EFFECT OF CUTTING ON SOLUTE UPTAKE BY PLASMA-MEMBRANE VESICLES FROM SUGAR-BEET (BETA-VULGARIS L) LEAVES

The uptake of sucrose, 3-0-methylglucose (3-0-MeG), and valine were st udied in discs and in purified plasma membrane vesicles (PMV) prepared from sugar beet (Beta vulgaris L.) exporting leaves. The uptake capac ities of freshly excised leaf discs were compared with the uptake in d iscs that had been floated for 12 h on a simple medium (aging) and wit h discs excised from leaves that had been cut from the plant 12 h befo re the experiments (cutting). After cutting, sucrose uptake amounted t o twice the uptake measured in fresh discs, whereas the uptake of 3-0- MeG and valine remained unaffected. In aged leaf discs, there was a ge neral stimulation of uptake, which represented 400, 300, and 400% of t he uptake measured in fresh discs for sucrose, 3-0-MeG, and valine, re spectively. Sucrose uptake in fresh discs was sensitive to N-ethylmale imide (NEM), to p-chloromercuribenzenesulfonic acid (PCMBS), and to me rsalyl acid (MA). Although cutting induced the appearance of a sucrose uptake system that is poorly sensitive to NEM but sensitive to PCMBS and MA, aging induced the development of an uptake system that is sens itive to NEM but poorly sensitive to PCMBS and MA. Autoradiographs of discs fed with [C-14]sucrose show that cutting resulted in an increase of vein labeling with little effect in the mesophyll, whereas aging i nduced an increase of labeling located mainly in the mesophyll. The da ta show that cutting is sufficient to induce dramatic and selective ch anges in the uptake properties of leaf tissues and that the effects of cutting and aging on the uptake of organic solutes are clearly differ ent. Parallel experiments were run with purified PMV prepared from fre sh and cut leaves. The uptake of sugars and amino acids was studied af ter imposition of an artificial proton motive force (pmf). Comparison of the uptake properties of PMV and of leaf tissues indicate that the recovery of the sucrose uptake system in PMV is better than the recove ry of the hexose and of the valine uptake systems. As observed with th e leaf discs, cutting induced a 2-fold increase of the initial rate of sucrose uptake in PMV but did not affect the uptake of valine and 3-0 -MeG. Cutting induced an increase of both V(max) and K(m) of the sucro se transport system in PMV. Measurements of the pmf imposed on the ves icles indicated that the increase of sucrose uptake induced by cutting was not due to a better integrity of the vesicles. Hexoses did not co mpete with sucrose for uptake in PMV from fresh and cut leaves, and ma ltose was a stronger inhibitor of sucrose uptake in PMV from cut leave s than in PMV from fresh leaves. The sensitivity of sucrose uptake to NEM, PCMBS, and MA in PMV from fresh and cut leaves paralleled that de scribed above for the corresponding leaf discs. These data show that ( a) the changes induced by cutting on sucrose uptake by leaf discs are due to membrane phenomena and not to the metabolism of sucrose; (b) th e study of sucrose uptake with PMV gives a good account of the physiol ogical situation; and (c) the specific effects induced by cutting on t he sucrose uptake system are not lost during the preparation of the PM V.