Studies on the Nature of the Primary Reactions of Photosystem II in Photosynthesis I. The Electrochromic 515 nm Absorption Change as an A ppropriate Indicator for the Functional State of the Photochemical Active Centers of System II in DCMU Poisoned Chloroplasts

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Abstract
The field indicating electrochromic 515 nm absorption change has been m easured under different excitation conditions in DCMU poisoned chloroplasts in the presence of benzylviologen as electron acceptor. It has been found : 1. The am plitude of the 515 nm absorption change is nearly completely suppressed under repetitive single turnover flash excitation conditions which kinetically block the back reaction around system II (P. Bennoun, Biochim. Biophys. Acta 216, 357 [1970]). 2. The am plitude of the 515 nm absorption change measured under repetitive single turnover flash excitation conditions which allow the completion of the back reaction during the dark time between the flashes (measuring light beam switched off) amounts in the presence of 2 /um DCMU nearly 50% of the electrochromic 515 nm am plitude obtained in the absence of DCMU. In DCMU poisoned chloroplasts this am plitude is significantly decreased by hydroxylaminhydrochloride, but nearly doubled in the presence of D C IP + ascorbate. 3. The dependence of the 515 nm amplitude on the time t& between the flashes kinetically resembles the back reaction around system II. The time course of the back reaction can be fairly described either by a second order reaction or by a two phase exponential kinetics. 4. 1,3-dinitrobenzene (DNB) or a-bromo-a-benzylmalodinitril (BBMD) reduce the 515 nm am plitude in DCMU poisoned chloroplasts, but seem to influence only slightly the kinetics of the back reaction. 5. The dependence of the 515 nm am plitude on the flash light intensity (the am plitude norm alized to 1 at 100% flash light intensity) is not changed by DNB. Based on these experimental data it has been concluded that in DCMU poisoned chloroplasts the am plitude of the 515 nm absorption change reflects the functional state of photosystem II centers (designated as photoelectric dipole generators II) under suitable excitation conditions. Furtherm ore, it is inferred th at in DCMU poisoned chloroplasts the photoelectric dipole generators II either cooperate (probably as twin-pairs) or exist in two functionally different forms. With respect to BBMD and DNB it is assumed that these agents transform the photoelectric dipole generators II into powerful nonphotochemical quenchers, which significantly reduce the variable fluorescence in DCMU-poisoned chloroplasts

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