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 angiosperm 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 (2x), or triple-ambient (3x) biologically effective UV-B radiation (UV-B-BE) as calculated for Pullman, Washington, at the summer solstice. Net photosynthesis increased in developing leaves (+ 10%, +36%), was unaffected in recently mature leaves, and decreased in older leaves (-20%, -20%) in plants under 2x and 3x treatments, respectively, compared to 1x plants. Similar effects were observed for stomatal conductance, transpiration rates, and chlorophyll concentrations. Dark respiration was unaffected by UV-B irradiation. Height, diameter, and biomass were reduced to the same degree by both 2x and 3x treatments. The 3x treatment stimulated production of lateral shoots and resulted in a significant shift in the shoot : root ratio in favor of shoots. Changes in growth and biomass allocation resulting from enhanced UV-B radiation could affect tolerance to competition and environmental stress, especially drought stress.