Multilevel UV-B Attenuance : Morphological and Chemical Adaptations of Vicia faba to Ultraviolet-B Radiation

Abstract

Due to anthropogenic reduction of stratospheric ozone, levels of potentially harmful solar UV-B radiation (280-315 nm) have been increasing on earth during the last three decades. The main aim of this thesis was to study growth responses and morphological and chemical adaptation mechanisms to harmful UV-B radiation. Two cultivars of Vicia faba, differing in UV-B sensitivity were used as a model system. Constitutive defence mechanisms appeared to function very well; even in the sensitive cv. Pistache, the negative effects of enhanced UV-B were limited. The increased growth in response to UV-B demonstrated that especially cv. Minica was well adapted to enhanced UV-B radiation. The differences in UV-B response between the two cultivars Pistache and Minica were mainly morphological differences of the whole plant (branching) and the amounts of UV-B induced quercetin. This caused differences in growth. The UV attenuation and total flavonoid accumulation and localisation of UV-B exposed leaves were comparable in both cultivars. Thus, I conclude that attenuation of UV-B radiation is very important and takes place at various hierarchical levels in the plant. First of all, the external UV-B exposure was reduced by adaptations in the plant architecture. In response to UV-B, plants showed a more compact stature with adventitious branches and shorter internode distance and plant height. This plant architecture leads to a denser canopy through which less UV-B radiation penetrated. Moreover, extra adventitious shoots with leaves compensated for the reduced length of the main stem. Secondly, leaf morphology, chemistry and localisation of flavonoids contributed to UV-B attenuation on leaf tissue level. Leaves became thicker and smaller in response to UV-B. Beside leaf thickness, kaempferol flavonoids in all leaf layers, nonsoluble phenolics and hydroxy cinnamic acids contributed to the constitutive UV attenuance. The UV-B induced attenuance was achieved by quercetin flavonoids in the adaxial epidermis. In general, a flavonoid concentration gradient occurred across V. faba leaves, with highest concentrations in the adaxial epidermis and lower concentrations in palisade parenchyma and much lower concentrations in spongy parenchyma. This distribution of flavonoids over the leaves contributed to an optimal filtering of UV-B radiation, which is supposed to be one of the most important functions of flavonoids in the leaves. The third level of attenuance was at the cellular level. Microscreens of flavonoids around nucleus and chloroplasts of palisade parenchyma provided additional UV-B protection. It was a third trap for UV-B radiation, which had penetrated through the epidermal layers and which could damage the targets such as DNA and chloroplasts. Among terrestrial flowering plants, legumes are generally considered to be relatively sensitive to enhanced solar UV-B radiation. However, the data of this thesis show that even UV-B sensitive plants such as V. faba are equipped with a multilevel UV attenuance system to deal with enhanced UV-B radiation. So, sensitivity to solar UV-B appears to be a rare phenomenon among terrestrial plant groups and research is needed to understand how well evolutionary older and younger plant groups are adapted to solar UV-B.Aerts, M.A.P.A. [Promotor]Ernst, W.H.O. [Promotor]Rozema, J. [Promotor