The effects of ultraviolet‐B radiation on loblolly pine. I. Growth, photosynthesis and pigment production in greenhouse‐grown seedlings

One‐year old loblolly pine (Pinus taeda L.) seedlings were grown in an unshaded greenhouse for 7 months under 4 levels of ultraviolet‐B (UV‐B) radiation simulating stratospheric ozone reductions of 16, 25 and 40% and included a control with no UV‐B radiation. Periodic measurements were made of growth and gas exchange characteristics and needle chlorophyll and UV‐B‐absorbing‐compound concentrations. The effectiveness of UV‐B radiation on seedling growth and physiology varied with the UV‐B irradiance level. Seedlings receiving the lowest supplemental UV‐B irradiance showed reductions in growth and photosynthetic capacity after only 1 month of irradiation. These reductions persisted and resulted in lower biomass production, while no increases in UV‐B‐absorbing compounds in needles were observed. Seedlings receiving UV‐B radiation which simulated a 25% stratospheric ozone reduction showed an increase in UV‐B‐absorbing‐compound concentrations after 6 months, which paralleled a recovery in photosynthesis and growth after an initial decrease in these characteristics. The seedlings grown at the highest UV‐B irradiance (40% stratospheric ozone reduction) showed a more rapid increase in the concentration of UV‐B‐absorbing compounds and no effects of UV‐B radiation on growth or photosynthetic capacity until after 4 months at this irradiance. Changes in photosynthetic capacity were probably the result of direct effects on light‐dependent processes, since no effects were observed on either needle chlorophyll concentrations or stomatal conductance. Further studies are necessary to determine whether these responses persist and accumulate over subsequent years.