Photosystem II Photosynthetic Unit Sizes from Fluorescence Induction in Leaves

Abstract

The use of fluorescence induction measurements in leaves Infiltrated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea has been evaluated as a routine method for estimation of the concentration of the reaction centers of photosystem II relative to total chlorophyll in a wide variety of plant species. The procedure is based on a simple theory that takes into account the attenuation of light in passing through the leaf and the linear dependence of the fluorescence induction time from different parts of the leaf on the inverse of the local ight intensity. A formula to calculate the reaction center concentration of photosystem II was obtained. The effect of the light attenuation is accounted for by a correction factor which could become practicafly insignificant by an optimal choice of the excitation and emission wavelengths and the geometry of the photodetector with respect to the sample. Estimation of quantum yields for pimary photochemistry and influence of light scattering were considered. The results demonstrate the effect of the above factors under various circumstances and are in agreement, to a first approximation, with the theory.The utility of the method is demonstrated by a detailed study of four desert plant species: estimation of reaction center concentrations of both photosystem I (by estimation of P700) and photosystem II (by the fluorescence induction method) were made and were compared to the rates of CO2 fixation. There was a good quantitative correlation between the photosynthetic rates and the concentration of photosystem II reaction centers (expressed as per chlorophyll or per unit area of the leaf), but no such correlation was found with photosystem I reaction centers.The ratio of total chlorophyll per reaction centers II varied in the range of about 200 to 800 in different species, but there was no variation of this parameter in any single species.It is well established that the primary charge transfer processes of photosynthesis take place at reaction centers served by a large number of light-collecting antennae pigments (13,15,17,32,33,40) plexes serving both PSII and PSI (1,2,26,27,38). These numbers are probably quite variable in different species of the plant kingdom and in different environments. Since the over-all rate of photosynthesis on a Chl basis may be related to these numbers (39), it follows that they are important in dealing with questions such as the efficiency of photosynthesis (24,28,30), including adaptation to the environment [r.g. ambient light intensity level (5, 6, 31) or other conditions (3, 7, 23, 37)].Fluorescence induction in chloroplasts was previously used to obtain the concentration of the PSII electron-acceptor pool (19,40), as well as the concentration of the reaction centers (10,11,40). The principle of the method lies in the equivalence between the number of quanta, which bring about the fluorescence change, and the magnitude ofthe electron acceptor pool. In the case where DCMU was added, the electron-acceptor pool is limited to the primary acceptor of th...

Economic photoprotection in photosystem II that retains a complete light-harvesting system with slow energy traps

“…Application of Malkin's method to this data gives information on the size of the PSII antenna, which is in energy transfer contact with RCII, that is, the PSII cross-section, because the fluorescence rise time from F o to F m is inversely proportional to the antenna/RCII chlorophyll ratio. A change in the cross-section can be quantified, as described previously 14,22 , by directly calculating the area enclosed between the fluorescence induction curve (F ¼ F(t)), the vertical axis (t ¼ 0) and the maximal fluorescence level (F ¼ F m ), after normalizing the value of the variable fluorescence (F v ¼ F m -F o ) to unity (Fig. 1b, the shaded area representing a relative value reciprocal to the RCII cross-section for a WT leaf).…”

The Arabidopsis nox Mutant Lacking Carotene Hydroxylase Activity Reveals a Critical Role for Xanthophylls in Photosystem I Biogenesis

“…WT, the wild type. taken as a measure of the functional antenna size of PSII (Malkin et al, 1981). It was reduced by 70% in nox with respect to the wild type ( Table 1), implying that the light-harvesting function is severely reduced in the absence of xanthophylls.…”

“…Fluorescence kinetics were measured with a home-built setup, in which leaves were vacuum infiltrated with 10 25 M DCMU 3.0 and excited with green light at 520 nm (Luxeon, Lumileds), and the emission was measured in the near far red (Rappaport et al, 2007). The t 2/3 of the fluorescence rise was taken as a measure of the functional antenna size of PSII (Malkin et al, 1981). The reoxidation kinetics of Q A were measured as the decay of chlorophyll a fluorescence with the PAM fluorometer.…”

Different Roles of α- and β-Branch Xanthophylls in Photosystem Assembly and Photoprotection

“…Variable fluorescence was induced in leaf discs, infiltrated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (30 M), sorbitol (150 mM), with a green light of 7 mol m Ϫ2 s Ϫ1 produced by a light-emitting diode. The time corresponding to two-thirds of the fluorescence rise (T2 ⁄3 )was taken as a measure of the functional antenna size of PSII (29). In 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated leaves, the rate of fluorescence rise depends on light intensity and functional antenna size of PSII.…”