Biochemical and Physiological Events following Exposure to Uv-b Radiation in Ice Plants

The common ice plant (Mesembryanthemum crystallinum L.) attained significance as a model organism for plant molecular biology studies on biochemical and physiological adaptation mechanisms, due to its extreme stress tolerance, drought and high salinity resistance, and its ability to shift from C3 photosynthesis to CAM in response to environmental stresses or as part of its developmental programme. At present minor attention has been paid to elevated light and UV rays effects on ice plant metabolism and its adaptation. Young ice plants have been exposed to 5 W m-2 UV-B light (3 to 9 kJ m-2 d-1 UV-BBE) for 10 – 30 min. On the next day leaves and roots have been collected, frozen and used for determination of total contents of osmolytes, chlorophyll, polyamines and free amino acid composition. A decrease in chlorophyll and net CO2 assimilation gave evidence that ice plant had high sensitivity to UV-B irradiation. The absence of proline in leaves accompanied with other modifications in free amino acids composition in both leaves and roots, the slight stimulation of polyamine biosynthesis as well as the lack of cadaverine in roots could be considered as indications for the damage of ice plants due to the poor metabolic adaptation to stress caused by UV-B irradiation. Our results suggest that ice plants have lower adaptive capacity to UV-B stress in comparison to drought and salinity. Data are discussed in relation to known hypotheses for M. crystallinum response mechanisms.