STUDY OF SUCROSE AND MANNITOL TRANSPORT IN PLASMA-MEMBRANE VESICLES FROM PHLOEM AND NONPHLOEM TISSUES OF CELERY (APIUM-GRAVEOLENS L) PETIOLES

The mature petiole of celery is an organ with versatile sink/source ca pacities where sucrose and mannitol are unloaded from and reloaded int o the phloem cells. Plasma-membrane vesicles were purified by two-phas e partitioning either from phloem strands isolated from mature petiole s of celery (Apium graveolens L.) or from mature petioles devoid of va scular bundles. Both types of vesicle were comparable in purity (more than 86% of plasma-membrane origin), size (135 nm diameter) and orient ation (72% right-side-out), Plasma-membrane vesicles from phloem tissu es had a higher vanadate-sensitive ATPase activity than plasma-membran e vesicles from petioles. Plasma-membrane vesicles from phloem tissues accumulated mannitol and sucrose in response to an artificial proton- motive force, in agreement with the existence of proton/substrate carr iers. Plasma-membrane vesicles from petioles devoid of vascular bundle s accumulated only mannitol following:application of an artificial pro ton-motive force. The data suggest the volvement of apo-plasmic transp ort events. The pathway for sucrose uptake in storage parenchyma cells is discussed in the light of the available physiological data.