The effects of 2,4-dichlorophenoxyacetic acid altered chloroplast development on photosynthesis [radish, Raphanus sativus, 2,4-D, electron transport, photophosphorylation, proton binding]
1981
McCracken, D.A. | Ort, D.R. | Nadakavukaren, M. (Illinois Univ., Urbana (USA). Dept. of Botany)
We studied the changes in function and physical properties of isolated radish (Raphanus sativus L. cv. Sparkler) lamellar membranes 48 h after chloroplast development was altered by 2,4-(dichlorophenoxy)acetic acid. The number of chlorophyll molecules attendant to each electron transport chain was approximately 25 per cent less in the chloroplasts from 2,4-(dichlorophenoxy)acetic acid-treated plants than in chloroplasts from untreated plants. The maximal turnover rate of Photosystem II 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 high 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-(dichlorophenoxy)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.
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