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

Abstract 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.