Sink tissues that store osmotically active compounds must osmoregulate to prevent excessively high turgor. The ability to regulate turgor may be related to membrane transport of solutes and thus sink strength. To study this possibility, the kinetics of sugar uptake were determined in sugar beet (Beta vulgaris L.) taproot tissue discs over a range of cell turgors. Sucrose uptake followed biphasic kinetics with a high affinity saturating component below 20 millimolar and a low affinity linear component at higher concentrations. Glucose uptake exhibited only simple saturation type kinetics. The high affinity saturating component of sucrose and glucose uptake was inhibited by increasing cell turgor (decreasing external mannitol concentrations). The inhibition was evident as a decrease in V,,,, but no effect on K,.. Sucrose uptake by tissue equilibrated in dilute buffer exhibited no saturating component. Ethylene glycol, a permeant osmoticum, had no effect on uptake kinetics, suggesting that the effect was due to changes in cell turgor and not due to decreased water potential per se. p-(Chloromercuri)benzene sulfonic acid (PCMBS) inhibited sucrose uptake at low but not high cell turgor. High cell turgor caused the tissue to become generally leaky to potassium, sucrose, amino acids, and reducing sugars. PCMBS had no effect on sucrose leakage, an indication that the turgor-induced leakage of sucrose was not via back flow through the carrier. The ability of the tissue to acidify the external media was turgor dependent with an optimum at 300 kilopascals. Acidification was sharply reduced at cell turgors above or below the optimum. The results suggest that the secondary transport of sucrose is reduced at high turgor as a result of inhibition of the plasma membrane ATPase. This inhibition of ATPase activity would explain the reduced V.,,, and leakiness to low molecular weight solutes. Cell turgor is an important regulator of sucrose uptake in this tissue and thus may be an important determinant of sink strength in tissues that store sucrose.Maintenance of cell turgor is an important aspect of plant tolerance to salt or water stress (16) and is frequently accom-'

Active sucrose uptake by discs of mature sugar beet (Bew vadai L cv GW-D2 and USH-20) root tssue shows a biphasc de nce on external sucrose. At concentrations up to 20 mill r sucroe, the active uptake mechanism appears to approach saturation, with an appaet K,, of 3.6 milhnmolar. At Sucrose gradients between source and sink regions can be maximized by increasing assimilate levels at the site of phloem loading in leaves (4) or by reducing the concentration of sucrose at the site of phloem unloading (13). Although the mechanism controlling phloem unloading of sucrose is not known, the concentration ofthe sucrose pool at the unloading site will presumably influence the rate of unloading. The sucrose pool into which phloem unloads may be in the cell wall free space in a number of economically important sinks, ie. soybean cotyledons (28), corn kernels (3), sugar beet taproot (27), and sugarcane (8). Therefore, the ability of economically important sinks to maintain low apoplastic sucrose concentrations either by metabolism or uptake should enhance their mobilizing ability relative to other sinks.An understanding of photosynthate movement into and within economically important sink regions of crop plants is necessary to evaluate some of the potential parameters controlling crop productivity and yield. In sugar beet, sucrose is not hydrolyzed during transport from the source leaves into the vacuoles of parenchyma cells of the root (6, 32). Wyse (32) found that sucrose uptake in sugar beet roots was linear between concentrations of I and 500 mi, occurred against a sucrose concentration gradient, and was sensitive to metabolic inhibitors. More recently, Saftner and Wyse (26) showed that sucrose is actively transported into the vacuoles of sugar beet root discs in a manner consistent with an alkali cation/sucrose co-transport mechanism. The objective of this research was to further characterize sucrose uptake into the cytoplasmic and vacuolar compartments of sugar beet taproot sink tissue.Translocation via the phloem conveys large quantities of photosynthates to specialized sink regions which compete for available assimilates. The ability of a sink region to assimilate translocated sucrose is limited by its relative ability to mobilize photosynthates from supply sources (12,28).The mobilizing ability of a sink region is a function of its relative size, potential growth rate, and capacity to take up and metabolize assimilates. This latter characteristic may influence sucrose gradients between various source/sink regions. These gradients are hypothesized to control carbon flux to sink regions (13,14). ' (25). Discs were incubated with [14Cjaorbitol for 1 h and the "C-sugar in the free space was then quantified by compartmental analysis. From the specific activity of the exteral media at the end of the incubation period and the "C in the free space, the volume of the free space was estimated. The volumes of the cytoplasm and vacuole were estimated from measurements made on electron micrographs of root parenchy...

Like other halophytic chenopods, sugar beet (Beta vulgaris L.) can accumulate high betaine levels in shoots and roots. N,N,N-trimethylglycine impedes sucrose crystallization and so lowers beet quality. among sugar beets and their relatives, about the role of betaine in the salt-tolerance of these plants, or about the synthesis and metabolism of betaine in the chenopods as a group (for reviews, see 7, 22). Betaine accumulation, whether constitutive or saltinduced, may be a specific adaptation for salt-tolerance in wild and cultivated members of the Chenopodiaceae, including Beta spp. (for reviews, see 21, 22). Histochemical, biochemical, and physiological evidence for such chenopods indicates that betaine is an inert and nontoxic cytoplasmic osmoticum that helps to maintain osmotic equilibrium between the cytoplasm and the vacuole as the vacuolar solute potential is lowered by salt accumulation (6,15,20,21). High betaine levels in sugar beet roots are undesirable to the beet sugar industry because betaine interferes with sucrose crystallization from the juice (e.g. 23). The molar concentration of betaine in juice can reach 5% to 10% that of sucrose (e.g. 17). High levels of betaine in sugar beet roots could conceivably reduce sugar yields in a second way: the synthesis of 1 mol of betaine requires about the same energy input as that of 1 mol of sucrose (11); therefore, photosynthate diverted to betaine represents an appreciable cost in energy and photosynthate that is neither available for storage as sucrose, nor for plant processes that contribute indirectly to economic yield. If useful genetic variability for root betaine level exists within the primary gene pool of B. vulgaris, in principle low-betaine types could be developed by breeding. Whether lowering betaine levels by genetic means would be worthwhile in practice hinges on several sets of considerations, including: (a) ecological and physiological considerations, which suggest that salt-tolerance (or more generally, ionic regulation) might be adversely affected; (b) metabolic considerations, which indicate that plant performance might suffer if betaine has some metabolic function and is not merely an inert osmoticum; (c) technological and bioenergetic considerations, which imply that sucrose yields would increase.In this paper, we present information that bears on considerations (a) and (b) in breeding for low-betaine sugar beet types.Very little is known about genetic variability for betaine3 level

The amino acid carriers in sugarcane suspension ceUs were characterized for amino acid specificity and the stoichiometry of proton and potassium flux during amino acid transport.Amino acid transport by sugarcane cells is dependent upon three distinct transport systems. One system is specific for neutral amino acids and transports all neutral amino acids including glutamine, asparagne, and histidine. The uptake of neutral amino acids is coupled to the uptake of one proton per amino acid; one potassium ion leaves the cells for chare compensation. Histidine is only taken up in the neutral form so that deprotonation of the charged imidazole nitrogen has to occur prior to uptake. The basic amino acids are transported by another system as uniport with charge-compensating efflux of protons and potassium. The acidic amino acids are transported by a third system. Acidic amino acids bind to the transport site only if the distal carboxyl group is in the dissociated form (i.e. if the acidic amino acid is anionic). Two protons are withdrawn from the medium and one potassium leaves the cell for charge compensation during the uptake of acid amino acids. Common to all three uptake systems is a monovalent positively charged amino acidproton carrier complex at the transport site.

Sucrose uptake by discs of mature sugar beet root tissue incubated in 114Cl-sucrose exhibited nonsaturating kinetics over the concentration range of 1 to 500 millimolar. Uptake Glucose and fructose uptake exhibited typical saturation kinetics but rates of uptake were lower than that of sucrose, particularly at high concentration. Glucose strongly inhibited the uptake of sucrose and fructose but sucrose and fructose had little effect on the rate of glucose uptake. It is proposed that a major portion of the sucrose movement between its free space and vacuole occurs via a nonsaturating carrier at sites where the plasmalemma and tonoplast are appressed.Two important aspects of a plant's productivity are: (a) its capacity to produce large amounts of photosynthates; and (b) its ability to transport and partition these photosynthates to appropriate sink areas. Of particular importance to food production is the ability of agronomic plants to partition a large proportion of assimilate to the economically important sink, ie. the grain, tuber, root, or foliage. Major research emphasis has been placed on photosynthesis and transport of assimilate, but relatively little information is available on the effect of the sink region on partitioning and thus agronomic productivity. An important aspect of sink metabolism is the capacity of the sink to assimilate translocated sucrose.