UV‐B EFFECTS ON TERRESTRIAL PLANTS

Abstract

The potential impacts of an increase in solar UV‐B radiation reaching the Earth's surface due to stratospheric ozone depletion have been investigated by several research groups during the last 15 years. Much of this research has centered on the effects of plant growth and physiology under artificial UV‐B irradiation supplied to plants in growth chambers or greenhouses. Since these artificial sources do not precisely match the solar spectrum and due to the wavelength dependency of photobiol‐ogical processes, weighting functions, based on action spectra for specific responses, have been developed to assess the biological effectiveness of the irradiation sources and of predicted ozone depletion. Recent experiments have also utilized artificially produced ozone cuvettes to filter natural solar radiation and simulate an environment of reduced UV‐B for comparative purposes. Overall, the effectiveness of UV‐B varies both among species and among cultivars of a given species. Sensitive plants often exhibit reduced growth (plant height, dry weight, leaf area, etc.), photosynthetic activity and flowering. Competitive interactions may also be altered indirectly by differential growth responses. Photosynthetic activity may be reduced by direct effects on photosynthetic enzymes, metabolic pathways or indirectly through effects on photosynthetic pigments or stomatal function. The fluence response of these changes has yet to be clearly demonstrated in most cases. Plants sensitive to UV‐B may also respond by accumulating UV‐absorbing compounds in their outer tissue layers, which presumably protect sensitive targets from UV damage. Several key enzymes in the biosynthetic pathways of these compounds have been shown to be specifically induced by UV‐B irradiation. Few studies have documented the effects of UV‐B on total plant yield under field conditions. One notable exception is a 6‐yr study with soybean demonstrating harvestable yield reductions under a simulated 25% ozone depletion. These effects are further modified by prevailing microclimatic conditions. Plants tend to be less sensitive to UV‐B radiation under drought or mineral deficiency, while sensitivity increases under low levels of visible light. Further studies are needed to understand the mechanisms of UV‐B effects and the interactions with present stresses and future projected changes in the environment.