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

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

Consequences of depletion of stratospheric ozone for terrestrial Antarctic ecosystem: the response of Deschampsia antarctica to enhanced UV-B radiation in a controlled environment.

Mini UV lamps were installed over antarctic plants at Leonie Island, Antarctic peninsula, and shoot length measurements of Deschampsia antarctica were performed during the austral summer January-February 1999. We studied the response of the antarctic hairgrass, Deschampsia antarctica to enhanced UV-B. In a climate room experiment we exposed tillers of Deschampsia antarctica, collected at Leonie Island, Antarctic peninsula, to ambient and enhanced levels of UV-B radiation. In this climate room experiment with 0, 2.5 and 5 kJ m(-2) day(-1) UV-B-BE treatments we observed that length growth of shoots at 2.5 and 5 kJ m(-2) day(-1) UV-B-BE was markedly reduced compared to 0 kJ m(-2) day(-1) UV-B-BE. In addition, there was an increased number of shoots and increased leaf thickness with enhanced UV- B. The Relative Growth Rate (RGR) was not affected by UV-B, possibly because reduced shoot length growth by enhanced UV-B was compensated by increased tillering. Light response curves of net leaf photosynthesis of plants exposed to 5 kJ m(-2) day(-)1 UV-BBE did not differ from those exposed to 0 kJ m(-)2 day(-1) UV-B-BE. The content of UV-B absorbing compounds of plants exposed to increasing UV-B did not significantly change. Mini UV-B lamp systems were installed in the field, to expose the terrestrial antarctic vegetation at Leonie Island to enhanced solar UV-B. In that study, the increment of shoot length of tagged plants of Deschampsia antarctica during the January-February 1999 at Leonie Island, was recorded and compared to shoot length growth under controlled conditions. The consequences of enhanced UV-B radiation as a result of ozone depletion for the terrestrial antarctic ecosytems are discussed. [KEYWORDS: Antarctica; climate change; Deschampsia antarctica; ecosystem; ozone depletion; UV-B; UV-B supplementation Solar ultraviolet-radiation; vascular plants; earths surface;dna-damage; photosynthesis; growth; reproduction; phytoplankton; temperature; peninsula