IS INCREASED UV-B A THREAT TO CROP PHOTOSYNTHESIS AND PRODUCTIVITY

It has been suggested that increases in ground-level UV-B, as a result of stratospheric ozone depletion, may have major deleterious effects on crop photosynthesis and productivity. The direct consequences of su ch effects have been projected by some as a world-wide decrease in cro p yields of 20-25%. Further losses, or unrealized gains, have also bee n suggested as a result of increased UV-B counteracting the beneficial effects of elevated atmospheric CO2. Deleterious W-B effects may be l argely partitioned between damage to the plant genome and damage to th e photosynthetic machinery. Direct damage to DNA is a common result of absorption of high energy UV-B photons. However, most plants possess repair mechanisms adequate to deal with the levels of damage expected from projected increases in ground-level UV-B. In addition, most plant s have the ability to increase production of UV-absorbing compounds in their leaves as a result of exposure to UV-B, UV-A and visible radiat ion. These compounds contribute substantially to reducing UV-B damage in situ. It has also been shown that in some plants, under the proper conditions, almost every facet of the photosynthetic machinery can be damaged directly by very high UV-B exposures. However, electron transp ort, mediated by Photosystem II (PS II) appears to be the most sensiti ve part of the system. Various laboratories have reported damage to vi rtually all parts of the PS II complex from the Mn binding site to the plastoquinone acceptor sites on the opposite surface of the thylakoid membrane. However, a critical review of the literature with emphasis on exposure protocols and characterization of the radiation environmen t, revealed that most growth chamber and greenhouse experiments and ve ry many field experiments have been conducted at unrealistic or indete rminate UV-B exposure levels, especially with regard to the spectral b alance of their normal radiation environment. Thus, these experiments have led directly to large overestimates of the potential for damage t o crop photosynthesis and yield within the context of 100 year project ions for stratospheric ozone depletion. Indeed, given the massive UVB exposures necessary to produce many of these effects, we suggest it is unlikely that they would occur in a natural setting and urge reconsid eration of the purported impacts of projected increases of UV-B on cro p productivity.