Seed growth rate and carbohydrate pool sizes of the soybean fruit

The relationships between various carbohydrate pools of the soybean (Glycine max [L.] Merrill) fruit and growth rate of seeds were evaluated. Plants during midpod-fill were subjected to various CO₂ concentrations or light intensities for 7 days to generate different rates of seed growth. Dry matter accumulation rates of seeds and pod wall, along with glucose, sucrose, and starch concentrations in the pod wall, seed coat, and embryo were measured in three-seeded fruits located from nodes six through ten. Seed growth rates ranged from 4 to 37 milligrams·day⁻¹·fruit⁻¹. When seed growth rates were greater than 12 milligrams·day⁻¹·fruit⁻¹, sucrose concentration remained relatively constant in the pod wall (1.5 milligrams·100 milligrams dry weight⁻¹), seed coat (8.5 milligrams·100 milligrams dry weight⁻¹), and embryo (5.0 milligrams·100 milligrams dry weight⁻¹). However, sucrose concentrations decreased in all three parts of the fruit as growth rate of the seeds fell below 12 milligrams·day⁻¹·fruit⁻¹. This relationship suggests that at high seed growth rates, flux of sucrose through the sucrose pools of the fruit was more important than pool size for growth. Starch concentration in the pod wall remained relatively constant (2 milligrams·100 milligrams dry weight⁻¹) at higher rates of seed growth but decreased as seed growth rates fell below 12 milligrams·day⁻¹·fruit⁻¹. This suggests that pod wall starch may buffer seed growth under conditions of limiting assimilate availability. There was no indication that carbohydrate pools of the fruit were a limitation to transport or growth processes of the soybean fruit.     

Relationship of endogenous abscisic Acid to sucrose level and seed growth rate of soybeans

It has been proposed that abscisic acid (ABA) may stimulate sucrose transport into filling seeds of legumes, potentially regulating seed growth rate. The objective of this study was to determine whether the rate of dry matter accumulation in seeds of soybeans (Glycine max L.) is correlated with the endogenous levels of ABA and sucrose in those sinks. The levels of ABA and sucrose in seed tissues were compared in nine diverse Plant Introduction lines having seed growth rates ranging from 2.5 to 10.0 milligrams dry weight per seed per day. At 14 days after anthesis (DAA), seeds of all genotypes contained less than 2 micrograms of ABA per gram fresh weight. Levels of ABA increased rapidly, however, reaching maxima at 20 to 30 DAA, depending upon tissue type and genotype. ABA accumulated first in seed coats and then in embryos, and ABA maxima were higher in seed coats (8 to 20 micrograms per gram fresh weight) than in embryos (4 to 9 micrograms per gram fresh weight. From 30 to 50 DAA, ABA levels in both tissues decreased to less than 2 micrograms per gram fresh weight. Levels of sucrose were also low early in development, less than 10 milligrams per gram fresh weight at 14 DAA. However, by 30 DAA, sucrose levels in seed coats had increased to 20 milligrams per gram fresh weight and remained fairly constant for the remainder of the filling period. In contrast, sucrose accumulated in embryos throughout the filling period, reaching levels greater than 40 milligrams per gram fresh weight by 50 DAA. Correlation analyses indicated that the level of ABA in seed coats and embryos was not directly correlated to the level of sucrose measured in those tissues or to the rate of seed dry matter accumulation during the linear filling period. Rather, the ubiquitous pattern of ABA accumulation early in development appeared to coincide with water uptake and the rapid expansion of cotyledons occurring at that time. Whole tissue sucrose levels in embryos and seed coats, as well as sucrose levels in the embryo apoplast, were generally not correlated with the rate of dry matter accumulation. Thus, it appears that, in this set of diverse soybean genotypes, seed growth rate was not limited by endogenous concentrations of ABA or sucrose in reproductive tissues.     

Differential response of carbon and nitrogen metabolism to fluoride application in fruiting structures of chickpea (Cicer arietinum)

The effect of sodium fluoride (10 and 50 mol·m⁻³) on the activities of sucrose metabolizing enzymes, transaminases and glutamine synthetase in relation to the transformation of free sugars to starch and protein in the fruiting structures (pod wall, seed coat, cotyledons) of chickpea was studied by culturing detached reproductive shoots in a liquid medium. Addition of fluoride to the culture medium drastically reduced starch content of the cotyledons and caused a marked build-up of total free sugars comprised mainly of reducing sugars in the pod wall and seed coat, and sucrose in the cotyledons. Concomitantly, the activity of soluble invertase was stimulated in the pod wall but reduced in the cotyledons. However, soluble protein content of both the pod wall and the cotyledons increased in conjunction with an increase in the activities of glutamate-oxaloacetate transaminase, glutamate-pyruvate transaminase and glutamine synthetase. Disruption of starch biosynthesis under the influence of fluoride and the resulting accumulation of free sugars possibly resulted in their favoured utilization in nitrogen metabolism. Labelling studies with [U-¹⁴C]-sucrose showed that the ¹⁴C incorporation into total free sugars was enhanced by fluoride in the pod wall but reduced in the seed coat and cotyledons, possibly due to an inhibitory effect on their translocation to the developing seeds.     

An Ontogenetic Study of Canavanine Formation in the Fruit of Jack Bean, Canavalia ensiformis (L.) DC

An ontogenetic study of canavanine formation in the fruit of jack bean, Canavalia ensiformis (L.) DC. was conducted. Evidence was presented to show that the ovary wall is the reservoir for seed canavanine. The testa possesses sufficient canavanine to account for the continued elevation in seed canavanine after the pod senesces. The seed canavanine concentration is not constant inasmuch as the canavanine content per milligram dry weight or soluble protein increases abruptly with seed growth and levels off only with the onset of fruit ripening.     

An in vivo technique for the study of Phloem unloading in seed coats of developing soybean seeds

A technique has been developed which permits mechanistic studies of phloem unloading in developing seeds of soybean (Glycine max cv Clark) and other legumes. An opening is cut in the pod wall and the embryo surgically removed from the seedcoat without diminishing the capacity of that tissue for assimilate import, phloem unloading, or efflux. The sites of phloem unloading were accessible via the seedcoat apoplast and were challenged with inhibitors, solutes, buffers, etc., to characterize the unloading process.

Unloading is stimulated by divalent metal chelators and diethylstilbestrol, and inhibited by metabolic uncouplers and sulfhydryl group modifiers. Solutes released from the seed coat had a carbon/nitrogen ratio of 31 milligrams carbon per milligram nitrogen; sucrose represented 90% of the carbon present and various nitrogenous solutes contributed the remaining 10%. Unloading could be maintained for up 8 hours at rates of 0.5 to 1.0 micromoles per hour, providing a valid, convenient in vivo technique for studies of phloem unloading and seed growth mechanisms.

     

Effect of Water Deficits on Seed Development in Soybean : II. Conservation of Seed Growth Rate

Water deficits during seed filling decrease seed size in soybean (Glycine max L.). This may result from a reduction in the supply of assimilates from the maternal plant and/or an inhibition of seed metabolism. To determine whether maternal or zygotic factors limited seed growth, we examined the effects of a plant water deficit on the supply of sucrose to and its utilization by developing embryos. Plants were grown in the greenhouse, and water deficits were imposed by withholding water for a period of 6 days during linear seed fill. When water was withheld, leaf water potential decreased rapidly, inhibiting canopy photosynthesis completely within 3 days. However, seed dry weight (nodes 7-11) continued to increase at or near the control rate. The level of total extractable carbohydrates in leaf, stem, and pericarp tissue decreased by 70, 50, and 45%, respectively, indicating that reserves were mobilized to support seed growth. Cotyledon sucrose content decreased from about 60 milligrams per gram dry weight to 30 milligrams per gram dry weight. Similarly, the concentration of sucrose in the interfacial apoplast of the cotyledons decreased from approximately 100 millimolar to 50 millimolar. However, the rate of sucrose accumulation by excised embryos, measured in a short-term in vitro assay, increased in response to the water deficit. These results indicate that both source and sink activity in soybean are altered by water deficits to maintain the flux of assimilates to the developing embryos. This may explain why seed growth is maintained, albeit for a shorter duration, when soybean is exposed to water deficits during the seed filling period.     

Rate of Increase in Size and Dry Weight of Individual Pods of Field Grown Soya Bean Plants

The dry weight of the whole fruit, the pod wall and an enclosedseed of randomly harvested soya beans is estimated from theexternal dimensions of the attached pod. The relations betweendimensions and dry weight are independent of cultivar and growthcondition and can be used on pods from 1 cm in length untilthe seeds reach their maximum fresh weight. Dimensions of tagged pods of three cultivars of field grownsoya beans differing in time to reach maturity were measuredevery 2–3 days from initial pod elongation until maturation.Dry weights for each pod were estimated from the dimensions,and the dry weight accumulation with time was fitted to thelogistic function to find the growth rate that best characterizedthe data for each pod. The final weight, the specific growthrate and the maximum growth rate of the whole fruit, the podwall and a single seed were subjected to analysis of variance. The most significant difference between pods of these cultivarswas the specific growth rate of individual seeds, which decreasedwith increasing maturity group. There were no differences ingrowth of the pod wall. However, most of the variation was betweenindividual pods within a cultivar, where the rate of dry weightaccumulation of the whole fruit, governed largely by the seedgrowth rate times the number of seeds, was highly correlatedwith the earlier growth of the pod wall. This suggests thatthe growth of individual whole fruit was determined early inpod development and was slightly influenced by factors appliedduring the period of rapid seed growth. Glycine max (L.) Merrill, Soya bean, seed growth analysis, specific growth rate     

Carbon Partitioning among Leaves, Fruits, and Seeds during Development of Phaseolus vulgaris L

Development of vegetative and floral buds was found to be a key factor in establishing the way carbon is distributed among growing leaves and fruits in Phaseolus vulgaris L. plants. Leaves emerged principally during a period 14 to 32 days after planting while flowers were produced during a 10- to 12-day period near the end of leaf emergence. Timing of anthesis established the sigmoidal time course for dry weight accumulated by the composite of all fruits on the plant. During the first 12 days following anthesis, fruit growth mainly consisted of elongation and dry weight accumulation by the pod wall. Thereafter, seed dry weight increased for about 1 week, decreased markedly for several days, and then increased again over the next 2 weeks. Accumulation of imported carbon in individual seeds, measured by steady-state labeling, confirmed the time course for dry weight accumulation observed during seed development. Seed respiration rate initially increased rapidly along with dry weight and then remained nearly steady until seed maturation. A number of developmental events described in the literature coincided with the different phases of diauxic growth. The results demonstrated the feasibility of relating current rates of carbon import in individual seeds measured with tracer ¹⁴C to the rates of conversion of imported sucrose and use of the products for specific developmental processes. The resulting data are useful for evaluating the roles of conversion and utilization of imported sucrose in regulating import by developing seeds.     

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     

Growth, sucrose synthase, and invertase activities of developing Phaseolus vulgaris L. fruits

Activities of the sucrose-cleaving enzymes, acid and neutral invertase and sucrose synthase, were measured in pods and seeds of developing snap bean (Phaseolus vulgaris L.) fruits, and compared with ¹⁴C-import, elongation and dry weight accumulation. During the first 10 d post-anthesis, pods elongated rapidly with pod dry weight increase lagging behind by several days. The temporal patterns of acid invertase activity and import coincided closely during the first part of pod development, consonant with a central role for this enzyme in converting imported sucrose during pod elongation and early dry weight accumulation. Later, sucrose synthase became the predominant enzyme of dry weight accumulation and was possibly associated with the development of phloem in pod walls. Sucrose synthase activity in seeds showed two peaks, corresponding to two phases of rapid import and dry weight accumulation; hence, sucrose synthase was associated with seed sink growth. Acid invertase activities in seeds were low and did not show a noticeable relationship with import or growth. All neutral invertase activities, during pod and seed development, were too low for it to have a dominant role in sucrose cleavage. Changes in activities of certain sucrose-cleaving enzymes appear to be correlated with certain sink functions, including import, storage of reserves, and biosynthetic activities. The data supports the association of specific sucrose-cleaving enzymes with the specific processes that occur in the developing pods and seeds of snap bean fruits; for example, acid invertase with pod elongation and sucrose synthase with fruit dry matter accumulation.     

Endosperm protein of wheat seed as a determinant of seedling growth

Seed of a Mexican semidwarf wheat (Triticum aestivum L. cv. Inia 66), was obtained from a nitrogen fertilizer field trial grown in Mexico. A high positive correlation was obtained between seed protein content and seedling dry weight after 3 weeks growth (r = +0.92^**). The seedling dry weight was positively related to the protein content of the aleurone layer and endosperm, but not to the embryo. Small, 35 milligrams, high protein seeds (4.7 milligrams protein per seed) produced larger seedlings than large, 45 milligrams, low protein seeds (4.3 milligram protein per seed). There was no difference in the weight or protein content of embryos from low and high protein seeds and their growth was similar. Composite seeds of the two protein levels were produced by transferring embryos from one endosperm type to the other. After 4 weeks, there was no difference between the different embryo types grown on the same endosperm type. High protein endosperm produced more vigorous seedlings regardless of the embryo type grown on it, indicating that the factor(s) responsible for the greater growth of high protein seed is in the endosperm.     

Nitrogen Nutrition of Nodules in Relation to ;N-Hunger' in Cowpea (Vigna unguiculata L. Walp)

Early growth, nodule development, and nitrogen fixation by two cultivars of cowpea (Vigna unguiculata L. Walp), one large-seeded (Vita 3; 146.0 ± 0.9 milligrams seed dry weight, 4.1 ± 0.2 milligrams seed N), the other small-seeded (Caloona; 57.5 ± 2.5 milligrams seed dry weight, 1.8 ± 0.1 milligrams seed N), were compared under conditions of sand culture with nutrient solution free of combined N. The seed stocks used had been obtained from plants uniformly labeled with ¹⁵N, thus enabling changes with time in distribution of cotyledon and fixed N among plant parts to be measured by isotope dilution. Caloona, but not Vita 3, showed physiological symptoms of `N hunger,' i.e. transient loss of chlorophyll (visible yellowing) and N from the first-formed unifoliolate leaves at or around the onset of symbiotic functioning and N<sub>2</sub> fixation. The smaller-seeded Caloona showed higher early nitrogenase activity than the larger-seeded Vita 3 and by 28 days had fixed 6.6 milligrams of N per milligram of seed N [mg N · (mg seed N)<sup>−1</sup>] versus only 3.5 mg N · (mg seed N)<sup>−1</sup> in Vita 3. Both cultivars lost around 30% of their initial seed N at germination, mostly as fallen cotyledons. Abscised cotyledons of Caloona contained 1.21 ± 0.17% N; those of Vita 3 contained 2.61 ± 0.37% N. When compared on the basis of cotyledon N available for seedling growth, Caloona was shown to have fixed 10.6 mg N · (mg seed N)<sup>−1</sup> and Vita 3 only 5.3 mg N · (mg seed N)<sup>−1</sup>. Most of the cotyledon N withdrawn from the unifoliolate leaf pair of Caloona during `N-hunger' was committed to early nodule growth and, in total, 20 to 25% of the cotyledon N resource of this cultivar was ultimately invested in establishment of symbiosis compared with only 7% in Vita 3.     

Carbohydrate Changes during Maturation of Cucumber Fruit : Implications for Sugar Metabolism and Transport

Changes in the carbohydrate profiles in the mesocarp, endocarp, and seeds of maturing cucumber (Cucumis sativus, L.) fruit were analyzed. Fruit maturity was measured by a decrease in endocarp pH, which was found to correlate with a loss in peel chlorophyll and an increase in citric acid content. Concentrations of glucose and fructose (8.6-10.3 milligrams per gram fresh weight, respectively) were found to be higher than the concentration of sucrose (0.3 milligrams per gram fresh weight) in both mesocarp and endocarp tissue. Neither raffinose nor stachyose were found in these tissues. The levels of glucose and fructose in seeds decreased during development, but sucrose, raffinose, and stachyose accumulated during the late stages of maturation. Both raffinose and stachyose were found in the seeds of six lines of Cucumis sativus L. This accumulation of raffinose saccharides coincided with an increase in galactinol synthase activity in the seeds. Funiculi from maturing fruit were found to be high in sucrose concentration (4.8 milligrams per gram fresh weight) but devoid of both raffinose and stachyose. The results indicated that sucrose is the transport sugar from the peduncle to seed, and that raffinose saccharide accumulation in the seed is the result of in situ biosynthesis and not from direct vascular transport of these oligosaccharides into the seeds.     

Pattern of dry matter accumulation in developing fruit parts of early- and late-maturing pigeon peas (Cajanus cajan (L.)Millsp.)

Pigeon pea cultivars AL 15 (early-) and T₂₁ (late-maturing) were compared for dry matter accumulation in fruit parts pod wall (PW), seed coats (SC) and seed at various seed developmental stages. Significant water loss and dry matter accumulation in the fruit parts commenced much earlier in cv. AL 15 as compared with cv. T₂₁. The pod wall accumulated starch, reducing sugars and N-substances up to 21 and 28 days after anthesis (DAA) in cultivars AL 15 and T₂₁, respectively, which was later distributed to the seed. Growth of pod wall and seed was sequential, not concurrent, as the pod wall lost significant dry matter when the seeds within them reached their maximum dry matter. The fruit parts of cv. AL 15 accumulated more photosynthates than cv. T₂₁ at all comparable stages.     

Sucrose Concentration at the Apoplastic Interface between Seed Coat and Cotyledons of Developing Soybean Seeds

The apoplastic sucrose concentration at the interface between cotyledons and surrounding seed coats of developing soybeans (Glycine max L. Merr. cv Wye) was found by three indirect methods to be in the range of 150 to 200 millimolar. This is an order of magnitude higher than has been reported elsewhere for soybean. It was also higher than the overall sucrose concentrations in the cotyledons and seed coats, each of which was approximately 90 millimolar. By defoliating plants 24 hours before measurement, both the overall sucrose concentration in the cotyledons and the interfacial apoplastic sucrose concentration were reduced by three-fourths. However, there was no day/night difference in overall tissue sucrose concentration of cotyledons or seed coats from intact plants suggesting the existence of a homeostatic mechanism compensating for the diurnal photosynthetic cycle. About 7 hours were required for a tritiated polyethylene glycol-900 solution to fully permeate developing cotyledons (from ~220 milligram fresh weight embryos), implying high diffusion resistance through the tissue.

These results indicate that a high interfacial sucrose concentration may exist in vivo. They suggest that the saturable carrier-mediated component of sucrose uptake may be of little physiological significance in the outermost cell layers of the cotyledons.

     

Internal recycling of respiratory CO2 in pods of chickpea (Cicer arietinum L.): the role of pod wall, seed coat, and embryo

It has previously been proposed that respiratory CO2 released from the embryo in grain legume pods is refixed by a layer of cells on the inner pod wall. In chickpea this refixation process is thought to be of significance to the seed carbon budget, particularly under drought. In this study it is reported that the excised embryo, seed coat, and pod wall in chickpea are all photosynthetically competent, but the pod wall alone is capable of net O2 evolution over and above respiration. The predominant role of the pod wall in refixation is supported by measurements of fixation of isotopically labelled CO2, which show that more than 80% of CO2 is fixed by this tissue when provided to the pod interior. Chlorophyll concentrations are of the same order for embryo, seed coat, and pod wall tissues in younger pods on both an area and a fresh weight basis, but decline differentially with development from 12-30 d after podding. Imaging of chlorophyll distribution in the pod wall suggests that less than 15% of chloroplasts are located in the inner layer of cells thought to refix CO2 in legumes; this would be sufficient to refix less than 40% of respired CO2. It is concluded that while all tissues of the pod are capable of refixing respiratory carbon, the entire pod wall is responsible for the majority of this process, rather than a specialized layer of cells on the inner epidermis. The role of this fixed carbon in the pod for reallocation to the seed is discussed     

Import of Sucrose and its Partitioning in the Synthesis of Galactomannan and Raffinose-oligosaccharides in the Developing Guar (Cyamopsis tetragonolobus) Seed

Import of sucrose and its transformation to galactomannan andraffinose-oligosaccharides have been studied in the developingguar seed. The amount of galactomannan gradually increased withthe ageing of the seed. During the entire period of pod development,sucrose constituted the major portion of the free sugars inthe seed (both endosperm and cotyledons) as well as in the podwall. Besides myo-inositol, the free sugars detected in thedeveloping endosperm and cotyledons were glucose, fructose,raffinose and stachyose. Some compounds, possibly glycosides(RG values higher than that of fructose), were also detectedin the endosperm. In the later stages of seed development, therelative proportion of raffinose in the free sugars increased,reaching 50% of the total free sugars in 77-d-old cotyledons.With pod maturity, the activities of soluble acid and boundacid invertases in the pod wall increased manifold with a concomitantdecline in the non-reducing sugar content. These enzymes seemto be involved in the mobilization of sucrose from this fruitingstructure into the seed. An increased synthesis of raffinose-oligosaccharidesboth in the endosperm and cotyledons was associated with highactivities of soluble acid invertase (pH 4.8) and sucrose-UDPglucosyl transferase in these tissues. Feeding uniformly labelled¹⁴C-sugars to the detached intact pods as well as to the isolatedendosperm and cotyledons resulted in labelling of all endogenousfree sugars and galactomannan. The uptake and incorporationinto galactomannan of ¹⁴C was stimulated by Co²⁺, Mn²⁺ and Mg²⁺.Except for mannose, a major proportion of the ¹⁴C from glucose,fructose and sucrose appeared in sucrose in both endosperm andcotyledons indicating a fast reconstitution of sucrose in situ.Based on the present results, a possible mode of transformationof sucrose to galactomannan and raffinose-oligosaccharides hasbeen proposed. Key words: Sucrose, galactomannan, raffinose-oligosaccharides, invertase, sucrose-UDP glucosyl transferase, ¹⁴C-incorporation, guar seed     

Seed-specific overexpression of a potato sucrose transporter increases sucrose uptake and growth rates of developing pea cotyledons

During the storage phase, cotyledons of developing pea seeds are nourished by nutrients released to the seed apoplasm by their maternal seed coats. Sucrose is transported into pea cotyledons by sucrose/H+ symport mediated by PsSUT1 and possibly other sucrose symporters. PsSUT1 is principally localised to plasma membranes of cotyledon epidermal and subepidermal transfer cells abutting the seed coat. We tested the hypothesis that endogenous sucrose/H+ symporter(s) regulate sucrose import into developing pea cotyledons. This was done by supplementing their transport activity with a potato sucrose symporter (StSUT1), selectively expressed in cotyledon storage parenchyma cells under control of a vicilin promoter. In segregating transgenic lines, enhanced [(14)C]sucrose influx into cotyledons above wild-type levels was found to be dependent on StSUT1 expression. The transgene significantly increased (approximately 2-fold) transport activity of cotyledon storage parenchyma tissues where it was selectively expressed. In contrast, sucrose influx into whole cotyledons through the endogenous epidermal transfer cell pathway was increased by only 23% in cotyledons expressing the transgene. A similar response was found for rates of biomass gain by intact cotyledons and by excised cotyledons cultured on a sucrose medium. These observations demonstrate that transport activities of sucrose symporters influence cotyledon growth rates. The attenuated effect of StSUT1 overexpression on sucrose and dry matter fluxes by whole cotyledons is consistent with a large proportion of sucrose being taken up at the cotyledonary surface. This indicates that the cellular location of sucrose transporter activity plays a key role in determining rates of sucrose import into cotyledons.     

Abscisic Acid and its relationship to seed filling in soybeans

The effect of exogenous abscisic acid (ABA) on the rate of sucrose uptake by soybean (Glycine max L. Merr.) embryos was evaluated in an in vitro system. In addition, the concentrations of endogenous ABA in seeds of three soybean Plant Introduction (PI) lines, differing in seed size, were commpared to their seed growth rates. ABA (10⁻⁷ molar) stimulated in vitro sucrose uptake in soybean (cv `Clay') embryos removed from plants grown in a controlled environment chamber, but not in embryos removed from field-grown plants of the three PI lines. However, the concentration of ABA in seeds of the three field-grown PI lines correlated well with their in situ seed growth rates and in vitro [¹⁴C] sucrose uptake rates.

Across genotypes, the concentration of ABA in seeds peaked at 8.5 micrograms per gram fresh weight, corresponding to the time of most rapid seed growth rate, and declined to 1.2 micrograms per gram at physiological maturity. Seeds of the large-seeded genotype maintained an ABA concentration at least 50% greater than that of the small-seeded genotype throughout the latter half of seed filling. A higher concentration of ABA was found in seed coats and cotyledons than in embryonic axes. Seed coats of the large-seeded genotype always had a higher concentration of ABA than seed coats of the small-seeded line. It is suggested that this higher concentration of ABA in seed coats of the large-seeded genotype stimulates sucrose unloading into the seed coat apoplast and that ABA in cotyledons may enhance sucrose uptake by the cotyledons.

     

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.