Kinetics of C-14 Translocation in Soybean

The pathway and kinetics of photosynthate unloading in developing seeds of bean (Phaseolus vulgaris L.) were investigated using steady-state labeling with '4C02. The continuous assimilation of 14CO2 at constant specific activity produced stable tracer fluxes that facilitated straighiforward analyses of photosynthate import and unloading in developing seeds. The kinetics of tracer equilibration within intact seeds were compatible with a symplastic route of photosynthate unloading in the seed coat. The import and partitioning of tracer within seeds were partially disrupted by the surgical excision of the distal halves of seeds as practiced during the preparation of "empty" seed coats for perfusion.The mechanisms that regulate photosynthate allocation within developing legume seeds have been the focus of many recent studies (for reviews see refs. 16, 25, 30). Photosynthates accumulated by the embryo must first be unloaded to the seed coat apoplast because the seed coat and embryo of legume seeds are not symplastically linked (25). The pathway of unloading to the seed coat apoplast has not been conclusively determined. Theoretically, photosynthates could unload directly from the phloem to the apoplast (apoplastic pathway) (29, 30), or they could move symplastically from the phloem throughout the tissues of the seed coat before unloading to the apoplast (symplastic pathway) (13,16,17,21). The symplastic unloading pathway has received the most support.The study of photosynthate unloading from legume seed coats has been advanced by the development of the "empty seed coat technique," in which the embryo of a developing seed is removed through an incision in the seed coat and a perfusion solution (or agar trap) is used to collect materials released from the seed coat for subsequent analysis (20,26,31). The use of the empty seed coat technique is complicated by the possibility that the surgical procedure, or the introduction of a solution into the seed coat "cup," may disturb phloem import into the seed coat and/or the process of unloading to the seed coat apoplast (15). Early studies using 'Supported by grant 87-CRCR-1-2397 from the U.S. Department of Agriculture (R.M.S.), grant DCB-8803837 from the National Science Foundation (R.T.), and funds provided by the Hatch Program.pulse-labeling of leaves (or petioles) indicated that the total amounts of labeled photosynthates imported to perfused and intact seeds were similar (20,26,31). However, quantitative comparisons of pulse-label accumulation by perfused versus intact seeds are unreliable when made over long time periods because the tracer fluxes produced by this technique are not sustained over time.Many problems inherent in pulse-labeling are circumvented by the use of steady-state labeling techniques. Steady-state labeling has been used for the quantitative analysis of various aspects of carbon partitioning in plants because the stable tracer fluxes produced by the continuous assimilation of "'CO2 at constant specific activity result in relatively simple trace...

Section: And Discussion Leaf Carbon Fixation Accumulation and Exportsupporting

confidence: 66%

Quantitative Analysis of Photosynthate Unloading in Developing Seeds of Phaseolus vulgaris L.

Ellis

Turgeon²,

Spanswick³

1992

“…The assumptions made in deriving this equation are emphasized because the model is only as valid as the assumptions, and because much of the work presented in the preceding papers (8,9) Model for a Rectangular Leaf. The model for a rectangular leaf, shown in Figure 3, consists essentially of two linear leaf models at right angles to a central channel (analogous to a midrib).…”

Section: Effects Of Leaf Size Leaf Shape and Translocation Velocitymentioning

confidence: 99%

“…The following mathematical considerations of translocation are an attempt to provide quantitative treatment of some of these factors, using data from the previous two papers (8,9) as a guide. Calculations are also made for the expected pressure drop for a simple mass flow translocation mechanism in soybean.…”

mentioning

confidence: 99%

Kinetics of C-14 Translocation in Soybean

Fisher

1970

Self Cite

Based largely on data from soybean, some mathematical models are derived to describe the transport kinetics of 14C-photosynthate. The effects of leaf size, leaf shape, and translocation velocity on the rate of tracer efflux from the leaf are considered, and it is shown that the duration of these effects will approximate the time required for tracer to reach the petiole from the farthest point of the leaf. This duration is designated as the "kinetic size" of the leaf. Although its effect will be slight in the case of soybean (about 2 to 3 minutes), a considerable effect of the kinetic size will be found in the case of large leaves, or when the translocation velocity is low.Source pool kinetics in soybean are described by a twocompartment model, one compartment representing a photosynthetic compartment and the second (the source pool) a nonphotosynthetic compartment next to the veins.The kinetics in the petiole are approximated by a twocompartment model representing the translocation stream and tissues outside the translocation stream. A combination of the models predicts fairly accurately the translocation kinetics observed in soybean.The models are compared with others in the literature. Although the assumptions are in substantial agreement with those made by Evans, Ebert, and Moorby, they are inconsistent with the model based on the movement of transcellular strands presented by Canny and Phillips.The use of radioactive tracers in translocation studies has encouraged the development of several mathematical models of translocation (5,6,12,17). Since, at least in some cases, the proposed theories for phloem transport would be expected to result in different kinetic observations, these models have often been concerned largely with mechanisms in the stem which would account for the data obtained there. With the exception of Geiger and Swanson's work (10), experimental data for tracer kinetics in the leaf were not utilized in the models, which relied to varying degrees on hypothetical rates of tracer efflux from the leaf (or, in the case of Spanner and Prebble's experiments [17], from a "3'Cs source applied directly to the stem) to solve equations for the

“…This significant difference in the N and B fractions of pulse and steady-state labeled plants probably reflects the relative flow of "C through the fractions (6,7,27). In steady-state experiments, the turnover of carbon during the 2-h labeling period was considered rapid enough so that recovered radioactivity represents the particular fraction's photosynthate pool size, whereas the 30-min pulse-labeling experiments probably did not permit equilibration of "C within the pools.…”

mentioning

confidence: 99%

Partitioning of ¹⁴C-Photosynthate, and Long Distance Translocation of Amino Acids in Preflowering and Flowering, Nodulated and Nonnodulated Soybeans

Housley¹,

Schrader²,

Miller³

et al. 1979

The influence of stage of development (preflowering versus flowering) in nodulated and nonnodulated soybeans (Glycine max IL.I Merr. cv. Wells) on partitioning of "C into assimilates following exposure of a soybean leaf to "CO2 by both steady-state and pulse-labeling techniques was studied.Blades on the second fully expanded leaf from the stem apex were exposed to "CO2. Radioactive assimilates were extracted from source leaf blades, petioles, and stems (both the path up and path down from source leaf), were separated into neutral (sugars), basic (amino acids), and acidic (organic acids, sugar phosphates) fractions by ion exchange chromatography. The basic fraction was further resolved using thin layer chromatography and the percentage of radioactivity recovered in each amino acid was determined.The distribution of radioactivity in the neutral, basic, and acidic fractions of the source leaf blades was significantly different from that of the transport path (petiole and stems). About 70% of the radioactivity in source leaf blades was recovered in the neutral fraction, whereas about 90% of the recovered radioactivity in the path was in the neutral fraction. "4C-Aminoacids constituted 8 to 17% and 2 to 7% of the recovered radioactivity in source leaves and paths, respectively. Recovered 14C in organic acids ranged from 13 to 20% and 2 to 7% in source leaves and paths, respectively.Partitioning of "4C-assimilates among the neutral, basic, and acidic fractions was not affected by the presence of nodules or flowers. However, when steady-state labeling was compared to pulse labeling, a significantly lower percentage of 14C was recovered in the neutral fraction with a concomitant increase in the basic fraction. Asparagine-arginine, serine, glutamate, y-aminobutyrate-alanine, and aspartate accounted for 69 to 85% of the recovered radioactivity in the basic fraction from the various treatments. 14CICSerine was significantly higher in pulse-labeling experiments, whereas glutamtate and proline were higher with steady-state labeling. 114CISerine was significantly higher in nonnodulated plants than in nodulated plants, whereas y-aminobutyrate-alanine was significantly higher in preflowering plants as compared to flowering plants. ' 94The transport of photosynthetically derived amino acids in soybean has not been extensively studied. The amino acids transported could differ in nonnodulated and nodulated soybeans, or perhaps with plant maturation. Plants without nodules are dependent on nitrate reduction as the sole source of reduced nitrogen, whereas plants with nodules can derive reduced nitrogen from both nitrate reduction and nitrogen fixation (14). During ontogeny, the amount of nitrogen reduced through these two processes varies (8,9), and may in fact be related to observed changes in amino acids found in soybean stem exudates (23). The relationship of these changes to photosynthesis is unknown because amino acids coupled to CO2 assimilation were not distinguishable in earlier studies.Tagging amino acids through...