Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 11, 1995
Publication Date: N/A
Interpretive Summary: Improved methods are required to assess environmental stress in crop plants in order 1) to diagnose incipient stress prior to obvious pathology, and 2) to identify stress-tolerant or resistant lines. Many compounds in plants emit light when irradiated (fluorescence). Fluorescence can be measured remotely in living plants or small tissue samples and is indicative of the amount, location or physico-chemical environment of the compounds in the plant. Changes in fluorescence are thus well-suited to detect stress and may provide diagnostic information on molecular responses. These experiments revealed changes in fluorescence consistent with accelerated protein breakdown and disruption of photosynthesis in cucumbers treated briefly (5 days) with ultraviolet-B radiation under physiological conditions.
Fluorescence emissions (excitation at 280 or 340 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 or without UV-B for 5 days with high photosynthetically active radiation. 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 fluorescence than Ashley for most wavelengths or spectral ratios, whether excited at 280 or 340 nm. The single dominant UV-A fluorescence peak observed for 280 nm excitation was shifted in DMSO from 340 to 350 nm in UV-B irradiated plants. UV-B exposed plants had higher blue/far-red fluorescence ratios (340 nm excitation), possibly due to enhanced production of a 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. UV-B irradiated plants exhibited a significantly lower far-red peak in DMSO and lower far- red/red fluorescence ratios in both media, indicating loss of chlorophyll. Chlorophyll (Chl) a and b were reduced and the Chl a/b and carotenoid/Chl 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.