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...