Growth and gas exchange in field-grown and greenhouse-grown Quercus rubra following three years of exposure to enhanced UV-B radiation

Abstract

Long-term effects of enhanced UV-B radiation were evaluated in field-grown and greenhouse-grown Quercus rubra L. (northern red oak), a species with a multiple flushing shoot growth habit. Seeds were germinated and grown in ambient, twice ambient (2x) or three times ambient (3x) biologically effective UV-B radiation from square-wave (greenhouse) or modulated (field) lamp systems for three growing seasons. Greenhouse plants in the 2x treatment had greater heights and diameters during the later part of the first year and into the second year, but by the third year there were no differences among treatments. There were no significant differences in growth among treatments for field plants. Enhanced UV-B radiation did not significantly reduce total biomass or distribution of biomass in either field or greenhouse plants. Net photosynthesis (3x), leaf conductance (2x and 3x) and water-use efficiency (3x) of greenhouse plants were greater in the enhanced UV-B radiation treatments in the second year but unaffected by the treatments in other years. None of the treatments affected these parameters in field plants. Dark respiration was increased by the 3x treatment in the first and third years in greenhouse plants, and by the 2x treatment during the second year in field plants. Enhanced UV-B had variable effects on apparent quantum yield and light compensation points. Chlorophylls were unaffected by enhanced UV-B radiation in both greenhouse and field conditions. Bulk methanol-extractable UV-absorbing compounds were increased only by the 3x treatment in greenhouse plants during the third year and by the 2x treatment in field plants during the second year. Overall, Q. rubra appears relatively resistant to potentially damaging enhanced UV-B radiation and is unlikely to be negatively impacted even in the predicted worst-case scenarios.