Transport of Photoassimilate in Pea Ovules

Interpretation of tracer washout from an attached empty seedcoat depends on whether photoassimilate within the apoplastof the seed coat is absorbed by the seed coat tissues. Usingsucrose trapping procedures, we were unable to see any evidencefor sucrose uptake from the seed coat apoplast which would beneeded to provide the seed coat with its carbohydrate requirementsif phloem unloading were into the apoplast. Once released intothe apoplast photoassimilate is unavailable to the seed coattissue. Changes between equimolar solutions of sorbitol andsorbitol/sucrose mixes induced small transient responses inseed coat unloading which suggest that sorbitol and sucrosehad different reflection coefficients and gave water relationresponses with rapid, and fatiguable, osmoregulation withinthe seed coat. Immediate inhibition of seed coat unloading with PCMBS is reported,followed by inhibition of import into the entire pod. PCMBSappears to be xylem mobile, thereby quickly being dispersedthroughout the entire experimental pod. A complex CCCP responseis reported, which is consistent with immediate inhibition ofsymplastic transport followed by membrane disruption. AlthoughCCCP inhibited seed coat unloading, there was no effect on ovuleimport. This has been interpreted as evidence that the seedcoat has an active role in control of photoassimilate importinto ovules. Key words: Pisum sativum, phloem unloading, seed coat unloading     

Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

Sugar release from the pedicel tissue of maize (Zea mays L.) kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of ¹⁴C-assimilate movement into treated versus intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 hours and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors, and solutes in order to characterize sugar unloading from this tissue.

Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 millimolar CaCl₂. Unloading was stimulated by 15 millimolar ethyleneglycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid which partially reversed the inhibitory effects of Ca²⁺. Based on these results, we suggest that passive efflux of sucrose occurs from the maize pedicel symplast followed by extracellular hydrolysis to hexoses.

     

Concentrations of sucrose and nitrogenous compounds in the apoplast of developing soybean seed coats and embryos

The apoplast of developing soybean (Glycine max cv Hodgson) embryos and seed coats was analyzed for sucrose, amino acids, ureides, nitrate, and ammonia. The apoplast concentration of amino acids and nitrate peaked during the most rapid stage of seed filling and declined sharply as the seed attained its maximum dry weight. Amino acids and nitrate accounted for 80 to 95% of the total nitrogen, with allantoin and allantoic acid either absent or present in only very small amounts. Aspartate, asparagine, glutamate, glutamine, serine, alanine, and γ-aminobutyric acid were the major amino acids, accounting for over 70% of the total amino acids present. There was a nearly quantitative conversion of glutamine to glutamate between the seed coat and embryo, most likely resulting from the activity of glutamate synthase found to be present in the seed coat tissue. This processing of glutamine suggests a partly symplastic route for solutes moving from the site of phloem unloading in the seed coat to the embryo.     

Sieve element unloading: cellular pathway, mechanism and control

The transport and distribution of phloem – mobile solutes is predominantly determined by transport processes located at the sink end of the source – transport – sink system. Transport across the sieve element boundary, sieve element unloading, is the first of a series of sink transport processes. Unloading of solutes from the sieve elements may follow an apo- or symplastic route. It is speculated that the unloading pathway is integrated with sink function and that apoplastic unloading is restricted to situations in which movement through the symplast is not compatible with sink function. These situations include axial transport and storage of osmotically active solutes against concentration and turgor gradients between the sieve elements and sink cells. Coupled with alteration in sink function, the cellular pathway of unloading can switch in stems and possibly other sinks. Experimental systems and approaches used to elucidate the mechanism of sieve element unloading are reviewed. Unloading fluxes to the apoplast can largely be accounted for by membrane diffusion in axial sinks. However, the higher fluxes in storage sinks suggests dependence on some form of facilitated transport. Proton sucrose symport is assessed to be a possible mechanism for facilitated efflux of solutes across the sieve element plasma membrane to the sink apoplast. Unloading through the symplast may occur by diffusion or mass flow. The latter mechanism serves to dissipate phloem water and hence prevent the potential elevation of sieve element turgor that would otherwise slow phloem import into the sink. The possibility of energised plasmodesmatal transport is raised. Sieve element unloading must be integrated with subsequent compartmentation and metabolism of the unloaded solute. Solute levels are an obvious basis for control of sieve element unloading, but are found to offer limited scope for a mass action mechanism. Apoplastic, cellular pathway, sieve element, solute transport, symplastic. Translated into a turgor signal, solute levels could regulate the rate of unloading, metabolism and compartmentation forming part of a turgor homeostat irrespective of the pathway of unloading.     

Turgor-dependent unloading of assimilates from coats of developing legume seed. Assessment of the significance of the phenomenon in the whole plant

The in vivo significance of turgor-dependent unloading was evaluated by examining assimilate transport to and within intact developing seeds of Phaseolus vulgaris (cv. Redland Pioneer) and Vicia faba (cv. Coles Prolific). The osmotic potentials of the seed apoplast were low. As a result, the osmotic gradients to the seed coat symplast were relatively small (i.e. 0.1 to 0.3 MPa). Sap concentrations of sucrose and potassium in the seed apoplast and coat symplast accounted for some 45 to 60% of the osmotic potentials of these compartments. Estimated turnover times of potassium and sucrose in the seed apoplast of < 1 h were some 5 to 13 times faster than the respective turnover times in the coat symplast pools. The small osmotic gradient between the seed apoplast and coat symplast combined with the relatively rapid turnover of solutes in the apoplast pool, confers the potential for a small change in assimilate uptake by the cotyledons to be rapidly translated into an amplified shift in the cell turgor of the seed coat. Observed adjustments in the osmotic potentials of solutions infused between the coat and cotyledons of intact seed were consistent with the in vivo operation of turgor-dependent unloading of solutes from the coat. Homeostatic regulation of turgor-dependent unloading was indicated by the maintenance of apoplast osmotic potentials of intact seeds when assimilate balance was manipulated by partial defoliation or elevating pod temperature. In contrast, osmotic potentials of the coat symplast adjusted upward to new steady values over a 2 to 4 h period. The resultant downward shift in coat cell turgor could serve to integrate phloem import into the seed coat with the new rates of efflux to the seed apoplast. Circumstantial evidence for this linkage was suggested by the approximate coincidence of the turgor changes with those in stem levels of ³²P used to monitor phloem transport. The results obtained provide qualified support for the in vivo operation of a turgor homeostat mechanism. It is proposed that the homeostat functions to integrate assimilate demand by the cotyledons with efflux from and phloem import into the coats of developing legume seed.     

Metabolism of Phloem-borne Amino Acids in Maternal Tissues2Pisum sativum L.)

The amino acid composition of the EDTA-induced phloem exudatereaching the fruit and the seed, and of the solutes releasedby the seed coat during fruit development were determined inglasshouse-grown pea (Pisum sativum L. cv. Finale) suppliedeither with nitrate-free nutrients (nodulated plants) or withcomplete medium (non-nodulated plants). The EDTA-promoted exudationtechnique was used supposedly to collect phloem sap and theempty seed technique supposedly to collect the solutes secretedby the seed coat to the embryo sac cavity. In young seeds embryosac liquid was sampled directly from the embryo sac. The maincarbohydrate transported and secreted was sucrose. The mainamino acids reaching the fruit were asparagine, glutamine, andhomoserine. Their proportions were steady during a day-nightcycle and throughout fruit development. Amino acid compositionchanges occurred first in the pathway from fruit stalk to seedfunicle, due to the formation of threonine (probably from homoserine)and in the seed coat due to production of glutamine, alanineand valine which, together with threonine were the main secretedamino acids. The temporary nitrogen reserves of the pod walland seed coat were remobilized as asparagine during senescence.Phloem exudate of nodulated plants showed a higher (about twice)proportion of asparagine but lower proportions of homoserineand glutamine than in EDTA-induced phloem exudate of nitrate-fedplants. The two types of nitrogen nutrition also produced somechanges in relative proportions of threonine and homoserinesecreted by the seed coat. Key words: Pisum sativum, phloem, amino acids, pod wall, seed coat     

Seed Coat Unloading in Pisum sativum--Osmotic Effects in Attached versus Excised Empty Ovules

Experiments were undertaken with embryo-less ovules of Pisumsativum to study the influence of apoplastic osmolality on seedcoat import and seed coat unloading.¹¹CO₂ pulse labelling alongwith collimated monitoring of plant tissues were used with attachedovules to measure continuously and simultaneously total podimport, import into a modified ovule and photo-assimilate washoutfrom the seed coat of the ovule into a flow-through bathingsolution.Our results indicated that seed coat import was immediatelyaffected by a change in the applied bathing solution osmolality,with a decrease in osmolality lowering seed coat import andan increase in osmolality increasing import. ¹¹C-photo-assimilatewashout from attached ovules was found to respond in a similarmanner to the apoplastic osmolality. However, the osmotic effecton ¹¹C-washout was a delayed response and it appears that themajority of this observed response was due to the alterationin seed coat tracer import. Further experiments with ¹⁴C-labelled,excised seed coat halves (i.e. no further import) supportedthis hypothesis by demonstrating that seed coat unloading (measuredas ¹⁴C-photo-assimilate washout) was actually enhanced at alow solution osmolality. PCMBS had no effect on seed coat importor washout in attached, modified ovules, suggesting that photo-assimilateunloading from seed coats of Pisum does not involve a carrierprotein. Studies of the spatial distribution of imported ¹⁴Cin Pisum seed coats further suggest that this unloading, intothe apoplast, occurs from non-phloem cell types, and that themovement of photo-assimilates from the sieve elements to theterminal unloading site occurs via symplastic transport. Key words: Pisum sativum, seed coat, seed coat unloading, phloem unloading     

Growth Regulators Have Rapid Effects on Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L

Of nine plant growth regulators (indoleacetic acid, 1-naphthalene acetic acid, gibberellic acid, giberellin 4/7, 6-benzylaminopurine, 6-furfurylaminopurine, abscisic acid, and 1-aminocyclopropane carboxylic acid) tested, only 6-benzylaminopurine and abscisic acid affected ¹⁴C-photosynthate unloading from excised seed coats of Phaseolus vulgaris L. Unloading, in the presence of KCl, was stimulated by 25 to 40%. Stimulation occurred immediately for 6-benzylaminopurine and for abscisic acid within 10 to 12 minutes of application.     

Role of source-to-sink transport of methionine in establishing seed protein quantity and quality in legumes

Grain legumes such as pea (Pisum sativum L.) are highly valued as a staple source of protein for human and animal nutrition. However, their seeds often contain limited amounts of high-quality, sulfur (S) rich proteins, caused by a shortage of the S-amino acids cysteine and methionine. It was hypothesized that legume seed quality is directly linked to the amount of organic S transported from leaves to seeds, and imported into the growing embryo. We expressed a high-affinity yeast (Saccharomyces cerevisiae) methionine/cysteine transporter (Methionine UPtake 1) in both the pea leaf phloem and seed cotyledons and found source-to-sink transport of methionine but not cysteine increased. Changes in methionine phloem loading triggered improvements in S uptake and assimilation and long-distance transport of the S compounds, S-methylmethionine and glutathione. In addition, nitrogen and carbon assimilation and source-to-sink allocation were upregulated, together resulting in increased plant biomass and seed yield. Further, methionine and amino acid delivery to individual seeds and uptake by the cotyledons improved, leading to increased accumulation of storage proteins by up to 23%, due to both higher levels of S-poor and, most importantly, S-rich proteins. Sulfate delivery to the embryo and S assimilation in the cotyledons were also upregulated, further contributing to the improved S-rich storage protein pools and seed quality. Overall, this work demonstrates that methionine transporter function in source and sink tissues presents a bottleneck in S allocation to seeds and that its targeted manipulation is essential for overcoming limitations in the accumulation of high-quality seed storage proteins.

Methionine transporter function in pea phloem and embryo affects sulfur, nitrogen, and carbon acquisition, metabolism, and partitioning, resulting in increased seed yield, protein levels, and quality.     

Temperature and oxygen effects on C-photosynthate unloading and accumulation in developing soybean seeds

The environmental sensitivity of the processes associated with the import of photosynthate by developing soybean seeds was investigated within intact fruit and with excised, immature embryos. Intact pods of field-grown (Glycine max [L.] Merr.) Amsoy 71 soybeans were subjected to localized regimes of 0, 21, or 100% O₂ and 15, 25, or 35°C during pulsechase translocation experiments and, 2.5 hours later, the uptake and distribution of ¹⁴C-photosynthate among dissected fruit tissues determined. In other experiments, excised embryos were incubated in [¹⁴C]sucrose solutions under various experimental conditions to separate the effects of these treatments on accumulation by the embryos from those which may operate on phloem unloading in the maternal seedcoat.     

Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L.--Nature and Cellular Location of Turgor-Sensitive Unloading

Patrick, J. W., Jacobs, E., Offler, C. E. and Cram, W. J. 1986.Photosynthate unloading from seed coats of Phaseolus vulgarisL.—Nature and cellular location of turgor-sensitive unloading—J.exp. Bot. 37: 1006–1019. Unloading rates of ¹⁴C-Photosynthates from excised seed-coathalves of Phaseolus vulgaris L. plants were sharply increasedat cell turgor potentials in excess of 5 ? 10⁵ Pa. Turgor-sensitiveunloading occurred in the absence of any change in the passivepermeability of, and active sucrose influx across, the plasmalemmaand tonoplast membranes. The proton ionophore CCCP, and lowtemperature significantly slowed turgor-sensitive unloadingwhile PCMBS, a non-permeating sulphydryl-modifying compound,was without effect. Turgor-sensitive unloading significantlydepressed the ¹⁴C-Photosynthate content of the ground and branchparenchyma, but had no effect on the ¹⁴C-Photosynthate levelsin the vascular tissues. Cycling of cell turgor potentials aboveand below 5 ? 10⁵ Pa elicited reproducible responses in theunloading rate of ¹⁴C-Photosynthates. Increasing turgor above5 ? 10⁵ Pa resulted in a burst of ¹⁴C-Photosynthate unloading.Reversal to turgors less than 5 ? 10⁵ Pa caused a rapid depressionin unloading rate. It is proposed that turgor-sensitive unloadingis facilitated by a specific turgor-sensitive porter locatedon the plasmalemma of the ground and/or branch parenchyma cellsof bean seed coats. Key words: Bean, seed coat, turgor-sensitive unloading, phloem     

Transporter SlSWEET15 unloads sucrose from phloem and seed coat for fruit and seed development in tomato

Tomato (Solanum lycopersium), an important fruit crop worldwide, requires efficient sugar allocation for fruit development. However, molecular mechanisms for sugar import to fruits remain poorly understood. Expression of sugars will eventually be exported transporters (SWEETs) proteins is closely linked to high fructose/glucose ratios in tomato fruits and may be involved in sugar allocation. Here, we discovered that SlSWEET15 is highly expressed in developing fruits compared to vegetative organs. In situ hybridization and β-glucuronidase fusion analyses revealed SlSWEET15 proteins accumulate in vascular tissues and seed coats, major sites of sucrose unloading in fruits. Localizing SlSWEET15-green fluorescent protein to the plasma membrane supported its putative role in apoplasmic sucrose unloading. The sucrose transport activity of SlSWEET15 was confirmed by complementary growth assays in a yeast (Saccharomyces cerevisiae) mutant. Elimination of SlSWEET15 function by clustered regularly interspaced short palindromic repeats (CRISPRs)/CRISPR-associated protein gene editing significantly decreased average sizes and weights of fruits, with severe defects in seed filling and embryo development. Altogether, our studies suggest a role of SlSWEET15 in mediating sucrose efflux from the releasing phloem cells to the fruit apoplasm and subsequent import into storage parenchyma cells during fruit development. Furthermore, SlSWEET15-mediated sucrose efflux is likely required for sucrose unloading from the seed coat to the developing embryo.

SlSWEET15, a specific sucrose uniporter in tomato, mediates apoplasmic sucrose unloading from phloem cells and seed coat to support fruit expansion and seed filling.     

A time-dependent mathematical expression of the münch hypothesis of phloem transport

A time-dependent mathematical expression of the Münch, osmotically driven mass flow hypothesis of phloem transport is presented. The dependent variables include concentration of solutes, pressure, velocity of phloem sap, osmotic flux of water, and concentration dependent unloading of solutes. The model meets conservation requirements during all iterations, and responds realistically to changes in independent variables. Given the same set of independent variables the time-dependent model converges to the same values as the closed-form steady-state model of Goeschl et al. (1976) regardless of the initial conditions.     

Quantitative Analysis of Photosynthate Unloading in Developing Seeds of Phaseolus vulgaris L. : II. Pathway and Turgor Sensitivity

Phloem import and unloading in perfused bean (Phaseolus vulgaris L.) seed coats were investigated using steady-state labeling. Though photosynthate import and unloading were significantly reduced by perfusion, measurements of photosynthate fluxes in perfused seed coats proved useful for the study of unloading mechanisms in vivo. Phloem import was stimulated by lowered seed coat cell turgor, as demonstrated by an increase in tracer and sucrose import to seed coats perfused with high concentrations of an osmoticum. The partitioning of photosynthates between retention in the seed coat and release to the perfusion solution also was turgor sensitive; increases in seed coat cell turgor stimulated photosynthate release to the apoplast at the expense of photosynthate retention within the seed coat. There was no evidence of a turgor-sensitive sucrose uptake mechanism in perfused seed coats. Thus, the turgor sensitivity of photosynthate partitioning within perfused seed coats was consistent with a turgor-sensitive efflux control mechanism. Measurements of tracer equilibration and sugar partitioning in perfused seed coats provided strong evidence for symplastic phloem unloading in seed coats.     

Tie-dyed1 encodes a novel, phloem-expressed transmembrane protein that functions in carbohydrate partitioning

Carbon is partitioned between export from the leaf and retention within the leaf, and this process is essential for all aspects of plant growth and development. In most plants, sucrose is loaded into the phloem of carbon-exporting leaves (sources), transported through the veins, and unloaded into carbon-importing tissues (sinks). We have taken a genetic approach to identify genes regulating carbon partitioning in maize (Zea mays). We identified a collection of mutants, called the tie-dyed (tdy) loci, that hyperaccumulate carbohydrates in regions of their leaves. To understand the molecular function of Tdy1, we cloned the gene. Tdy1 encodes a novel transmembrane protein present only in grasses, although two protein domains are conserved across angiosperms. We found that Tdy1 is expressed exclusively in phloem cells of both source and sink tissues, suggesting that Tdy1 may play a role in phloem loading and unloading processes. In addition, Tdy1 RNA accumulates in protophloem cells upon differentiation, suggesting that Tdy1 may function as soon as phloem cells become competent to transport assimilates. Monitoring the movement of a fluorescent, soluble dye showed that tdy1 leaves have retarded phloem loading. However, once the dye entered into the phloem, solute transport appeared equal in wild-type and tdy1 mutant plants, suggesting that tdy1 plants are not defective in phloem unloading. Therefore, even though Tdy1 RNA accumulates in source and sink tissues, we propose that TDY1 functions in carbon partitioning by promoting phloem loading. Possible roles for TDY1 are discussed.     

Effect of KCN and p-chloromercuribenzenesulfonic acid on the release of sucrose and 2-amino(1-14C)isobutyric acid by the seed coat of Pisum sativum

The process of sugar and amino acid release by the seed coat of Pisum sativum L. cv. Marzia was studied. Prior to measuring the release of solutes by the seed coat of developing ovules, the embryo was removed from each ovule studied. After this surgical treatment, each "empty" seed coat was filled with the appropriate solution (pH 5.5) with or without inhibitor. Both KCN and p-chloromercuribenzenesulfonic acid (PCMBS) strongly inhibited the release of sucrose and p -aminoisobutyric acid (AIB) by the seed-coat. These data support the view that phloem unloading is an energy-dependent process sensitive to the sulfhydryl group modifier PCMBS. In pulse-labelling experiments, addition of high concentrations of unlabelled sucrose (200 m M ) and AIB (25 m M ) to the solution filling the seed coat cavity did not diminish the release of labelled solutes by the unloading sites of the seed coat. This observation presents evidence against the view that phloem unloading into a strong sink is related to low sugar concentrations in the apoplast.     

Role of nitrogen assimilation in seed development of soybean

A nondestructive acetylene reduction assay for nitrogenase activity of soybean (Glycine max L. Merr) field plots is presented. Plots consisted of 120 × 150 × 30 centimeter boxes containing 65 plants. The plants were grown in a medium grade sand under controlled nutrient, moisture, and root temperature conditions. Acetylene at a concentration of 10 milliliters per liter was circulated through manifolds in the chambers; equilibration required 5 minutes, and activity was linear with time. Optimum growth and assay environments resulted in activity of 70 micromoles ethylene per plant per hour. Plant development and yield were comparable to soil-grown companion plots.

The well accepted hypothesis that developing seeds deprive the nodules of carbohydrate was not substantiated. The nondestructive acetylene reduction profile did not decline until 30 days after the onset of seed development (R-5). This result was consistent with reports from the literature which indicated that 60% of seasonal nitrogen was fixed after R-5. Further, a high correlation shown between integrated seasonal acetylene reduction and yield (r = 0.999) suggested a cooperative relationship between the roots and shoot. A reduction in source:sink ratio (60% defoliation) after R-5 had no effect on acetylene reduction. This showed that neither an increase in sink demand by the pods nor a carbon shortage during podfill decreased dinitrogen fixation. A conceptual model relating seed growth with carbon and nitrogen assimilation is proposed.

     

Solute fluxes from tobacco to the parasitic angiosperm Orobanche cernua and the influence of infection on host carbon and nitrogen relations

Orobanche species are holoparasites which are very efficient sinks for host-derived solutes. Here, we report the use of direct measurements of xylem sap solute concentrations and water fluxes, together with a modelling procedure to calculate element fluxes within an association between Orobanche cernua and its tobacco host. Infection of tobacco by the parasite markedly influenced carbon acquisition and partitioning; net fixation of carbon was 20% higher in infected tobacco compared with controls. Orobanche cernua caused a 84% increase in net carbon flux moving downward from the tobacco shoot and 73% of this carbon was intercepted by the parasite, almost entirely through the phloem (>99%). Further, the parasite also exerted a large impact on the nitrogen relations of the plant, notably nitrate uptake was stimulated and the amino acid content of xylem sap was lower. The parasite also relied heavily on host phloem for the supply of other resources, with only 5 to 15% of N, and 16% of K, 23% of Na, 63% of Mg and 13% of S being derived from the xylem. Thus, we provide quantitative information on the phloem dependency of the parasite and show that host carbon and nitrogen metabolism is stimulated as a consequence of infection.     

Changes in Photosynthate Unloading from Perfused Seed Coats of Phaseolus vulgaris L. Induced by Osmoticum and Ethylenediaminetetraacetate (EDTA)

Photosynthate unloading in Phaseolus vulgaris L. seed coatswas studied by treating perfused seed coats with differing concentrationsof an osmoticum and ethylenediaminetetraacetate (EDTA). Largechanges in osmoticum concentration typically produced rapidchanges in efflux of unlabelled sugar and steady-state-labelled¹⁴C-photosynthate. Osmoticum-induced changes in photosynthateefflux were caused by phloem import stimulation at low cellturgor and net efflux stimulation by high cell turgor. Eventhough rapid changes in sugar and tracer efflux were often inducedby osmoticum treatments, the specific activity of sugar releasedfrom seed coats was not greatly affected by these treatmentsand was similar to the specific activity of sugar remainingin the seed coat after perfusion. Thus, tracer was transportedfrom the phloem throughout the seed coat sugar pool before itwas released to the apoplast. This result is most consistentwith symplastic phloem unloading throughout perfused seed coats,because apoplastic transport between cells within the seed coatwas blocked by perfusion. Photosynthate efflux was stimulatedby simultaneous treatment of seed coats with EDTA and differentconcentrations of an osmoticum; loss of photosynthate from seedcoats did not appear to be tissue-specific. Key words: Phaseolus vulgaris, seed coat, photosynthate unloading, turgor, EDTA     

Role of amides, amino acids, and ureides in the nutrition of developing soybean seeds

The various nitrogenous solutes important to embryo development in symbiotic soybean plants were determined during the midpodfilling stage. Glutamine was the principal form of nitrogen, contributing 55% of the embryo nitrogen requirement. Asparagine was the second most important, contributing 20%. The ureides allantoin and allantoic acid directly contributed only insignificantly to the total nitrogen requirement of the embryo. These conclusions were based upon analyses of tissue extracts, translocation studies of radiolabeled solutes, analysis of in vivo seed coat exudate collected from the freespace of attached, surgically altered seeds, and the in vitro culture of isolated immature soybean embryos.