Sensitivity of white clover to UV-B radiation depends on water availability, plant productivity and duration of stress

Nine populations of white clover (Trifolium repens L.) were grown for 12 weeks with supplemental application of 13.3 kJ m−2 d−1 ultraviolet‐B (UV‐B) radiation under controlled environmental conditions. Drought was applied during the last four weeks of the experiment. Under well‐watered conditions, UV‐B decreased white clover growth on average by 20%. Cultivars bred for agricultural performance were sensitive to UV‐B, while slow‐growing ecotypes were UV‐B‐tolerant. After four weeks of water stress, there were no significant population differences in UV‐B responsiveness. UV‐B sensitivity decreased with increasing exposure to drought and with longer duration of UV‐B irradiation, suggesting that the direction and extent of the UV‐B 3× drought interaction depends on the duration of stress. The population comparisons indicate that low constitutive growth rate and adaptation to other forms of stress may be related to UV‐B tolerance under well‐watered conditions, but not during extended periods of drought.