An analysis of the inhibitory effects of linolenic acid upon photosystem II of higher plants

Abstract

This study utilizes steady state fluorescence measurements, flash-induced P68Q+ absorption transients, and DCIP reduction kinetics to study the inhibitory effects of linolenic acid (LA) upon Photosystem II (PSII) in whole spinach chloroplasts and insideout wheat thylakoids. It confirms the presence within PSII of LA-induced inhibition of energy trapping and/or primary charge separation (i.e., primary inhibition), in addition to 2 donor side inhibition. The latter is diminished in the presence of 1,5-Diphenylcarbohydrazide (DPC) and probably takes place at the oxygen evolving complex. Primary inhibition, which is more controversial, probably occurs between Ph and ~, with a likely contribution at the level of PSil energy trapping. In addition, the ability of Mg2+ to delay a drop in steady state fluorescence intensity normally associated with thylakoid exposure to LA is explained by the ability of this cation to confer resistance to LA-induced destacking of thylakoid membranes.Steady state fluorescence results in the presence of DCMU, dithionite and LA also support the presence of an additional acceptor between Ph and QA. This acceptor, designated here as "~." is proposed not to be a sequential member of the transport chain, but may be accessible to it via QA when the chain blocked, such as with DCMU. ~-is proposed to exert a coulombic effect upon Ph, thereby affecting the degree of primary charge recombination. It may be related to one of the several acceptors already proposed by others and the need for more study is stressed in order to confirm or refute its existence. I also would like to express appreciation to Chuck Haymond and to the science support services for their assistance in designing and building the log converter amplifier used in the DCIP kinetic experiments. Finally, I would like to thank those around me, both within the university and without, who offered many helpful suggestions and who were a constant source of encouragement. AN ANALYSIS OF THE INHIBITORY EFFECTS OF LINOLENICThis reaction involves the net reduction of NADP+ at the expense of water. It also involves the phosphorylation of ADP to form ATP (not shown) by utilizing a free energy gradient produced in part as protons generated from (2) accumulate on the inner side of the thylakoid membrane. The enzymes in the dark reactions are not membrane-bound, but require the products formed in the light reactions in addition to carbon dioxide:where F-6-P is an abbreviation for fructose-6-phosphate. Equation ( blocking electron transport between QA and QB ( Fig. 1 b ). Atrazine and other triazine herbicides function in a similar manner. Structural similarities relating the physiological activity of these two types of compounds include a carbonyl or equivalent group and a positively charged nitrogen in each [Trebst, 1987].Linolenic Acid. It has long been known that fatty acids, especially unsaturated fatty acids, inhibit photosynthesis when added to functioning systems [Spikes et al., 1955][ Krogmann and Jagendorf, 1959]. This may seem surpri...