhanced UV-B radiation could affect tolerance to competition and environmental stress, especially drought stress.