Photosystem Electron-Transport Capacity and Light-Harvesting Antenna Size in Maize Chloroplasts

Abstract

Spectrophotometric and kinetic measurements were applied to yield photosystem (PS) stoichiometries and the functional antenna size of PSI, PSII,,, and PSIIO in Zea mays chloroplasts in situ. Concentrations of PSII and PSI reaction centers were determined from the amplitude of the light-induced absorbance change at 320 and 700 nm, which reflect the photoreduction of the primary electron acceptor Q of PSII and the photooxidation of the reaction center P700 of PSI, respectively. Determination of the functional chlorophyll antenna size (N) for each photosystem was obtained from the measurement of the rate of light absorption by the respective reaction center. Under the experimental conditions employed, the rate of light absorption by each reaction center was directly proportional to the number of light-harvesting chlorophyll molecules associated with the respective photosystem. We determined Npm = 195, N, = 230, N, = 50 for the number of chlorophyll molecules in the lightharvesting antenna of PSI, PSII,, and PSII,,, respectively. The above values were used to estimate the PSII/PSI electron-transport capacity ratio (C) in maize chloroplasts. In mesophyll chloroplasts C > 1.4, indicating that, under green actinic excitation when Chl a and Chi b molecules absorb nearly equal amounts of excitation, PSII has a capacity to turn over electrons faster than PSI. In bundle sheath chloroplasts C < 1, suggesting that such chloroplasts are not optimally poised for linear electron transport and reductant generation.Under light-limiting conditions, the measured rate of PSI and PSII electron transport in chloroplasts is a function of the rate of excitation transfer to the reaction center and the number of reaction centers in the thylakoid membrane. The rate of excitation transfer to the reaction center of a photosystem (K) depends on three parameters: the intensity of the actinic excitation (I), the effective absorption cross-section of the light-harvesting pigments of that PS2 (a) (12) have employed an O2 flash yield method by which they determined the absorption cross-section of PSII in Chlorella vulgaris.A method developed in this laboratory, based on the steady-state kinetic measurements of Q photoreduction and P700 photooxidation, has permitted the estimation of the antenna size of both PSI and PSII in situ (15,22,28).In the present work we applied spectrophotometric and kinetic measurements to obtain PS stoichiometries and the functional antenna size of PSI, PSII,,, and PSII, in maize chloroplasts. We also present a quantitative evaluation of the overall electron transport capacity (C) of maize chloroplast PS, and compare it to the values reported for C3 plants. MATERIALS AND METHODSLeaves from 2-to 3-week-old Zea mays L. plants, grown under controlled conditions in the greenhouse, were cut in small pieces and ground for 40 s in a Waring Blendor at 0C, in 50 mm Tricine buffer, pH 7.9, which contained 0.4 M sucrose, 10 mM NaCl, and 5 mm MgClI. The slurry was filtered through 8 layers of cheesecloth; cell debris were r...

Photochemical Apparatus Organization in the Chloroplasts of Two Beta vulgaris Genotypes

“…The PSI antenna size (calculated from spectroscopic measurements) was also lower than other reported values, i.e. 144 molecules of Chl per PSI reaction center in the sugar beet hybrid compared to 195 in maize (11) and 209 to 222 in spinach and pea (17,18). PSII antenna sizes were comparable to those reported previously ( 17,18 (10,15,24).…”

“…The rate of light absorption by PSI was determined under the same excitation conditions from the kinetic analysis of the absorbance change at 700 nm of KCN-treated (plastocyanin inhibited) chloroplasts in the presence of methylviologen (11,18). For plastocyanin inhibition, a procedure modified from Izawa et al (13) Figure 1.…”

The Generation of Singlet Oxygen (1O2) by the Nitrodiphenyl Ether Herbicide Oxyfluorfen Is Independent of Photosynthesis

“…One possible explanation for the additional effect upon the oxyfluorfen dependent production of 102 could be linked to the overall redox state of the photosynthetic membrane in the presence or absence of an electron acceptor. In the absence of an electron acceptor the thylakoid membrane will develop a higher redox (more negative) potential than when the acceptor is present, due to a build up of electrons in the plastoquinone pool (10). Thus experiments using oxyfluorfen alone utilize a membrane system with more reducing power than a system in which paraquat is included.…”

The Photosystem II Light-Harvesting Protein Lhcb3 Affects the Macrostructure of Photosystem II and the Rate of State Transitions in Arabidopsis

“…The P700 oxidation traces were fitted with first-order kinetics, and the rate of oxidation was determined. Under these experimental conditions, the rate of P700 oxidation is proportional with the antenna size (Ghirardi and Melis, 1984). For fluorescence induction, the data obtained during the 20-ms induction curves (2000 points) were numerically fitted by the function F(t, I, sPSII, J) based on a sigmoidal fluorescence induction model (Koblizek et al, 2001), where J is the PSII connectivity parameter that determines the shape of the induction curve.…”