POTENTIAL EFFECTS OF UV-B ON PHOTOSYNTHES IS AND PHOTOSYNTHETIC PRODUCTIVITY OF HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 139

field with soybean exposed to modulated UV-B that a reduction in biomass was seen
only when PPFD and UV-A were reduced to below half of the full sunlight levels. The
higher ratio of UV-B to PPFD and UV-A irradiation is the likely cause of the much
larger UV-B effects observed when plants are exposed to “square-wave” UV-B
supplementation in controlled environments, glasshouses or in the field, compared to
modulated UV-B field experiments.
The absence of any significant supplemental UV-A effect on photosynthesis
(Fig. 1) or biomass (Table II) suggests that under these conditions the small increase in
ambient UV-A produced by the lamps (< 2 %) did not have any appreciable effect. It is
clear that a realistic increase in UV-B radiation will not have any impact on
photosynthesis or on photosynthesis productivity of field-grown plants.
5. Conclusions
Although glasshouse studies are useful in identifying the mechanism by which
UV-B radiation affects the photosynthetic capacity of leaves, they do not predict the
impact of increase UV-B radiation on plants. In glasshouse studies, high UV-B
irradiances affected two main processes of photosynthesis (stomata and Calvin cycle
enzymes) and decreased leaf area and productivity. When the plants were acclimated to
high UV-B radiation, UV-B absorbing compounds increased and UV-B effects were
only found in adaxial stomata (because these cells are not screened by flavonoids).
Interestedly, field studies using UV-B levels that may be realistically expected in the
future found no effects on plants. It is concluded that an increase in UV-B radiation,
associated with depletion in the stratospheric ozone layer, is unlikely to have any
significant impact on photosynthesis or on the photosynthetic productivity of higher
plants.
Acknowledgement. We are grateful to J.I.L. Morison for discussions on A/ci
relationship and stomata.
References
1. Allen, D.J., Nogués, S. and Baker, N.R. (1998) Ozone depletion and increased UV-B radiation: is there a
real threat to photosynthesis?, J. Exp. Bot. , 49: 1775-1788.
2. Jordan, B.R. (1996) The Effects of Ultraviolet-B Radiation on Plants: A Molecular Perspective, Adv. Bot.
Res. , 22: 97-162.
3. Cen, Y.-P. and Bornman, J.F. (1993) The effect of exposure to enhanced UV-B radiation on the
penetration of monochromatic and polychromatic UV-B radiation in leaves of Brassica napus , Physiologia
Plantarum , 87: 249-255.
4. Ålenius, C.M., Vogelmann, T.C. and Bornman, J.F. (1995) A three-dimensional representation of the
relationship between penetration of UV-B radiation and UV-screening pigments in leaves of Brassica
napus , New Phytologist , 131: 297-302.
5. Bornman, J.F. and Teramura, A.H. (1993) Effects of Ultraviolet-B Radiation on Terrestrial Plants. In
Environmental UV Photobiology (A.R. Young, L.O. Björn, J. Moan and W. Nultsch, eds), Plenum Press,
New York, pp 427-472.
6. Beggs, C.J. and Wellmann, E. (1994) Photocontrol of flavonoid biosynthesis. In Photomorphogenesis in
Plants (R.E. Kendrick and G.H.M. Kronenberg, eds), Kluwer Academic Publishers, The Netherlands,
pp 733-751.