Ozone depletion and increased UV-B radiation: is there a real threat to photosynthesis?

Abstract

This critical review of recent literature questions earlier predictions that photosynthetic productivity of higher plants is vulnerable to increased ultraviolet-B (UV-B) radiation as a result of stratospheric ozone (O3) depletion. Direct UV-B-induced inhibition of photosynthetic competence is observed only at high UV-B irradiances and primarily involves the loss of soluble Calvin cycle enzymes and adaxial stomatal closure in amphistomatous plants. However, even under these extreme UV-B exposures, acclimation (e.g. induction of UV-B absorbing flavonoids) can protect the photosynthetic processes. In plants irradiated with UV-B throughout development a reduction in productivity is usually associated with a reduced ability to intercept light (i.e. smaller leaf area) and not an inhibition of photosynthetic competence. Finally, a review of field experiments utilizing realistic UV-B enhancement is made to evaluate whether the mechanisms involved in UV-B-induced depressions of photosynthesis are likely to impact on the photosynthetic productivity of crops and natural vegetation in the future. Predictions of plant responses to O3 depletion are suspect from squarewave irradiance experiments in the field and controlled environments due to the increased sensitivity of plants to UV-B at relatively low photosynthetically-active photon flux densities (PPFD) and ultraviolet-A (UV-A) irradiances. Realistic modulated UV-B irradiances in the field do not appear to have any significant effects on photosynthetic competence or light-interception. It is concluded that O3 depletion and the concurrent rise in UV-B irradiance is not a direct threat to photosynthetic productivity of crops and natural vegetation.