Study of the non-photochemical dark rise in chlorophyll fluorescence in pre-illuminated leaves of various C3 and C4 plants submitted to partial anaerobiosis

Abstract

Changes in chlorophyll fluorescence yield were studied during a dark period in pre-illuminated leaves of various C3 and C4 plants. The oxygen content in the gaseous atmosphere was either normal (21 kPa) or low (1.5 or 0.36 kPa). C3 and C4 plants of the NAD malic enzyme subgroup showed an initial rise in fluorescence at the onset of the dark period with an amplitude depending on the O 2 level in the gas. In C4 plants belonging to the other two subgroups, the slow rise was absent or of very low size. At high [O 2], the fluorescence level decreased in some minutes to the initial F 0 level (determined in dark-adapted leaves). Conversely at low [O 2], the fluorescence yield remained higher than F 0 in all the C4 plants studied, whereas it decreased slowly to the F 0 level in the different C3 plants. At low [O 2], the fluorescence level decreased rapidly to F 0 when introducing for 30 s, a high O 2 level or when giving a 15-s far-red illumination. At the end of these treatments, the fluorescence level re-increased. These results demonstrate the presence at low [O 2] of highly fluorescent `closed' photosystem II centres containing Q - A in equilibrium with reduced plastoquinone molecules of the chloroplastic pool. Reoxidation of the plastoquinone pool would be dependent on the functioning of an oxidase probably dependent on a chlororespiration process fully active at O 2 levels higher than 2 kPa. The source of reducing equivalents for the plastoquinone pool is discussed.