Untitled

Abstract

The adverse effect of low intensity, small band UV-B irradiation (lambda = 305 +/- 5 nm, I = 300 mW m(-2)) on PS II has been studied by comparative measurements of laser flash-induced changes of the absorption at 325 nm, DeltaA(325)(t), as an indicator of redox changes in Q(A), and of the relative fluorescence quantum yield, F(t)/F(o), in PS II membrane fragments. The properties of untreated control were compared with those of samples where the oxygen evolution rate under illumination with continuous saturating light was inhibited by up to 95%. The following results were obtained: a) the detectable initial amplitude (at a time resolution of 30 mus) of the 325 nm absorption changes, DeltaA(325), remained virtually invariant whereas the relaxation kinetics exhibit significant changes, b) the 300 mus kinetics of DeltaA(325) dominating the relaxation in UV-B treated samples was largely replaced by a 1.3 ms kinetics after addition of MnCl(2), c) the extent of the flash induced rise of the relative fluorescence quantum yield was severely diminished in UV-B treated PS II membrane fragments but the relaxation kinetics remain virtually unaffected. Based on these results the water oxidizing complex (WOC) is inferred to be the primary target of UV-B impairment of PS II while the formation of the 'stable' radical pair P680(+*)Q(A) (-) (*) is almost invariant to this UV-B treatment.

Modulations in Chlorophyll a Fluorescence Based on Intensity and Spectral Variations of Light

“…In a recent study, phy A and phy B mutants of A. thaliana were imparted with 311 nm UV-B for 1 h, and this caused a reduction in photosynthesis, F V /F M , respiration rates, and PI ABS in all plants, but the phy B mutants exhibited a decrease in photosynthetic activity. Moreover, this was due to the mutant’s lower content of carotenoids and UV-absorbing pigments [ 75 ]. From all these studies, it is evident that UV radiation can cause specific photomorphogenic responses and certain deleterious effects, especially in reducing the photosynthetic pigments and photosynthetic efficiency in plants.…”

Transcription factor TGA2 is essential for UV-B stress tolerance controlling oxidative stress in Arabidopsis

“…Particularly under UV-B stress, ROS accumulation occurs in the apoplast, due to activation of the NADPH oxidase AtRBOHD and AtRBOHF subunits (Rao et al, 1996;Casati and Walbot, 2003;Kalbina and Strid, 2006); and in the chloroplasts, due to an effect of UV-B on photosystem (PS) II function (Kulandaivelu and Noorudeen, 1983;Larkum et al, 2001).…”