INITIAL ASSESSMENT OF PHYSIOLOGICAL-RESPONSE TO UV-B IRRADIATION USING FLUORESCENCE MEASUREMENTS

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 (C ucumis sativus L.) cultivars, Poinsett (UV-B sensitive) and Ashley (in sensitive). Plants were grown in chambers with controlled spectral irr adiation, including biologically effective UV-B irradiation (21 or 0.3 kJ m(-2) d(-1)) provided for 5 days with photosynthetically active ra diation (similar to 38 mol m(-2) d(-1)). Differentiating UV-B induced effects and cultivar differences proved more successful with a dimethy l sulfoxide (DMSO) leaf extract than with freshly excised, intact leav es. Poinsett exhibited significantly lower (P less than or equal to 0. 01) fluorescence for most wavelengths or spectral ratios, whether exci ted at 280 or 340 nm. The single dominant UV-A fluorescence peak obser ved in all 280EX emission spectra (330-350 nm) was shifted in DMSO fro m 340 to 350 nm m UV-B irradiated plants (with a significantly higher F350/F475 ratio, P less than or equal to 0.001). This could indicate t hat UV-B irradiation altered the relative amounts of soluble protein i nvested in enzymes for photosynthesis (e.g., rubisco) versus UV-B prot ective compounds. In 340EX spectra, UV-B exposed plants also had highe r blue/far-red ratios, possibly due to enhanced production of an antio xidant, blue fluorescing compound known to accumulate after UV-B induc ed degradation of rubisco. In DMSO, this ratio (F450/F730) was linearl y related to the total carotenoid/Chl pigment ratio, with qualitativel y different responses for the two cultivars. For 340EX spectra, UV-B e ffects were most successfully discriminated by the far-red peak, alone or included in a ratio with either red or blue fluorescence. UV-B irr adiated plants exhibited a significantly lower (P less than or equal t o 0.001) far-red peak in DMSO and lower far-red/red fluorescence ratio s in both media, indicating loss of chlorophyll. The F730/F680 ratio f or DMSO was log-linearly dependent on total chlorophyll content (r(2) = 0.58; P less than or equal to 0.001). Chlorophyll (Chl) a and b were reduced (P less than or equal to 0.01) and the Chl a/b (P less than o r equal to 0.05) and carotenoid/Chl (P less than or equal to 0.001) ra tios were higher under UV-B irradiation. This experiment provides furt her evidence that UV-B induced damage includes degradation of Chi and photosynthetic function, while photoprotection involves the antioxidan t defense system.