Uncoupling and photoinhibition in chloroplasts from a triazine-resistant and a susceptible Chenopodium album biotype

Abstract

The photosynthetic control and photoinhibitory damage were investigated in chloroplasts isolated from a susceptible and a triazine-resistant biotype of Chenopodium album. Photosynthetic control was defined as the rate of electron flow after addition of an uncoupler divided by the rate before addition (coupling ratio). It was studied by measuring electron flow before and after the addition of the uncoupling agents carbonylcyanide- m-chlorophenylhydrazone (CCCP), ammonium chloride, a combination of nigericin, valinomycin, and potassium chloride, and by addition of adenosine diphosphate and inorganic phosphate. In all cases the photosynthetic control in the resistant biotype is about half that in the susceptible one. CCCP is more active in the inhibition of uncoupled electron flow in resistant choroplasts, compared with that in susceptible ones. The photoinhibitory damage to electron flow from water to silicomolybdic acid (in the presence of diuron) is comparable for the two biotypes. It is concluded that the resistant chloroplasts utilize the pH gradient less efficiently for phosphorylation than do the susceptible ones. There is probably a modification of the oxygen-evolving complex in the resistant biotype. The alteration in the resistant biotype which confines resistance to triazine herbicides has no influence on photoinhibitory damage.