During the greening of etiolated barley seedlings the accumulation of chlorophylls and carotenoids was determined in primary leaves together with the chlorophyll fluorescence ratio F690/F735 (at maximum and steady-state fluorescence) as well as the variable fluorescence decrease ratio (Rfd-values in 690 nm and 735 nm) as calculated from the fluorescence induction kinetics recorded by the LITWaF fluorometer. The variable chlorophyll fluorescence parameters (Fv/Fm, Fv/Fo, Delta F/Fm') were monitored using the pulse amplitude modulation chlorophyll fluorometer PAM, and photosynthetic net CO2 fixation by using a CO2/H2O porometer. All parameters were used to characterize the development of photosynthetic activity under illumination with continuous white light in the upper (oldest), middle and lower (youngest) part of the primary leaf blade of etiolated barley seedlings. The time course of changes in the studied parameters provided information on the gradual and age-dependent development of photosynthetic activity in the three leaf parts of different age. During the greening process the chlorophyll fluorescence ratio F690/F735 at maximum and steady-state fluorescence, fm and fs, strongly correlated with the total chlorophyll content (in an inverse curvilinear relationship) in the upper, middle and lower part of etiolated leaves. Though the absolute values were different, there also existed a linear correlation between the Rfd-values measured at 690 nm and 735 nm. The time dependence of the variable fluorescence parameters Fv/Fm, Fv/Fo, Delta F/Fm' yielded a saturation curve, with maximum values at about 12 h of illumination. These variable fluorescence ratios characterized the changes of the efficiency of exciton capture by open PSII reaction centres and the quantum yield of non-cyclic electron transport during the greening of etiolated barley seedlings in the different parts of primary leaves. Fv/Fo exhibited a curvilinear relationship to Fv/Fm and Delta Fv/Fm', but a linear relationship to Rfd-values measured at 690 nm and 735 nm during the greening period. Fv/Fo and Rfd-values represent a similar pair of variable fluorescence ratios at the dark-adapted (state 1) and the light-adapted state (state 2) of the photosynthetic apparatus as the ratios Fv/Fm and Delta F/Fm'. The variation of net CO2 fixation in the middle part of the leaf blade demonstrated that during the first 6 h of greening the net CO2 fixation remained near the zero line, but then rose very fast. In contrast, the variable chlorophyll fluorescence parameters had reached already after 6 h illumination 90 % (Fv/Fm), 88 % Delta F/Fm'), 77% (Fv/Fo) and 60 to 77% (Rfd values at 690 and 735 nm, respectively) of the maximum value. A saturation level of these variable fluorescence ratios was obtained at or around an illumination time of 12 h. These results indicated that the photosynthetic quantum conversion and electron transport within the leaf proceeded at closed stomata and much earlier than measurable CO2 fixation, apparently using internal respiratory CO2 for photosynthesis. The greening process and development of photosynthetic quantum conversion was faster in the lower and the middle leaf blade parts, but was very much retarded in the upper, oldest part of the leaf blade.
