Direct versus indirect effects of p‐chloromercuribenzenesulphonic acid on sucrose uptake by plant tissues: The electrophysiological evidence

The short‐term effects of p‐chloromercuribenzenesulphonic acid (PCMBS) on the transmembrane potential difference (PD) of broad bean (Vicia faba L. cv. Aguadulce) cotyledon cells and on sugar beet (Beta vulgaris L. cv. Klein E.) leaf cells were studied by the electrophysiological method. These effects were compared with that of the permeant thiol reagents N‐ethylmaleimide and HgCl2. N‐Ethylmaleimide and HgCl2 markedly and rapidly depolarised the PD of all the material studied, while PCMBS caused either a slight depolarisation (cotyledon cells) or no depolarisation (leaf cells) during the first 30 min of treatment. In cotyledons, PCMBS markedly inhibited sucrose uptake (89%) and the sucrose‐induced depolarisation associated with the proton‐sucrose symport (67%), while it decreased the proton‐motive force only marginally (7%). It is concluded that during short treatments (30 min or less). PCMBS inhibits sucrose uptake directly by blocking the sucrose carrier, and not the proton pump. For longer treatments, indirect effects cannot be excluded.