Effects of cadmium and enhanced UV radiation on the

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Effects of cadmium and enhanced UV radiation on the physiology and the concentration of UV-absorbing compounds of the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia S. Otero, E. N´un˜ ez-Olivera, J. Mart´ınez-Abaigar,* R. Tom´as, M. Arr´oniz-Crespo and N. Beaucourt Received 25th January 2006, Accepted 7th June 2006 First published as an Advance Article on the web 20th June 2006 DOI: 10.1039/b601105e The aquatic liverwort Jungermannia exsertifolia subsp. cordifolia was cultivated for 15 d under controlled conditions to study the single and combined effects of cadmium and enhanced ultraviolet (UV) radiation. Both cadmium and UV radiation caused chlorophyll degradation and a decrease in the maximum quantum yield of photosystem II (PSII), together with an increase in the mechanisms of nonphotochemical dissipation of energy (increase in the xanthophyll index). Cadmium was more stressing than UV radiation, since the metal also influenced photosynthesis globally and caused a decrease in net photosynthetic rates, in the effective quantum yield of photosynthetic energy conversion of PSII, and in the maximal apparent electron transport rate through PSII. Ultraviolet radiation increased the level of trans-p-coumaroylmalic acid and cadmium increased trans-phaselic and feruloylmalic acids. The increase in these compounds was probably related to both a more efficient absorption of harmful UV radiation and an enhanced protection against oxidative stress. DNA damage was specifically caused by UV-B radiation, but was intensified under the presence of cadmium, probably because the metal impairs the DNA enzymatic repair mechanisms. Ultraviolet radiation and cadmium seemed to operate additively on some physiological processes, while other responses were probably due to either factor alone.

Introduction Cadmium (Cd) is an important and widespread heavy metal, released into the environment by power stations, heating systems, metal industries, waste incinerators, urban traffic, cement factories, and agricultural fertilizers.1 It occurs also in natural environments as an accompanying metal in Pb and Zn minerals. The general effects of Cd toxicity in higher plants include root alterations, leaf roll, chlorosis, and reduced growth. More specifically, Cd interacts with water balance, damages the photosynthetic apparatus, lowers chlorophyll and carotenoid content, inhibits stomatal opening, affects the activity of several enzymes through replacement of other metal ions, and produces oxidative stress.1 In response to Cd stress, plants can resort to a number of defence systems, such as exudation of complexing agents, immobilization in the cell wall, exclusion through the action of the plasma membrane, compartmentalization in the vacuole, formation of metal-resistant enzymes, and synthesis of phytochelatins and stress proteins.1,2 Cadmium accumulation in plants may be dangerous for animal life and human health because Cd enters the food chains in this way. Ultraviolet (UV) radiation has many harmful effects in all living organisms, including humans. UV-C radiation (