No direct linkage between proton pumping and photosynthate unloading from seed coats of Phaseolus vulgaris L.

The energization of the active sucrose release from bean seed-coat halves was investigated. For this purpose, seed coat tissues adjacent to the apoplastic space were exposed to a variety of treatments and proton and photosynthate release were measured. Fusicoccin (10^–5 moll^–1) stimulated proton pump activities. Orthovanadate (2×10^–4 moll^–1) and abscisic acid (10^–5 moll^–1) diminished the proton extrusion evoked by fusicoccin. Fusicoccin inhibited sucrose release, whereas orthovanadate and abscisic acid stimulated it. Addition of 100 mmoll^–1 K⁺ had a promotory effect on photosynthate unloading, fading away with time. This extra unloading was linearly related to an enhanced proton loss. It was concluded that the photosynthate unloading apparently is not a proton/sucrose antiport and that a pump-leak system for photosynthate release is unlikely. A tentative model for photosynthate/proton symport not directly linked to proton pumping is presented as the mechanism of unloading.Abbreviations ABA abscisic acid - CCCP carbonyl cyanide m-chlorophenyl hydrazone - DTE diethioerythritol - FC fusicoccin - MES 2-(N-morpholino) ethanesulfonic acid monohydrate - NEM n-ethylmaleimide - PCMBS p-chloromercuriphenylsulfonic acid - TRIS 2-amino-2-(hydroxymethyl) propane-1,3 diol - VAN sodium orthovanadate