Effect of Potassium on Sucrose and Amino Acid Release from the Seed Coat of Developing Seeds of Pisum sativum

After removal of the embryo from developing seeds of Pisum sativum,the ‘empty’ ovules (seed coats without enclosedembryo) were filled with a solution (pH 5.5) containing mannitol(usually 400 mM) to which various salts were added. A solutioncontaining two isotopes ((a) [²H]-sucrose/[–¹⁴C]aminoisobutyricacid (AIB) or (b) [³H]valine/[₁₄C]asparagine mixture) was administeredto the plant via the petiole subtending the fruiting node, and[²H]solute and [¹⁴C]solute unloading from the seed coat wasmeasured, in pulse-labelling experiments of about 5 h. The presenceof 25 or 50 mM K⁺ in the ‘empty’ ovule enhancedthe release of sucrose from the seed coat particularly duringthe first hours of the experiment, but the stimulating effectof K⁺ on the release of labelled solutes derived from aminoacids was much smaller. The presence of 25 mM CaCl₂ did notaffect the release of sucrose or amino acids from the seed coat.The effect of K⁺ on sucrose and amino acid release is explainedas an inhibition of sucrose and amino acid resorption from theseed coat apoplast into seed coat cells, after unloading fromthe seed coat unloading sites. It is suggested that amino acidrelease is much less affected by K⁺ than sucrose release, becausefar less resorption of amino acids by seed coat parenchyma cellstakes place during amino acid transport into the seed coat cavity. Pisum sativum, pea, assimilate transport, assimilate unloading, seed-coat exudate, seed development, sucrose resorption, surgical treatment     

The Effect of Sucrose Supply on the Energization of Amino Acid Uptake by Cotyledons of Euphorbia lathyris L

The cotyledons of Euphorbia lathyris L. take up sucrose andamino acids from the endosperm. The interaction between theuptake of sucrose and that of amino acids by cotyledons of intactseedlings was investigated. Sucrose (100 mol m^–3) reducedvaline uptake to 75% of the control rate; the active uptakecomponent of valine uptake was reduced from 45 to 25 % of thetotal uptake rate. In a reverse experiment, 100 mol m^–3valine inhibited sucrose uptake by 25%. At 500 mol m^–3sucrose, valine uptake was completely restored to the controlrate, whereas high valine concentrations failed to restore sucroseuptake. The stimulation of valine uptake by sucrose is linkedto the role of sucrose as a primary respiratory substrate. Whenthe cotyledons were bathed in sucrose concentrations rangingfrom 0 to 100 mol m^–3 (these concentrations are non-saturatingwith respect to sucrose uptake), a constant 1.8% of the sucrosetaken up was respired. The K_m of the concentration-dependentsucrose oxidation (44±6 mol m^–3) agreed reasonablywell with that for sucrose uptake (29±6 mol m^–3).When the external sucrose concentration was increased from 100to 600 mol m^–3, the sucrose uptake increased by 30% again,while sucrose oxidation was increased by 300%. This increasewas not due to an increased engagement of the alternative (cyanide-resistant)pathway for respiration. Alternative pathway, Euphorbia lathyris L., fermentation, seedling, sucrose uptake, valine uptake     

Seed Water Relations and the Regulation of the Duration of Seed Growth in Soybean

Soybean [Glycine max (L.) Merrill] seeds and cotyledons weregrown in an in vitro culture system to investigate the relationshipsbetween cell expansion (net water uptake by the seed) and drymatter accumulation. Seeds or cotyledons grown in a completenutrient medium containing 200 mol m^–3 sucrose continueddry matter accumulation for up to 16 d after in planta seedsreached physiological maturity (maximum seed dry weight). Seedor cotyledon water content increased throughout the cultureperiod and the water concentration remained above 600 g kg^–1fresh weight. These data indicate that the cessation of seeddry matter accumulation is controlled by the physiological environmentof the seed and is not a pre-determined seed characteristic.Adding 600 mol m^–3 mannitol to the medium caused a decreasein seed water content and concentration. Seeds in this mediumstopped accumulating dry matter at a water concentration ofapproximately 550 g kg^–1. The data suggest that dry matteraccumulation by soybean seeds can continue only as long as thereis a net uptake of water to drive cell expansion. In the absenceof a net water uptake, continued dry matter accumulation causesdesiccation which triggers maturation. Key words: Glycine max (L.) Merrill, solution culture, duration of seed growth, water content, dry matter accumulation     

Turgor-sensitive transport in developing seeds of legumes: the role of the stage of development and the use of excised vs. attached seed coats

Abstract After removal of the embryo from developing seeds of Vicia faba L. and Pisum sativum L., the ‘empty’ ovules were filled with a substitute medium (pH 5.5) and the effect of the osmolality of this solution on assimilate transport was exandned. In pulse-labelling experiments with a mixture of [³H]sucrose and [¹⁴C]α-andnoisobutyric acid (AIB), a solute concentration of 400 mol m^?3 (100 mol m^3? sucrose + 300 mol m^?3 mannitol) was too low to maintain sugar and andno acid transport into empty ovules of V. faba in a very early stage of development (embryo dry weight < 100 mg) on the same level as transport into intact ovules within the same fruit. A 550-mol m^?3 solution could maintain the normal rate of transport. In experiments with seeds in a more advanced stage of development (embryo dry weight > 250 mg), transport of labelled sucrose and AIB into empty ovules filled with a 400-mol m^?3 solution was practically equal to transport into intact ovules within the same fruit. Experiments without isotopes, on sugar and andno acid release from the seed coat, confirmed the important role of the osmotic environment. A very low osmolality of the solution (e.g. 50 mol m^?3 mannitol) enhanced net efflux of assimilates from excised seed coats and cotyledons, by inhibiting resorption from the apoplast.     

Mobilization of Endosperm Reserves and Uptake of Sucrose and Valine by the Cotyledons of Euphorbia lathyris L.

Upon germination, the endosperm triacylglycerols and proteinswere converted to sucrose and amino acids. During early postgerminativegrowth, the rate of sucrose and amino acid production exceededthe rate of uptake by the cotyledons. As a result, the levelsof total amino acid and sucrose in the endosperm increased;maximum levels were reached at 7 d and 10 d after imbibition(DAI), respectively. Intact seedlings were used to measure thedevelopment of valine, arginine, glutamic acid, and sucroseuptake rate throughout the course of endosperm depletion. Maximumamino acid uptake rates were measured at around 9 DAI, the highestuptake rate for sucrose was obtained at 12 DAI (just beforedepletion of the endosperm). The daily increase of sucrose andamino acid uptake could be manipulated, by replacing the endospermwith a pre-incubation solution during 1 d. The increase in sucroseuptake in vitro was equal to that measured with intact seedlingswhen the cotyledons were pre-incubated in 10 mol m^–3 sucrose.Higher sucrose concentrations reduced the increase of sucroseuptake; at 300 mol m^–3 sucrose (corresponding to the meanendosperm sucrose concentration) sucrose uptake after pre-incubationwas even lower than before. This reduction was largely counteractedwhen the pre-incubation solution was supplemented with minerals.The development of the valine uptake was hardly affected bysucrose, but was inhibited by several amino acids. Key words: Euphorbia lathyris seedling, sucrose uptake, amino acid uptake, reserve mobilization     

Sugar Efflux from Attached Seed Coats of Glycine max (L.) Men.

Ellis, E. C. and Spanswick, R. M. 1987. Sugar efflux from attachedseed coats of Glycine max (L.) Merr.—J. exp. Bot. 38:1470—1483. Sugar efflux (sucrose + glucose) from attached seed coats ofGlycine max (L.) Merr. was measured at high sampling rates toimprove the kinetic characterization of seed coat exudation.This study confirms that sugar efflux in seed coats has at leasttwo components, and demonstrates that the concentration of mannitolosmoticum bathing the seed coat may influence one or both ofthese components. High leaf irradiance increased sugar effluxrelative to a low leaf irradiance at the same mannitol concentration.A high concentration of mannitol (500 mol m³) enhanced sugarefflux relative to a medium concentration (100 mol m³) underboth high and low leaf irradiance. A low mannitol concentration(10 mol m³) stimulated sugar efflux (relative to 100 mol m³)to a greater extent when leaf irradiance was high. Rapid changesin mannitol concentration produced immediate stimulations ofsugar efflux. Effects of osmoticum on sugar efflux are explainedby simultaneous turgor-mediated effects on import of sucroseby the phloem and retrieval of apoplastic sucrose, presumablyby seed coat parenchyma.     

Effect of the Osmotic Environment on Assimilate Transport in Isolated Developing Embryos of Maize (Zea mays)

Embryos obtained from developing kernels of maize (Zea maysL.) were incubated in a bathing medium, to measure the effectof the osmotic environment on the balance between uptake andrelease of assimilates by the embryo. Net efflux of sucroseand amino acids from the embryo decreased with increasing mannitolconcentration in the bathing medium and net uptake of [¹⁴C]valine increased with increasing mannitol concentration. Therole of a high osmolality of the seed apoplast in seed developmentis discussed Zea mays, maize, embryo, seed development, assimilate transport, turgor-sensitive transport     

Effect of a pretreatment of seed coats with a low-osmolality solution on subsequent [14C] sucrose transport into attached empty ovules of pea

This paper discusses the question as to whether or not the seed coat tissues can‘adapt’to a treatment with a solution containing a low osmoticum concentration, representing an environment which is sub-optimal for assimilate transport into attached surgically modified ovules. Before the start of a pulse-labelling procedure, in experiments on [¹⁴C] sucrose transport into fruits of pea (Pisum sativum) with four empty ovules, two empty ovules were filled with a low-osmolality solution (a 200 mol m^?3 mannitol medium or a solution without mannitol) and the other two ovules were filled with a 400 mol m^?3 mannitol medium. Pretreatment with a low-osmolality medium, during a period of 2–3 h, enhanced subsequent transport of [¹⁴C] sucrose into empty ovules filled with a low-osmolality medium, in comparison with [¹⁴C] sucrose transport into empty ovules filled with a 400mol m^?3 mannitol medium during the pretreatment period. This partial recovery of sink strength of attached empty ovules can be explained as the result of a stimulation of solute efflux from seed coat cells at high cell turgor.     

Photoperiodically Induced Changes in Seed Growth Rate of Soybean as Related to Endogenous Concentrations of ABA and Sucrose in Seed Tissues

Short-day photoperiods can increase the partitioning of assimilatesto filling seeds of soybean (Glycine max L. Merr.), resultingin higher seed growth rates. The plant growth substance ABAhas been implicated in the regulation of assimilate transferwithin filling soybean seeds. Thus, we hypothesized that anincreased concentration of endogenous ABA in seeds may enhancesucrose accumulation and seed growth rate of soybeans exposedto short-day photoperiods. Plants of cv. Hood 75 were grownin a greenhouse under an 8-h short-day photoperiod (SD) until11 d after anthesis (DAA) of the first flower, when half ofthe plants were transferred to a night-interruption (NI) treatment(3 h of low-intensity light inserted into the middle of thedark period). Plants remaining in SD throughout seed developmenthad seed growth rates 43% higher than that of plants shiftedto NI (7·6 mg seed^–1 d^–1 vs. 5·3 mgseed^–1 d^–1). On a tissue-water basis, the concentrationof ABA in SD seeds increased rapidly from 7.6 µmol l^–1at 11 DAA to 65·2 µmol l^–1 at 18 DAA, butthen declined to 6·6 µmol l^–1 by 39 DAA.In contrast, the concentration of ABA increased more slowlyin NI seeds, reaching only 47·4 µmol l^–1by 18 DAA, peaking at 57·0 µmol l^–1 on 25DAA, and declining to 10·2 µmol l^–1 by 39DAA. The concentration of sucrose in SD embryos peaked at 73·5mmol l^–1 on 25 DAA and remained relatively constant forthe remainder of the seed-filling period. In NI, the concentrationof sucrose reached only 38·3 mmol 1^–1 by 25 DAA,and peaked at 61·5 µmol l^–1 on 32 DAA. Thusin both SD and NI, sucrose accumulated in embryos only afterthe peak in ABA concentration, suggesting that ABA may havestimulated sucrose movement to the seeds. The earlier accumulationof ABA and sucrose in SD suggests that ABA may have increasedassimilate availability during the critical cell-division period,thus regulating cotyledon cell number and subsequent seed growthrate for the remainder of the seed-filling period. Glycine max L. Merr. cv. Hood 75, soybean, assimilate partitioning, abscisic acid, photoperiod, source-sink     

The Role of Maturation Drying in the Transition from Seed Development to Germination: VII. EFFECTS OF PARTIAL AND COMPLETE DESICCATION ON ABSCISIC ACID LEVELS AND SENSITIVITY IN RICINUS COMMUNIS L. SEEDS

During mid-development (25–40 d after pollination: DAP)of the castor bean seed the amount of abscisic acid (ABA) increasesin both the endosperm and the embryo, declining substantiallythereafter until there is little present in the mature dry (60DAP) seed. Premature desiccation of the seed at 35 DAP alsoleads to a major decline in ABA within the embryo and endosperm.Partial water loss from the seed at 35 DAP which, like naturaland premature desiccation, leads to subsequent germination uponreturn of the seed to full hydration, causes a much smallerdecline in ABA levels. In contrast, ABA declines substantiallyin the non-dried (hydrated) control at 35 DAP, but the seedsdo not germinate. Hence, a clear negative correlation betweenABA content and germinability is not observed. Both drying,whether natural or imposed prematurely, and partial drying decreasethe sensitivity of the isolated embryo to exogenous ABA by about10-fold. The protein synthetic response of the castor bean embryo exposedto 0.1 mol m^–3 ABA following premature desiccation exhibitssome similarity to the response of the non-dried developingembryo—in both cases the synthesis of some developmentalproteins is enhanced by ABA, and germination is suppressed.Germination of mature seeds is also suppressed by 0.1 mol m^–3ABA, but the same developmental proteins are not synthesized.In the cotyledons of prematurely-desiccated seed, some proteinsare hydrolysed upon imbibition in 0.1 mol m^–3 ABA, a phenomenonthat occurs also in the cotyledons of similarly treated matureembryos, but not in developing non-dried embryos. Hence theembryo exhibits an ‘intermediate’ response uponrehydration in 0.1 mol m^–3 ABA following premature desiccation;viz. some of the responses are developmental and some germinative.Following natural or imposed drying, the isolated embryo becomesrelatively insensitive to 0.01 mol m^–3 ABA: germinationis elicited and post-germinative reserve breakdown occurs inthe radicle and cotyledons. The reduced sensitivity of the embryoto ABA as a consequence of desiccation may be an important factorin eliciting the switch to germination and growth within thewhole seed. Key words: Abscisic acid, desiccation, astor bean endosperm, seed development, germination, protein synthesis, isolated embryos, hormone sensitivity     

Mechanism of Photosynthate Efflux from Vicia faba L. Seed Coats

In order to develop a tentative model of the mechanism of photosynthateefflux from the vascular region of Vicia faba L. seed coats,wash-out experiments were performed after removal of the embryo. The sulphydryl group modifiers, pCMBS and NEM, reduced ¹⁴C-photosynthateefflux by 40% and 50%, respectively. Their inhibitory effectcould be prevented or reduced (in the latter case) by includingDTT in the bathing solution. Maltose competed with sucrose forefflux; a concentration of 300 mol m^–3 inhibited ₁₄C-photosynthaterelease by 35%. The cations K⁺ , Na⁺ Mg²⁺ and TPP⁺ enhancedefflux significantly, whereas the countenon Cl^– had noeffect. The presence of the protonophore CCCP (0·1 molm^–3) led to a reduction of efflux by 50% net proton extrusiondropped by 34%. To a lesser extent, an efflux inhibition wasalso achieved by decreasing the cytoplasmic pH with the weakacid DM0. In contrast, alterations in the external pH causedonly a feeble response. The ATPase inhibitor, EB, decreasedphotosynthate efflux and H⁺ extrusion. DES reduced efflux slightly,presumably by affecting ATPase activity as well as energy metabolism. Based on these findings, it is proposed that a sucrose/protonantiport mechanism could be responsible for photosynthate effluxfrom Vicia faba seed coats. Key words: Photosynthate efflux, proton extrusion, proton/sucrose antiport, seed coat, Vicia faba L.     

Response of Wheat Seedlings to Low O2 Concentrations in Nutrient Solution: I. GROWTH, O2 UPTAKE AND SYNTHESIS OF FERMENTATIVE END-PRODUCTS BY ROOT SEGMENTS

Root growth of 7-d-old wheat (Triticum aestivum cv. Gamenya)seedlings was impaired at dissolved O₂ concentrations of 0.01and 0.055 mol m^–3 O₂, while growth at 0.115 mol m^–3O₂ was the same as that in continuously aerated controls (0.26mol m^–3 O2). Oxygen uptake by apical (0–2 mm), expanding (2–4mm) and expanded (10–12 mm) tissues of the roots decreasedbelow 0.16, 0.09 and 0.05 mol m^–3 O₂, respectively. Thishierarchy is consistent with the metabolic rates of these tissues.There was a small (c. 9%) inhibition of O₂ uptake and some netsynthesis of ethanol and alanine in root apices at 0.115 molm^–3 O₂. Significant amounts of anaerobic end-productsaccumulated at 0.055 mol m^–3 O₂ and even more so at 0.01mol m^–3 O₂, indicating that oxidative phosphorylationwas strongly inhibited. Net alanine synthesis increased in fully expanded (10–16mm) tissues exposed to <0.003–0.01 mol m^–3 O₂,and this increase was accompanied either by a proportionallysmaller increase in the concentration of other free amino acidsor by a net decrease in free amino acid levels excluding alanine.This suggests that alanine was synthesized as an end-productof anaerobic catabolism and did not accumulate simply becauseof decreased net protein synthesis. Comparing the carbon flow to CO₂, ethanol, lactate and alaninein roots at 0.01 mol m^–3 O₂ with carbon loss as CO₂ inaerated roots suggests that carbon flow to products of metabolismwas not greatly enhanced due to O₂ deficiency. This infers,but does not prove that, in wheat, generation of energy duringperiods of O₂ deficiency is not enhanced due to a Pasteur effect. Key words: Anaerobic, fermentation, oxygen, wheat     

Melibiose, A Suitable, Non-Permeating Osmoticum for Suspension-Cultured Tobacco Cells

Dracup, M., Gibbs, J. and Greenway, H. 1986. Melibiose, a suitablenon-permeating osmoticum for suspension-cultured tobacco cells.—J.exp. Bot. 37: 1079–1089. A neutral, non-permeating osmoticum with low molecular weightwas required for studies involving responses to water deficitand salinity by suspension-cultured cells of tobacco. For thispurpose, raffinose, sorbitol, mannitol and melibiose were evaluated. Raffinose was hydrolysed by cells which were then able to useone of the products, namely fructose, for growth. Sorbitol andmannitol were not used for growth but were taken up by cells.After 96 h in media containing 50 mol m^–3 of sorbitolor mannitol as the carbon source, cells contained 85 mol m^–3sorbitol or 45 mol m^–3 mannitol. At least part of theuptake of sorbitol would have been active as sorbitol was transportedagainst a concentration gradient. Melibiose was one of the products of hydrolysis of raffinoseand proved to be an effective osmoticum. When supplied as thesole source of sugar for cells, melibiose was neither hydrolysednor taken up by cells. Furthermore, melibiose was not toxicsince adding 50 mol m^–3 to a culture medium containingglucose did not affect growth of cells. Key words: Sorbitol, mannitol, uptake     

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

Influx and Efflux of Phosphorus in Roots of Wheat Plants in Non-Growth-Limiting Concentrations of Phosphorus

The regulation of net phosphorus uptake was studied in wheatplants at ambient non-growth-limiting P-concentrations. Wheat(Triticum aestivum cv. Klein Atalaya) seedlings were grown fromgermination in culture solutions containing 0.05, 0.5 and 5.0mol m–3 phosphate. Only small increments in plant P-concentrationand specific accumulation rate for phosphorus were found whenambient P-concentration was increased 100 times. P-influx, estimatedby ³²P-uptake, was markedly greater with increased externalP-concentration, but only small changes in V_max and no changesin K_m were found. Indirect estimation of P-efflux in a time-courseof ³²P-uptake, and direct P-efflux measurements in ‘washout’ experiments indicated that P-efflux markedly increasedin higher ambient P-concentration. The increase in P-effluxalmost completely neutralized the higher P-influx observed in5.0 mol m^–3 relative to 0.05 mol m^–3 phosphate.It is postulated that in non-limiting P-concentration net P-uptakeis mainly controlled by P-efflux. Key words: Net P uptake, ³²P, kinetic parameters     

Effects of Nitrate Withdrawal and Resupply on the Assimilation of Nitrate in Leaves of Tomato

Nitrate reduction in leaves of tomato occurred at the same ratein plants grown in 8.0 mol m^–3 nitrate as in plants grownin 2.0 mol m^–3 nitrate, but at a much slower rate in plantsgrown in 0.1 mol m^–3 nitrate. However, the plants grownin 8.0 mol m^–3 nitrate had a larger leaf system than theplants grown in 2.0 mol m^–3 nitrate, and so the totalcapacity to assimilate nitrate was greater in the plants grownin the higher concentration. It was shown that plants grownin 8.0 mol m^–3 nitrate were better buffered against nitratewithdrawal than plants grown in 2.0 mol m^–3 nitrate asthe rate of nitrate reduction declined more slowly when plantswere transferred to 0.1 mol m^–3 nitrate from the higherconcentration than from the lower concentration. Furthermore,leaf expansion continued in the plants transferred from thehigher concentration, whereas it ceased abruptly in the plantstransferred from the lower concentration. It was concluded thatboth continuing expansion and continuing nitrate reduction wereaccompanied, and possibly caused by, a release of nitrate fromstorage pools in the lower part of the stem or the roots. Duringwithdrawal of nitrate the leaves were shown to maintain potentialactivity of the enzyme nitrate reductase although there wasno nitrate to be reduced. When nitrate was resupplied it couldbe reduced very quickly and reduction in the leaves was seento increase within 5 h of resupply. By 3 d after resupply furtherenzyme activity had been induced. Key words: Lycopersicon esculentum Mill, nitrate assimilation, nitrate reductase activity, nitrate withdrawal     

Characteristics of Transport of L-Leucine and Glycine in Pea Protoplasts

The uptake of L-leucine and glycine into pea protoplasts wasstudied under various conditions. The uptake of both L-leucineand glycine was pH dependent with the optimal pH being 4.0 and5.0 for L-leucine and glycine, respectively. A kinetic studyof L-leucine uptake showed that uptake is multiphasic; K_m valuesof different phases were 1.1 mol m^–3, 33.3 mol m^–3and 100 mol m³. A similar analysis for glycine at a concentrationrange of 0.1–10 mol m^–3 also showed a multiphasictransport system for it. The uptake of L-leucine at lower concentrations(between0.1–2.0 mol m^–3) was energy dependent, since arsenate,azide, dinitrophenol and iodoacetate inhibited the uptake. However,the uptake of L-leucine was not inhibited by ouabain at anyconcentration of L-leucine employed. The uptake of glycine wasnot inhibited by any of these inhibitors suggesting that glycineuptake was not mediated by an active process. Key words: Pea protoplast, L-Leucine, Glycine transport, Active transport, Mediated transport     

Malate Accumulates in the Vacuole of Chara australis During Uptake of Ammonium From Chloride-free Solution

Internodal cells of Chara australis can accumulate ammoniumto high concentrations (10 to 70 mol m^–3) in their vacuoles.When Cl^– is included in the bathing solution, changesin the cellular concentrations of ammonium, K⁺, Cl^– andNa⁺ have been shown to meet the requirements for electroneutralityand to account for the changes in vacuolar osmotic pressureassociated with ammonium uptake. If accumulation occurs in theabsence of external Cl^–, however; changes in the inorganicions do not meet these criteria. Malate is found in the vacuolesof cells accumulating amine in the absence of external Cl^–and its presence (at 0·5 to 8·5 mol m^–3)allows us to account for electroneutrality and for changes inthe osmotic potential. Key words: Malate, Chara, electroneutrality, ammonium     

Turgor-sensitive sucrose and amino acid transport into developing seeds of Pisum sativum. Effect of a high sucrose or mannitol concentration in experiments with empty ovules

Sugar and amino acid transport into empty ovules of Pisum sativum L. cv. Marzia was examined. In fruits containing 4–6 developing seeds, the embryo was removed from four ovules. After this surgical treatment, each empty seed coat was filled with a solution (pH 5.5) containing a low (0, 50 or 200 m M ), medium (350, 400 or 500 m M ) or high (0.7 or 1 M ) concentration of sucrose and/or mannitol. In pulse-labelling experiments with sucrose and α-aminoisobutyric acid (AIB), transport of sucrose and AIB into an empty ovule filled with a solution containing a high sucrose concentration was the same as transport into an ovule filled with a mannitol solution of similar osmolarity, demonstrating that a high sucrose concentration in the seed coat apoplast affects phloem transport of sucrose and AIB into the seed coat only by the osmotic effect. The osmolarity of a given solution filling the seed coat cavity appeared to be important for phloem transport of sucrose and AIB into empty ovules.
In our experiments, 350 m M appeared to be the optimal concentration for sucrose and AIB transport into the cavity within an empty ovule, giving results comparable with transport into intact ovules. A lower osmolarity of the solution induced less transport. Very high sucrose or mannitol concentrations caused a strong inhibition of sucrose and AIB unloading from the seed coat, so that transport into the empty ovules was inhibited. A low (strongly negative) but not too low osmotic potential of the solution in the seed coat apoplast seems necessary to maintain a normal rate of phloem transport into developing seeds. Apparently, the "sink strength" of developing seeds is turgor-sensitive.     

Rapid, Reversible Inhibition of Nitrate Influx in Barley by Ammonium

The rate of influx of nitrate into the roots of intact barleyplants was measured over a period of 3–5 min from externalnitrate concentrations of 1–150 mmol m^–3, using¹³N-labelled nitrate as tracer. Ammonium at external concentrationsof 0.005–50 mol m^–3 inhibited nitrate influx ina manner which did not conform to a simple kinetic model butincreased approximately as the logarithm of the ammonium concentration.At any particular ammonium concentration, inhibition of nitrateinflux reached its full extent within 3 min of the ammoniumbeing supplied and was not made more severe by up to 17 minpre-treatment with ammonium. On removing the external ammonium,nitrate influx returned to its original rate within about 3min. Potassium at 0.005–50 mol m^–3 did not reproducethe rapid effect of ammonium on nitrate influx. Net uptake of nitrate also decreased when ammonium was supplied,over a similar timescale and to a similar extent as nitrateinflux. The decrease in nitrate influx caused by ammonium wassufficient to account for the observed reduction in net uptake,without necessitating any acceleration of nitrate efflux. Key words: Hordeum vulgare, roots, ion transport, short-lived isotopes, ¹³N