Leaf-level responses to ultraviolet-B radiation in Trifolium repens populations under defoliation pressure

Abstract

Pasture plants such as Trifolium repens L. (white clover) are exposed to high levels of ultraviolet-B (UV-B) radiation in summer, as well as to frequent defoliation events from grazing animals and pests. This study examined responses in two T. repens populations exposed to 16 weeks supplementation of 0 or 13.3kJm−2d−1 UV-B radiation under controlled environmental conditions. During that period, plants were exposed to two large defoliation events that lasted two and three weeks, respectively. We investigated a number of leaf morphological characteristics, photochemical attributes, as well as aspects of cellular leaf structure. In particular, we sought to explore whether possible differences in these attributes between the two T. repens populations could be related to their UV-B responsiveness. Leaf dry mass decreased by 16% in the UV-B-sensitive cultivar ‘Huia’ under UV-B, whereas the tolerant ecotype ‘Tienshan’ was unaffected. This differential UV-B response was related to constitutive differences in leaf mass and in leaf area between the two populations. UV-B did not affect specific leaf mass, whereas leaf dry matter content was reduced by 8% in response to UV-B. Measurements of chlorophyll fluorescence revealed no significant effects of UV-B on photochemistry. Results from light microscopy showed that the cellular leaf structure of the T. repens populations was not damaged by UV-B. Population-specific structural features included more dome-shaped epidermal cells for ‘Tienshan’. We conclude that differential UV-B-responses in T. repens populations can occur after defoliation pressure and can be related to differences in leaf characteristics.