Short-term treatment of pea leaves with supplementary UV-B at different oxygen concentrations: impacts on chloroplast and plasma membrane bound processes

Abstract

The effect of short-term supplementary ultraviolet-B (UV-B, biologically effective irradiance 1.2 W m −2) radiation on membrane function of detached leaves of pea ( Pisum sativum L.) was studied at low (2%), normal (22%) and high (60%) oxygen concentration to evaluate if photo-oxidative processes are a primary cause of UV-B-induced damage. The effect of supplementary UV-B on thylakoids was investigated by measuring the maximal photochemical efficiency of photosystem II ( Fv Fm ), the relaxation half-time ( t 1 2 ) of the rapid flash-induced decrease in light transmission at 515 nm and polyunsaturated lipid peroxidation, determined as malondialdehyde (MDA) formation. Damage to the plasma membrane was assessed by measuring electrolyte leakage from leaf disks. Fv Fm decreased linearly during exposure to supplementary UV-B due to a concomitant increase of initial fluorescence, Fo and decrease of maximal fluorescence, Fm. The decrease of Fv Fm in 60% oxygen was similar to that in air, but was enhanced in 2% oxygen compared with air. The relaxation half-time ( t 1 2 ) decreased both in treated and control leaves, but the decrease was about 2 times faster in UV-B exposed leaves. The UV-B-induced decrease of t 1 2 was not influenced by high or low oxygen concentration. Supplementary UV-B induced an increase of lipid peroxidation as a delayed response after exposure, indicating that UV-B-induced lipid peroxidation was a secondary effect. Lipid peroxidation was not differentially influenced by high or low oxygen concentration during exposure to UV-B. Exposing leaf disks to supplementary UV-B for 8 h caused a 2.5-fold higher electrolyte leakage than in controls; leakage of 8 h-exposed disks further increased during storage in the dark following UV-B exposure, while leakage of control disks slightly decreased during dark storage. Low oxygen concentration (2%) did not protect against UV-B-induced damage of the plasma membrane. These results do not support a direct involvement of photo-oxidative processes as a primary cause of UV-B-induced membrane damage in pea leaves.