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 = 1×), twice‐ambient (2×), or triple‐ambient (3×) biologically effective UV‐B radiation (UV‐BBE) 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 2× and 3× treatments, respectively, compared to 1× 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 2× and 3× treatments. The 3× 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.