THE IMPORTANCE OF THE RATIO UV-B PHOTOSYNTHETIC ACTIVE RADIATION (PAR) DURING LEAF DEVELOPMENT AS DETERMINING FACTOR OF PLANT-SENSITIVITY TO INCREASED UV-B IRRADIANCE - EFFECTS ON GROWTH, GAS-EXCHANGE AND PIGMENTATION OF BEAN-PLANTS (PHASEOLUS-VULGARIS CV LABEL)/

To evaluate the effect of different naturally occurring irradiation co nditions on the sensitivity of bean (Phaseolus vulgaris cv. Label) to increased UV-B levels, plants were grown under six different light tre atments. In the control series (at ambient levels of UV-B), UV-B and v isible light were decreased in parallel, resulting in three different total irradiation treatments with the same UV-B/PAR ratio. A second se ries with a 15% increase in UV-B irradiation at each PAR level was use d to investigate the effect of UV-B under the varying total irradiance levels. The different total irradiance levels resulted in large diffe rences in total dry weight, specific leaf weight, photosynthesis-light response and pigment concentrations. Nevertheless, the 15% increase i n UV-B resulted in equal reductions in total dry weight (from 24.5 to 34.3%) and effective photosynthesis for all light levels. The accumula tion of protective pigments in the primary bean leaves was strongly co rrelated to the total irradiance level (200% increase from the lowest to the highest light level), but was not influenced by increasing UV-B levels. As the UV-B/PAR ratio outside increases with decreasing total irradiance (when induced by cloud cover) this implies that low radiat ion levels are potentially dangerous to some plants, even though the U V-B levels may seem negligible.