Sucrose uptake by excised developing soybean cotyledons shows a biphasic dependence on sucrose concentration. At concentrations less than about 50 millimolar external sucrose, uptake can be described as a carriermediated process, with a Km of 8 milimolar. At higher external sucrose concentrations, a linear dependence becomes apparent, which suggests the participation of a nonsaturable component in total uptake. Sucrose absorption is dependent on the presence of an electrochemical potential gradient for protons since agents interfering with the generation or maintenance of this gradient (NaN3 or carbonylcyanide-m-chlorophenyl hydrazone) decrease sucrose transport to a level at or below that predicted from the operation of the noncarrier-mediated process alone. redistribution of photoassimilate during seed-fill (27), either sucrose itself or its invert products (glucose and fructose) should be transported into the developing cotyledons. Starch is transiently accumulated in the plastids of the cotyledonary mesophyll, disappearing by maturation and seed desiccation as protein and lipid bodies are formed. Besides utilizing carbohydrates as carbon sources ior amino acids and lipids, accumulated sugars are a means of osmotic adjustment, providing a driving force for water uptake, turgor maintenance, cellular enlargement, and substrate for cell wall components.Soybean fruits act as principal sinks for photoassimilate during reproductive growth. A recent investigation describes sucrose uptake by the cotyledons as a necessary step in the maintenance of a source-sink concentration gradient (27).An understanding of the mechanisms of sucrose movement into the developing fruit also is necessary to evaluate limitations to seed productivity and yield. The final weight of the seed, that is, the yield, is in part determined by its ability to draw nutrients from supply sources in the plant, i.e., by its sink strength. Sink strength, the product of sink size and sink activity, is defined as the net gain in dry weight per unit time per unit dry weight and necessarily includes factors relating to transport and metabolism of the assimilates. In source/sink relationships, an evaluation of the specific conductances at points along the pathway from the region of synthesis to that of accumulation can indicate which site(s) limit the movement of assimilate, and thus, may limit yield. This type of analysis has been applied to wheat (12, 13) where assimilate movement has been extensively studied. In wheat, distinct reductions in sucrose concentration were noted between the vascular bundle in the grain furrow and the endosperm cells.Such concentration gradients imply that movement of sugar from sites of phloem unloading to the endosperm cavity (across the plasmalemma ofthe endosperm cell) meets considerable resistance and hence, may be limiting sucrose accumulation. Such a sharp profile in sugar concentration also exists between the liquid endosperm and the cotyledons in developing Phaseolus vulgaris seeds (26). Recent experiments (27...

Addition of sucrose to a solution bathing an excised developing soybean cotyledon causes a transient depolarization of the membrane potential, as measured using standard electrophysiological techniques. The magnitude of the depolarization is dependent on the concentration of both sucrose and protons in a manner which suggests carrier mediation; this process has an apparent Km for sucrose of about 10 milimolar. Agents interfering with the generation or maintenance of a proton electrochemical gradient eliminate these depolarizations. Electrogenic sugar transport is sensitive to sulfhydryl-modifying reagents; their effect appears to be through a direct interaction with the carrier protein and/or with the process establishing the proton electrochemical gradient across the plasma membrane. p-Chloromercuribenzene sulfonate appears to be a selective inhibitor of the carrier-mediated process itself.and protons inside and outside the cell, respectively, and A4A is the membrane potential.In physical terms, transport across the cell membrane is facilitated by a carrier which alternately exposes its binding sites for proton(s) and solute on the external and internal side of the membrane (Fig. 1). The movement of protons into the cell upon addition of substrate causes a transient change in the voltage difference across the plasma membrane (8, 10, 22, 26, 29; see 22 for additional references); the value towards which the membrane potential moves is Ecotra. (16), the voltage at which no net current flows through the co-transport system. It can be derived from equation (1) (2) The concept that the energy stored in an existing ion gradient can be used to accomplish the accumulation of organic solutes is based on the early ideas of Crane and Mitchell, now commonly regarded as an outgrowth of the chemiosmotic theory. A history of the development of these ideas has been presented by Crane (4). In this scheme, the movement of an ion down its electrochemical gradient provides energy for the coupled uptake of another solute; the process is termed co-transport or symport. If transport occurs by the system outlined above, several criteria should be fulfilled: first, the uptake velocity should show carriermediated kinetic behavior with respect to substrate and proton concentrations; second, transport should be sensitive to those agents interfering with the generation and maintenance of the proton electrochemical gradient; third, transport should be affected by reagents interfering with the membrane-bound carrier protein.Developing soybean seeds offer an excellent experimental system for investigating transport processes. Since there are no symplasmic connections between parent plant and embryo (37), assim- LICHTNER AND SPANSWICK ilate must negotiate at least one plasma membrane between the site of phloem unloading and that of storage. Photosynthate, 90 to 95% in the form of sucrose (39), reaches the seeds by the ventral phloem bundles traversing the carpel (pod) and anastomosing through the outer integument. Phloem translocate...

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The administration of the sulfhydryl reagent N-ethyl-maleimide (NEM) to internodal cells of Chara coralina caused alterations in the biophysical properties of the plasmalemma, as measured with electrophysiological and radioactive tracer techniques. The membrane potential depolarized to, or near, the calculated Nermst potential for potassium (EK) after 30 seconds' exposure to 0.1 millimolar NEM. During this time, the ATP level did not decrease below the control value, and the specific membrane resistance did not increase; only upon further exposure to NEM did the resistance approach the value observed in the dark. In the depolarized state, the membrane potential responded to changes in the external potassium concentration in the manner of a K -electrode, but it retained it's relative insensitivity to external sodium.These results are interpreted in the following manner. NEM causes a) an increase in the membrane permeability to K+ (ie. an increase in K+ conductance); and b) perturbation of the electrogenic transport system(s) of the plasma membrane. The latter effect is manifested in a manner that is not consistent with an inhibition of ATP catalysis by a voltage-dependent ATPase possessing conductance. The nonpermeant sulfhydryl modifier, pchloromercuribenzenesulfonic acid, appeared to affect membrane properties in a similar, but reversible, way.The biochemical and biophysical properties of the plasmalemma play an undisputed key role in the organization and function of membrane transport phenomena. Increasingly, heavy metals and sulfhydryl reagents have been used to probe the interrelationships of these processes (4-6, 12, 18, 31 amino-1,3-naphthalene disulfonic acid; Rm, specific membrane resistance (K _-cm2); gm, total membrane conductance; PK, potassium permeability. tion of the external medium. Because the electrophysiology and ionic relations of Chara corallina are relatively well understood and since certain transport processes in this alga appear to be differentially susceptible to -SH reactive agents (18), we investigated the biophysical properties in C. corallina before and after treatment with thiol-modifying reagents. This approach has allowed us to conclude that sulfhydryl reagents affect the passive permeability of the membrane by increasing potassium conductance and act upon the electrogenic transport processes of the cell membrane. Experimental Solutions. Solutions were prepared using high purity glass-distilled H20. The CPW contained 1.0 mm NaCl, 0.2 mm KCI, 0.2 mm CaSO4, 1.0 mm TAPS (pH 8.0); modifications to this medium will be detailed in the text. Bicarbonate-free solutions were prepared by bubbling solutions with air that had been passed through C02-absorbent material (Ascarite; Arthur H. Thomas Co.). All chemicals were of Analar reagent grade and, except for PCMBS, were not purified further. Stock (100 mm) NEM solutions were titrated to pH 7, using fresh NaOH solution, and stored at 4 C for a maximum of 4 days. Because hydrolytic ring opening in the NEM molecule occurs at alkaline...