OZONE DEPLETION AND INCREASED UV-B RADIATION - IS THERE A REAL THREATTO PHOTOSYNTHESIS

This critical review of recent literature questions earlier prediction s that photosynthetic productivity of higher plants is vulnerable to i ncreased ultraviolet-B (UV-B) radiation as a result of stratospheric o zone (O-3) depletion. Direct UV-B-induced inhibition of photosynthetic competence is observed only at high UV-B irradiances and primarily in volves the loss of soluble Calvin cycle enzymes and adaxial stomatal c losure 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 U V-B throughout development a reduction in productivity is usually asso ciated with a reduced ability to intercept light (i.e. smaller leaf ar ea) and not an inhibition of photosynthetic competence. Finally, a rev iew of field experiments utilizing realistic UV-B enhancement is made to evaluate whether the mechanisms involved in UV-B-induced depression s of photosynthesis are likely to impact on the photosynthetic product ivity of crops and natural vegetation in the future. Predictions of pl ant responses to O-3 depletion are suspect from square-wave irradiance experiments in the field and controlled environments due to the incre ased sensitivity of plants to UV-B a at relatively low photosynthetica lly-active photon flux densities (PPFD) and ultraviolet-A (UV-A) irrad iances. Realistic modulated UV-B irradiances in the field do not appea r to have any significant effects on photosynthetic competence or ligh t-interception. it is concluded that O-3 depletion and the concurrent rise in UV-B irradiance is not a direct threat to photosynthetic produ ctivity of crops and natural vegetation.