The effects of 2, 4-dichIorophenoxyacetk acid altered chloroplast development on photosynthesis

We studied the changes in function and physical properties of isolated radish ( Raphonus sativus L. cv. Sparkler) lamellar membranes 48 h after chloroplast development was altered by 2, 4-(dichlorophenoxy)acet, tc acid. The number of chlorophyll molecules attendant to each electron transport chain was approximately 25% less in the chloroplasts from 2, 4-(dichlorophenoxy)acetic acid-treated plants than in chloroplasts from untreated plants. The maximal turnover rate of Photosystem I] in the treated chloroplasts was slightly less than half the turnover rate in normal chloroplasts. The efficiency of coupling between electron flux and ATP formation was not significantly different in the two chloroplast types. This hight efficiency of photophosphorylation in addition to normal membrane conductance to hydrogen ions indicates that the herbicide has not brought about a general deterioration of the membrane. A dramatic increase in the proton binding capacity of the lamellar membrane was observed in the treated chloroplasts. This increase in hydrogen ion buffering groups was largely accounted for by extrinsic membrane proteins bound to the exterior surface of the lamellar membrane. Although the addition of 2, 4-(dichloro-phenoxy) acetic acid to chloroplasts isolated from untreated plants caused concurrent uncoupling of ATP formation and inhibition of electron transport, our data show that these direct effects of the compound have little to do with its herbicidal action.