ABSTRACT
We have investigated the effect of heat-treatment of chloroplast thylakoid membranes on photosystem I-mediated electron transport. Spectroscopic techniques, oxidation of dichlorophenolindophenol (donor side) and reduction of NADP or methyl purple (acceptor side), showed no indication of an increased activity of photosystem I electron transport. Enhancement of oxygen uptake in the heat-treated (40 degrees C-48 degrees C) samples could largely be accounted for by decline in the activity of superoxide dismutase.
Subject(s)
Chloroplasts/metabolism , Oxygen Consumption , Photosynthetic Reaction Center Complex Proteins/metabolism , Superoxide Dismutase/metabolism , Diuron/pharmacology , Electron Transport , Hot Temperature , Indicators and Reagents , Intracellular Membranes/metabolism , Kinetics , Oxygen Consumption/drug effects , Photosystem I Protein ComplexABSTRACT
Flash-induced primary charge separation, detected as electrochromic absorbance change, the operation of the cytochrome b/f complex and the redox state of the plastoquinone pool were measured in leaves, protoplasts and open-cell preparations of tobacco (Nicotiana tabacum L.), and in isolated intact chloroplasts of peas (Pisum sativum L.). Addition of 0.5-5 mM KCN to these samples resulted in a large increase in the slow electrochromic rise originating from the electrogenic activity of the cytochrome b/f complex. The enhancement was also demonstrated by monitoring the absorbance transients of cytochrome f and b 6 between 540 and 572 nm. In isolated, intact chloroplasts with an inhibited photosystem (PS) II, low concentrations of dithionite or ascorbate rendered turnover of only 60% of the PSI reaction centers, KCN being required to reactivate the remainder. "Silent" PSI reaction centers which could be reactivated by KCN were shown to occur in protoplasts both in the absence and presence of a PSII inhibitor. Contrasting spectroscopic data obtained for chloroplasts before and after isolation indicated the existence of a continuous supply of reducing equivalents from the cytosol.Our data indicate that: (i) A respiratory electron-transport pathway involving a cyanide-sensitive component is located in chloroplasts and competes with photosynthetic electron transport for reducing equivalents from the plastoquinone pool. This chlororespiratory pathway appears to be similar to that found in photosynthetic prokaryotes and green algae. (ii) There is an influx of reducing equivalents from the cytosol to the plastoquinone pool. These may be indicative of a complex respiratory control of photosynthetic electron transport in higher-plant cells.
