THE RATIO UV-B PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR) DETERMINES THE SENSITIVITY OF RYE TO INCREASED UV-B RADIATION

To evaluate the effect of different naturally occurring irradiation co nditions on the sensitivity of rye (Secale cereale L.) to increased UV -B levels, plants were grown under eight different light treatments. I n the control series (at ambient levels of UV-B), UV-B and visible lig ht were decreased in parallel, resulting in four different total irrad iation treatments with the same UV-B/photosynthetically active radiati on (PAR) ratios. A second series with a 30% increase in UV-B irradiati on at each PAR level was used to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance le vels resulted in significant differences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations . A 30% increase in UV-B resulted in equal reductions in total dry wei ght (from 20.0 to 28.6%) and effective photosynthesis for all light le vels. The accumulation of protective pigments in the leaves was stimul ated by PAR and even more by UV-B, except at the highest UV-B irradiat ion, where a small decrease was noted. These results indicate that rye plants are able to adapt to changes in the natural light environment as long as the ratio UV-B/PAR is constant. (C) 1997 Elsevier Science B .V.