Sugar Beet Taproots Research Articles

Influence of phloem transport, N-fertilization and ion accumulation on sucrose storage in the taproots of fodder beet and sugar beet

To investigate the factors governing the accumulation of sucrose and amino acids in the taproots of sugar beet, their contents were measured in the leaves, phloem sap and the taproots of sugar beet, fodder beet and a hybrid between both, grown on either 3.0 or 0.5 mM nitrate. In the taproots the contents of malate, citrate and inorganic ions were also determined. For the high sucrose accumulation in sugar beet as compared to the other varieties three factors were found, (a) In sugar beet, less amino acids and more sucrose are taken up into the phloem than in fodder beet, (b) In sugar beet, the sucrose and amino acid syntheses are less sensitive to the nitrate concentrations that are required for optimal plant growth than in other varieties. In fodder beet, upon raising the nitrate concentration from 0.5 mM to 3 mM, the synthesis and storage of sucrose is decreased and that of amino acids increased. The corresponding values in sugar beet (0.5 mM) are similar to those in fodder beet and are not much affected by an increase of nitrate, (c) The sucrose accumulation is limited by the acccumulation of inorganic ions in the taproots. The sucrose content in the taproots is negatively correlated to the total ion content. Whereas sucrose represents two-thirds of all solutes in the taproots of sugar beet, it amounts to only one-third of the solutes in fodder beet taproots.

Protons and calcium modulate SV-type channels in the vacuolar-lysosomal compartment ? channel interaction with calmodulin inhibitors

Slowly activating vacuolar (SV-type; Hedrich and Neher 1987, Nature 329: 833–835) ion channels provide the predominant membrane conductance of the vacuolar-lysosomal compartment of Vicia faba L. guard cells and sugar beet (Beta vulgaris L.) taproots. Applying the patch-clamp technique to isolated vacuoles of both tissues, the electrical and pharmacological properties of guard-cell SV-type currents were studied and compared to the sugar beet channel with regard to its modulation by cytoplasmic Ca2+ and pH. This outward rectifier of V. faba guard cells showed a half-maximum activation at 55–60 mV with an apparent gating charge equivalent of z ≈ 4. Studies on the single-channel and whole-vacuole level revealed an extremely high conductance of 280 pS for the guard-cell channels at a mean density of 0.37 μm-2 compared to taproots (120–140 pS at about 0.16 channels per μm2). Guard-cell SV-type channels are weakly selective for cations over anions and lack saturation at KC1 concentrations of up to 1 M. Since in the absence of physiological K+ concentrations, Ca2+ is the major permeable ion, relative changes in the amounts of the two ions might control the permeation process. In spite of their different origins and physiological functions, in guard cells and beet taproot cells, cytoplasmic Ca2+ and protons, both considered as candidates for intracellular signalling in plants, modulate the voltage dependence of SV-type channels. While the two effectors do not alter the single-channel conductance, they strongly interact with the voltage sensor. The calmodulin (CaM) antagonists N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide hydrochloride (W-7), trifluoperazine (TFP) and calmidazolium hydrochloride (R 24571) effectively blocked the channel in an antagonist-specific manner. In agreement with the properties of a Ca2+-permeable channel, CaM could be involved in the modulation of the activation threshold of the SV-type channel. We therefore conclude that guard-cell SV-type channels, which might be responsible for the release of K+, Cl- and to a smaller extent Ca2+ during stomatal closure, could serve as an intracellular sensor for changes in cytosolic calcium (calcium-CaM) and pH.

Partial purification and characterization of fructokinase from developing taproots of sugar beet ( Beta vulgaris)

Fructokinase (FK) has been purified from developing sugar beet ( Beta vulgaris L.) taproots by ion exchange chromatography and gel filtration. One major isoform was identified. The protein appears to be a dimer ( M r 38 000). Kinetically, the purified sugar beet fructokinase has a pH optimum of 8.5 and a high specificity for fructose ( K m = 0.068 mM). The enzyme can utilise a range of nucleotide triphosphates, although ATP is the most effective. Sugar beet fructokinase is inhibited by fructose concentrations in excess of 0.6 mM. Fructose-6-phosphate and Mg ADP are also inhibitory, but at relatively high concentrations. K + at 10 mM stimulates activity by 30%. Fructokinase activity and the level of FK protein remain high throughout taproot development. Tissue-blots showed that high levels of FK protein were associated with conducting tissues.

Lumenal calcium modulates unitary conductance and gating of a plant vacuolar calcium release channel

The patch clamp technique has been used to investigate ion permeation and Ca(2+)-dependent gating of a voltage-sensitive Ca2+ release channel in the vacuolar membrane of sugar beet tap roots. Reversal potential measurements in bi-ionic conditions revealed a sequence for permeability ratios of Ca2+ approximately Sr2+ approximately Ba2+ > Mg2+ >> K+ which is inversely related to the size of the unitary conductances K+ >> Mg2+ approximately Ba2+ > Sr2+ approximately Ca2+, suggesting that ion movement is not independent. In the presence of Ca2+, the unitary K+ current is reduced in a concentration- and voltage-dependent manner by Ca2+ binding at a high affinity site (K0.5 = 0.29 mM at 0 mV) which is located 9% along the electric field of the membrane from the vacuolar side. Comparison of reversal potentials obtained under strictly bi-ionic conditions with those obtained in the presence of mixtures of the two ions indicates that the channel forms a multi-ion pore. Lumenal Ca2+ also has an effect on voltage-dependent channel gating. Stepwise increases of vacuolar Ca2+ from micromolar to millimolar concentrations resulted in a dramatic increase in channel openings over the physiological voltage range via a shift in threshold for channel activation to less negative membrane potentials. The steepness of the concentration dependence of channel activation by Ca2+ at -41 mV predicts that two Ca2+ ions need to bind to open the gate. The implications of the results for ion permeation and channel gating are discussed.

The voltage-gated Ca 2+ release channel in the vacuolar membrane of sugar beet resides in two activity states

The voltage-gated Ca 2+ release channel in the vacuolar membrane of sugar beet tap roots resides in two states which show similar reversal potentials in biionic conditions but differ dramatically in their single channel current and open frequency. State I has a unitary Ca 2+ conductance of 3.9 ± 0.5 pS and channel openings are rare at vacuolar resting potentials. After spontaneous and reversible transition into State II the unitary Ca 2+ conductance increases three-fold, accompanied by a concomitant rise in channel activity over the physiological voltage range. The 15- to 19-fold increase in total Ca 2+ current after the State I to State II transition could play an essential role in channel activation during signal transduction.

Localization of phenolic compounds in crowns and roots of healthy and Rhizoctonia solani-infected sugar beets

A histochemical test employing nitrous acid, that produces red nitroso derivatives of some phenolic compounds, was modified for use in the field and was used to demonstrate localization of phenolic compounds in healthy and Rhizoctonia solani-infected sugar beet ( Beta vulgaris) plants. Nitroso-reactive materials were present in petioles and around mature crown cavities of healthy plants. They were absent from tap roots, except that they occasionally were observed in lateral root tissue contained within tap roots. Nitroso-reactive materials were localized in healthy tissues surrounding quiescent, but previously active rot lesions caused by R. solani, but were not found around actively developing lesions or in association with wounds. The intensity of staining diminished late in the growing season. Preformed phenolic compounds may be involved in prophylaxis against pathogens in petioles and crown cavities and in active resistance against R. solani in sugar beet tap roots.

Characterization of the Major Integral Protein of Vacuolar Membrane

The vacuolar membrane of radish (Raphanus sativus) taproot contained a large quantity of a protein of 23 kilodaltons that accounted for more than 25% of the total membrane proteins. The protein, tentatively named VM 23, was purified and characterized. VM 23 tends to aggregate at high temperature even in the presence of 1% sodium dodecyl sulfate. The apparent molecular size of VM 23 was estimated to be about 400 kilodaltons by polyacrylamide gel electrophoresis in the presence of 0.1% Triton X-100. VM 23 was partially extracted from the vacuolar membranes with chloroform:methanol, indicating its high hydrophobicity. The hydrophobic carboxyl modifier N,N'-dicyclohexylcarbodiimide bound covalently to VM 23. The results suggest that VM 23 may act as a secondary transport system coupled with the proton transport. The antibody against radish VM 23 reacted with the major proteins in the vacuolar membranes of mung bean (Vigna radiata) and castor bean (Ricinus communis) hypocotyls and pumpkin (Cucurbita moschata) epicotyl, but not with that of sugar beet (Beta vulgaris) taproot. VM 23 comigrated with vacuolar H(+)-pyrophosphatase on sucrose density gradient centrifugation after sonication of membranes, indicating that it is associated with the vacuolar membrane.

Hydraulic conductivity of cell walls in sugar beet tissue

Parenchymal tissue from sugar beet tap root ( Beta vulgaris) was investigated. After destruction of the cell membranes by heat or action of tetrachloromethane, the tissue is shown to be highly compressible. The water flow proceeds exclusively from a zone adjacent to the outflow surface while no flow takes place in the remaining part of the tissue. During the dewatering process this zone increases until the total probe is compressed. Subsequently a further dewatering (consolidation period) is observed. The first period is equivalent to a filtration. By transferring the filtration theory to the compression of cells both the overall hydraulic conductivity L p and the specific hydraulic conductivity of the cell wall material L pp can be calculated. The value L pp ≈ 10 −3 cm 2 bar −1 s −1 is one order of magnitude higher than that postulated by Steudle and Boyer for soy bean tissue. The role of the intercellular space is not yet elucidated.

Evidence for the Presence of a Sucrose Carrier in Immature Sugar Beet Tap Roots

The objectives of this work were to determine the path of phloem unloading and if a sucrose carrier was present in young sugar beet (Beta vulgaris L.) taproots. The approach was to exploit the characteristics of the sucrose analog, 1'-fluorosucrose (F-sucrose) which is a poor substrate for acid invertase but is a substrate for sucrose synthase. Ten millimolar each of [(3)H]sucrose and [(14)C]F-sucrose were applied in a 1:1 ratio to an abraded region of an attached leaf for 6 hours. [(14)C]F-sucrose was translocated and accumulated in the roots at a higher rate than [(3)H]sucrose. This was due to [(3)H]sucrose hydrolysis along the translocation path. Presence of [(3)H]hexose and [(14)C]F-sucrose in the root apoplast suggested apoplastic sucrose unloading with its subsequent hydrolysis. Labeled F-sucrose uptake by root tissue discs exhibited biphasic kinetics and was inhibited by unlabeled sucrose, indicating that immature roots have the ability for carrier-mediated sucrose transport from the apoplast. Collectively, in vivo and in vitro data indicate that despite sucrose hydrolysis by the wall-bound invertase, sucrose hydrolysis is not entirely essential for sugar accumulation in this tissue.

Leaf Diffusive Conductance and Tap Root Cell Turgor Pressure of Sugarbeet

Abstract. The interrelationships of leaf diffusive conductance, tap root cell turgor pressure and the diameter of the tap root of sugarbeet were studied. The study was conducted on well‐watered plants growing in pots under artificial light in the glasshouse. In a typical experiment, on illumination (400 μmol m−2 s−1) leaf conductance increased from 0.6 to 7.4 mm s−1. Cell turgor pressure in the tap root decreased from 0.8 MPa to 0.45 MPa and the root diameter (9.0 cm) contracted by 145μm. Removal of light resulted in the reversal of each of the above parameters to their previous values. Quantitively similar results were obtained when sugar beet plants were uprooted and the response of each of the parameters was measured. The sequence of events however was different. On stimulation by light, changes in leaf diffusive conductance preceded the turgor and root diameter changes (which were simultaneous) by some 15–20min. In contrast, on uprooting the simultaneous changes in root turgor pressure and diameter preceded the changes in leaf conductance. The lag times between changes in diffusive conductance and turgor pressure in the root were between 20 and 30 min.Tap root turgor pressure and diameter correlated strongly and permitted the calculation of an apparent whole root volumetric elastic modules (55–63 MPa). The small changes in tissue volume relative to the transpiration rate suggest that the tap root is not a significant source of transpirational water during the day.

Membrane Transport in Isolated Vesicles from Sugarbeet Taproot

The effects of fluoride on the tonoplast type ATPase and transport activities associated with sealed membrane vesicles isolated from sugarbeet (Beta vulgaris L.) storage tissue were examined. This anion had two distinct effects upon the proton-pumping vesicles. When ATP hydrolysis was measured in the presence of gramicidin D, significant inhibition (approximately 50%) only occurred when the fluoride concentration approached 50 millimolar. In contrast, the same degree of inhibition of proton transport occurred when the fluoride concentration was about 24 millimolar. Effects on proton pumping at this concentration of fluoride could be attributed to an inhibition of chloride movement which serves to dissipate the vesicle membrane potential. Valinomycin could partially restore ATPase activity in sealed vesicles which were inhibited by fluoride and this restoration occurred with a reduction in the membrane potential. Fluoride demonstrated a competitive interaction with chloride-stimulation of proton transport and inhibited the uptake of radioactive chloride into sealed vesicles. When the vesicles were allowed to develop a pH gradient in the absence of KCl, and KCl was subsequently added, fluoride reduced enhancement of the existing pH gradient by KCl. The results are consistent with a chloride carrier that is inhibited by fluoride.

Electrical Evidence for Turgor Inhibition of Proton Extrusion in Sugar Beet Taproot

Sections of sugar beet (Beta vulgaris L.) taproot were incubated in various concentrations of mannitol. At 0.4, 0.6, and 0.8 molar, the membrane electrical potential difference (E(m)) averaged about -130 millivolts; at 0.2 molar, about -90 millivolts; and at 0 molar, between -60 and -80 millivolts. Additions of 10 millivolts acetate to the incubation solutions (all at pH 5) enhanced the membrane polarity to about -200 millivolts. We conclude from these and previous findings that high turgor inhibits proton extrusion in the sugar beet, but that proton extrusion can be activated in fully turgid tissue by acidification of the cytoplasm. A possible function of this turgor effect may be the control of turgor itself.

Turgor Regulation of Sucrose Transport in Sugar Beet Taproot Tissue

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(max) but no effect on K(m). 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(max) 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.

Plasma membrane ATPase of sugarbeet

A membrane fraction enriched with magnesium-dependent ATPase activity was isolated from sugarbeet ( Beta vulgaris L.) taproot by a combination of differential centrifugation, extraction with KI and sucrose density gradient centrifugation. This activity was inhibited by vanadate, N, N′-dicyclohexylcarbodiimide and diethylstilbestrol, but was insensitive to molybdate, azide, oligomycin, ouabain, and nitrate, suggesting enrichment in plasma membrane ATPase. The enzyme was substrate specific for ATP, had a pH optimum of 7.0, but showed little stimulation by 50 mM KCl. The sugarbeet ATPase preparation contained endogenous protein kinase activity which could be reduced by extraction of the membranes with 0.1% (w/v) sodium deoxycholate. Reduction of protein kinase activity allowed the demonstration of a rapidly turning over phosphorylated intermediate on a M r 105000 polypeptide, most likely representing the catalytic subunit of the ATPase. Phosphorylation was magnesium dependent, sensitive to diethylstilbestrol and vanadate but insensitive to oligomycin and azide. Neither the ATPase activity nor phosphoenzyme level were affected by combinations of sodium and potassium in the assay. These results argue against the presence of a synergistically stimulated NaK-ATPase at the plasma membrane of sugarbeet.

Membrane Transport in Isolated Vesicles from Sugarbeet Taproot

The process of sucrose transport was investigated in sealed putative tonoplast vesicles isolated from sugarbeet (Beta vulgaris L.) taproot. If the vesicles were allowed to develop a steady state pH gradient by the associated transport ATPase and 10 millimolar sucrose was added, a transient flux of protons out of the vesicles was observed. The presence of an ATPase produced pH gradient allowed [(14)C]sucrose transport into the vesicles to occur at a rate 10-fold higher than the rate observed in the absence of an imposed pH gradient. Labeled sucrose accumulated into the sealed vesicles could be released back to the external medium if the pH gradient was dissipated with carbonylcyanide-m-chlorophenyl hydrazone (CCCP). When the kinetics of ATP dependent [(14)C]sucrose uptake were examined, the kinetic profile followed the simple Michaelis-Menten relationship and a Michaelis constant of 12.1 millimolar was found. When a transient, inwardly directed sucrose gradient was imposed on the vesicles in the absence of charge compensating ions, a transient interior negative membrane potential was observed. This membrane potential could be prevented by the addition of CCCP prior to sucrose or dissipated by the addition of CCCP after sucrose was added. These results suggest that an electrogenic H(+)/sucrose antiport may be operating on the vesicle membrane.

Membrane Transport in Isolated Vesicles from Sugarbeet Taproot

Sealed membrane vesicles were isolated from homogenates of sugarbeet (Beta vulgaris L.) taproot by a combination of differential centrifugation, extraction with KI, and dextran gradient centrifugation. Relative to the KI-extracted microsomes, the content of plasma membranes, mitochondrial membranes, and Golgi membranes was much reduced in the final vesicle fraction. A component of ATPase activity that was inhibited by nitrate co-enriched with the capacity of the vesicles to form a steady state pH gradient during the purification procedure. This suggests that the nitrate-sensitive ATPase may be involved in driving H(+)-transport, and this is consistent with the observation that H(+)-transport, in the final vesicle fraction was inhibited by nitrate. Proton transport in the sugarbeet vesicles was substrate specific for ATP, insensitive to sodium vanadate and oligomycin but was inhibited by diethylstilbestrol and N,N'-dicyclohexylcarbodiimide. The formation of a pH gradient in the vesicles was enhanced by halide ions in the sequence I(-) > Br(-) > Cl(-) while F(-) was inhibitory. These stimulatory effects occur from both a direct stimulation of the ATPase by anions and a reduction in the vesicle membrane potential. In the presence of Cl(-), alkali cations reduce the pH gradient relative to that observed with bis-tris-propane, possibly by H(+)/alkali cation exchange. Based upon the properties of the H(+)-transporting vesicles, it is proposed that they are most likely derived from the tonoplast so that this vesicle preparation would represent a convenient system for studying the mechanism of transport at this membrane boundary.

Sucrose Uptake and Compartmentation in Sugar Beet Taproot Tissue

Active sucrose uptake by discs of mature sugar beet (Beta vulgaris L. cv GW-D2 and USH-20) root tissue shows a biphasic dependence on external sucrose. At concentrations up to 20 millimolar sucrose, the active uptake mechanism appears to approach saturation, with an apparent K(m) of 3.6 millimolar. At higher external sucrose concentrations, a linear dependence becomes obvious indicating the probable presence of a nonsaturable, metabolically dependent uptake component. Active transport was not observed at external sucrose concentrations that caused tissue plasmolysis. Passive sucrose uptake in unplasmolyzed tissue showed a linear dependence on external sucrose concentration. The mitochondrial and/or suspected vacuolar ATPase inhibitors oligomycin, diethylstilbestrol, and N,N-dicyclohexylcarbodiimide strongly inhibited active sucrose uptake, whereas the putative plasmalemma-specific ATPase inhibitor orthovanadate was without effect.Sucrose efflux patterns from root discs indicated three distinct sucrose compartments having efflux kinetics consistent with those for cell wall, cytoplasm, and vacuole with the vacuole being the slowest releasing compartment. The sucrose contents and volumes of the compartments indicated that sucrose uptake into the vacuole was against a concentration gradient. Combined sucrose uptake/efflux analyses indicated that sucrose uptake into the vacuole is primarily an active transport process while transport into the cytoplasm is apparently passive, at least at external sucrose concentrations above 20 millimolar. We discuss the possibility that active sucrose uptake into the vacuoles of sugar beet storage cells is rate limited by passive sucrose transport to the active uptake site.

Efficient Deep Tillage for Sugarbeets on Pullman Clay Loam

INTRODUCTION Sugarbeets in the Texas Panhandle are grown largely on soils with a naturally occurring, very dense, clay B22t horizon at about a 20to 50-cm depth. The clay loam surface horizon is low in organic matter and subject to severe compaction. Tillage to a 30or 40-cm depth in these soils has been shown to increase sugar yield by increasing water infiltration, improving aeration, and reducing bulk density and Rhizoctonia root rot ( 2 ) . In Ohio, sugarbeets were very sensitive to poor aeration associated with non-capillary pore space below 8% (1). Commonly practiced sub-soiling, followed by disking and leveling, is very energy intensive and inefficient because operations after sub-soiling strongly recompact the soil. A new tillage tool, the hipper-ripper, seems to have several advantages. With this implement, deep tillage is limited to the center of the bed where the sugarbeet taproot will penetrate. Since all subsequent traffic is limited to the water furrows between beds, recompaction of the sub-soiled zone is not a .problem. This research was undertaken to test the hypothesis that the hipper-ripper would reduce fuel requirement for tillage while producing acceptable sugarbeet yield. MATERIALS AND METHODS Four tillage treatments were compared in 1980 and 1981 at Bushland, Texas on Pullman clay loam (fine, mixed,

Partitioning within the Taproot Sink of Sugarbeet: Effect of Photosynthate Supply1

Genotypes of sugarbeet (Beta vulgaris L.) differ widely in partitioning of photosynthate between sucrose and nonsucrose dry matter in the taproot sink. Differences in root (sink) morphology between various genotypes and agronomic practices (cell size and distance between vascular rings) have been found to be related to photosynthate partitioning. The objective of this research was to determine the effect of photosynthate supply on partitioning within the root sink. It was hypothesized that altering the photosynthate supply would not affect partitioning, but would rather affect the growth rate of the root with no change in relative root morphology. Photosynthetic rates of field‐grown sugarbeets were increased by CO2 enrichment and reduced by shading. Carbon dioxide levels within the canopy were increased to 600 to 850 ppm using a propane‐fired CO2 generator with dry ice. In the shading treatments, light levels were reduced by 50% with a nylon mesh screen suspended above the canopy. Shading reduced total dry matter production by 29%, but had no effect on either the root:shoot ratio or the sucrose content of the taproot on a dry weight basis. Although the size of root was altered in the two treatments, the differences were compensated by increased or decreased frequency of vascular rings, resulting in no change in vascular density or in cell size. These results are consistent with previously proposed models for photosynthate partitioning within the taproot of sugarbeet and indicate that the partitioning mechanism maintains a constant partitioning ratio between sucrose and nonsucrose dry matter independent of photosynthate supply.

Comparative Enzymic Studies of Sucrose Metabolism in the Taproots and Fibrous Roots of Beta vulgaris L

Comparative enzymic studies of sugar beet (Beta vulgaris L.) taproots and fibrous roots revealed differences in invertase (EC 3.2.1.26) and sucrose synthetase (EC 2.4.1.13) activity. Invertase activity of the two root forms differs with respect to specific activity, pH optimum, and enzyme solubility. Acid invertase (pH 4.5) in the taproot was restricted to the peripheral meristematic tissue which produces cells for both taproot and fibrous root growth. This finding supports the hypothesis that the enzyme regulates sucrose partitioning between the taproot and fibrous roots. A distinct alkaline invertase (pH 8.0) was detected in sucrose storage tissues of the taproot.The V(max) of taproot sucrose synthetase (sucrose cleavage reaction) was highest in the presence of UDP. However, the fibrous root enzyme had the highest V(max) with ADP as substrate. Differential nucleoside diphosphate substrate affinities may provide for compartmentation and separate regulation of sucrose cleavage and resultant hexose utilization in adjoining taproot and fibrous root tissues.