Light sensitivity of the dark-grown pea (Pisum sativum L.) epicotyl, the role of the aggregated nature of the NADPH:protochlorophyllide oxidoreductase (POR)

Unlike etiolated leaves, epicotyls contain the majority of protochlorophyllide (Pchlide) in monomer state emitting at 629 and 636 nm at 77 K. The longer wavelength, 644 and 655 nm emitting forms are in small quantities. Flash illumination converts only a small amount of Pchlide. Dark-grown epicotyls can be greened with continuous light of low irradiance only; natural sunlight causes bleaching of the pigments, loos of turgor and wilting. In this work, details of the photosensitivity were studied on intact epicotyls and in their homogenates. Under illumination, an increased amount of water was found in the intracellular spaces in stereo microscopic studies and light activated water-loss of the epicotyls was measured with a potometer. Immediately after preparation, the spectral properties and the photochemical activity of the homogenates were identical to those of intact epicotyls. The bleaching of the samples could be prevented with Na-ascorbate. When homogenates were stored at ? 20 °C for 4 days, the amplitude of the emission band at 636 nm decreased while the 655 nm band increased. Flash illumination caused full photoconversion of this latter form into chlorophyllide emitting at 692 nm. The efficiency of this process was higher than that of intact epicotyls. The light stress reactions, i.e. bleaching and water loss, suggest a free-radical chain reaction process in vivo, which can be stopped with free-radical trap. The spectral changes at ? 20 °C refer to the aggregation of POR subunits, which can be the reason of the increased phototransformation and decreased photosensitivity.