Development of the Two Heterogeneous Photosystem II Units in Etiolated Bean Leaves

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Abstract
The development of the photosystem II units in relation to the heterogeneity of their photochemical centers was studied in etiolated bean leaves (Phaseolus vulgaris variety Red Kidney) greened under continuous or intermittent light. The study was done to see whether grana are the loci of units with the efficient Photosystem II activity (.alpha. units), while the stroma thylakoids are the loci of units with the less efficient Photosystem II activity (.beta. units), as was proposed. The interrelations between .alpha. and .beta. centers were investigated. The .alpha. and .beta. centers of Photosystem II apparently were present in the 1st photosynthetic membranes irrespective of the mode of greening of leaves. The magnitude of their respective photochemical rate constants, K''.alpha. and K.beta., increased with time in continuous light and reached the steady-state values of mature chloroplasts within 16 h, while in intermittent light it remained smaller. The differentiation of the System II units in .alpha. and .beta. centers containing units is more evident under conditions of intermittent illumination, i.e., when the rate of chlorophyll biosynthesis is the limiting step for chloroplast development. The heterogeneity of the photochemical centers in System II apparently is an endogenous property of chlorplast lamellae. The .alpha. and .beta. centers develop independently of each other from the beginning of light-induced greening. They do not share the same pigment beds. The presence of grana, chlorophyll b and chlorophyll-protein complex II is not a prerequisite for the formation or development of .alpha. centers. The formation of these centers precedes grana formation in greening plastids.

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