Initial Assessment of Physiological Response to UV-B Irradiation Using Fluorescence Measurements

Abstract

Fluorescence emissions obtained by excitation at 280 and 340 nm (280EX 300-520 nm ; 340EX 360-800 nm) were used to discriminate physiological change induced by ultraviolet B (UV-B) irradiation in two cucumber (Cucumis sativus L.) cultivars, Poinsett (UV-B sensitive) and Ashley (insensitive). Plants were grown in chambers with controlled spectral irradiation, including biologically effective UV-B irradiation (21 or 0.3 kJ m -2 d -1 ) provided for 5 days with photosynthetically active radiation (-38 mol m -2 d -1 ), Differentiating UV-B induced effects and cultivar differences proved more successful with a dimethyl sulfoxide (DMSO) leaf extract than with freshly excised, intact leaves. Poinsett exhibited significantly lower (P≤0.01) fluorescence for most wavelengths or spectral ratios, whether excited at 280 or 340 nm. The single dominant UV-A fluorescence peak observed in all 280EX emission spectra (330-350 nm) was shifted in DMSO from 340 to 350 nm in UV-B irradiated plants (with a significantly higher F350/F475 ratio, P ≤ 0.001). This could indicate that UV-B irradiation altered the relative amounts of soluble protein invested in enzymes for photosynthesis (e.g., rubisco) versus UV-B protective compounds. In 340EX spectra, UV-B exposed plants also had higher blue/far-red ratios, possibly due to enhanced production of an antioxidant, blue fluorescing compound known to accumulate after UV-B induced degradation of rubisco. In DMSO, this ratio (F450/F730) was linearly related to the total carotenoid/Chl pigment ratio, with qualitatively different responses for the two cultivars. For 340EX spectra, UV-B effects were most successfully discriminated by the far-red peak, alone or included in a ratio with either red or blue fluorescence. UV-B irradiated plants exhibited a significantly lower (P ≤0.001) far-red peak in DMSO and lower far-red/red fluorescence ratios in both media, indicating loss of chlorophyll. The F730/F680 ratio for DMSO was log-linearly dependent on total chlorophyll content (r 2 =0.58 ; P≤0.001). Chlorophyll (Chl) a and b were reduced (P≤0.01) and the Chl a/b (P ≤0.05) and carotenoid/Chl (P≤0.001) ratios were higher under UV-B irradiation. This experiment provides further evidence that UV-B induced damage includes degradation of Chl and photosynthetic function, while photoprotection involves the antioxidant defense system.