Photosystem II Photosynthetic Unit Sizes from Fluorescence Induction in Leaves

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Abstract
The use of fluorescence induction measurements in leaves infiltrated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea was 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 light 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 practically 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 primary 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 1st approximation, with the theory. The utility of the method is demonstrated by a detailed study of the desert plant species Camissonia brevipes, Lupinus sparsiflorus, Datura meteloides and Perityle emoryi: estimation of reaction center concentrations of both photosystem I (by estimation 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 .apprx. 200-800 in different species but there was no variation of this parameter in any single species.

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