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 conditions on the sensitivity of bean ( Phaseolus vulgaris cv. Label) to increased UV‐B levels, plants were grown under six different light treatments. In the control series (at ambient levels of UV‐B), UV‐B and visible light were decreased in parallel, resulting in three different total irradiation treatments with the same UV‐B/PAR ratio. A second series with a 15% increase in UV‐B irradiation at each PAR level was used to investigate the effect of UV‐B under the varying total irradiance levels. The different total irradiance levels resulted in large differences in total dry weight, specific leaf weight, photosynthesis‐light response and pigment concentrations. Nevertheless, the 15% increase in UV‐B resulted in equal reductions in total dry weight (from 24.5 to 34.3%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the primary bean leaves was strongly correlated 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 radiation levels are potentially dangerous to some plants, even though the UV‐B levels may seem negligible.