Action of UV‐B radiation on photosynthetic primary reactions in spinach chloroplasts

Abstract

Spinach ( Spinacia oleracea L. cv. Matador) chloroplasts were irradiated with several levels of UV‐B radiation. Measurements which reflect characteristic steps of photosynthetic electron transport were made to localize the site of impairment of photosynthesis by UV‐B radiation. Variable fluorescence, the μs‐kinetics of the 320 nm absorption changes and also oxygen evolution were substantially reduced in chloroplasts irradiated with UV‐B. It was not possible to restore the amplitude of the 320 nm absorption changes nor the signal of the transmembrane electric field measured at 520 nm by adding the photo‐system II donor couple hydroquinone/ascorbate to UV‐B treated chloroplast samples. This indicates that impairment of photosystem II activity is not caused by selective inhibition of the water‐splitting enzyme system Y, but rather is due to blockage of photosystem II reaction centers. Photosystem 1 is inferred to be highly resistant to UV‐B radiation. These results suggest that the reaction centers of photosystem II are transformed into dissipative sinks for excitation energy by action of UV‐B radiation.