Sugar Accumulation in Sugarcane

The involvement of the free space in phloem loading of sucrose was studied in sugar beet source leaves (Beta vulgaris, L.). Sucrose, supplied exogenously to the abraded upper surface of leaves at a concentration of 20 mM, was available for translocation at rates siniilar to those obtained with photosynthesis. The exogenous sucrose substituted as a source of translocate for assimilate derived from photosynthesis when the latter process was disrupted by plasmolysis of the leaf with 0.8 M mannitol. The mesophyll symplast was not completely disrupted by this treatment, however. Data from the sugar uptake experiments indicate that phloem loading can occur from the free space.Isotope trapping of labeled sugars derived from "CO2 was used to intercept and identify sugars passing through the free space prior to phloem loading. Increased translocation rates induced by 4 mM ATP or increased light intensity were accompanied by increased trapping of sucrose but not of glucose. The data support the view that sucrose passes into the free space prior to phloem loading.In previous studies, it was shown that phloem loading is an active process which accompanies translocation of sucrose and, at least in some part, is responsible for causing it (3, 13). Active uptake of sucrose was found to occur when exogenous sucrose was supplied (13)

Section: Discussionmentioning

confidence: 81%

“…The equilibration period was of the same order as observed by Glasziou and Gayler (7) when they studied free space sucrose concentration. The level of 14C in the bathing solution in this type of experiment is considered to be an indication of the concentration of the labeled components in the free space rather than their turnover rates (7).…”

Section: Resultsmentioning

confidence: 99%

Role of Free Space in Translocation in Sugar Beet

Geiger

Sovonick²,

Shock³

et al. 1974

“…The lack of any detectable solute reabsorption may be due to the fact that cut cells comprise a smaller percentage of the total tissue in large slices than in small slices. Therefore, the effect of cut surfaces on the bulk tissue T measurements is probably less in large slices used in this experiment than in small tissue slices used by previous investigators (8,9).…”

Section: Discussionmentioning

confidence: 86%

Compartmentation of Solutes and Water in Developing Sugarcane Stalk Tissue

Welbaum¹,

Meinzer²

1990

136

108

Previous studies have suggested that the apoplast solution of sugarcane stalk tissue contains high concentrations of sucrose, but the accuracy of these reports has been questioned because sucrose leakage from damaged cells may have influenced the results. In this study, the solute potential of the apoplast and symplast of the second (immature), tenth, twentieth, thirtieth, and fortieth internodes of field-grown sugarcane ( determine the extent of solute accumulation in the apoplast as well as the symplast. Two independent methods were used to quantify the water relations of both the symplast and apoplast in terms of i'', ',, and i&p. Specific objectives were to measure 'I, /,, and ,6p for both mature and immature tissue, to determine how the partitioning of solutes and water between the apoplast and symplast changes with development, to identify the solutes present in each compartment, and to determine the relative volume of the apoplast solution in order to assess its importance as a storage compartment for sucrose. MATERIALS AND METHODS Plant Material.Sugarcane (Saccharum spp. hybrid, cv H65-7052) plants were field-grown in a silty-clay loam (Typic Torrox) soil near Waipahu, Oahu, HI. Well-watered 1.5-to 2-year-old plants were excised prior to sunrise when guttation water was present on leaf margins, indicating that the plants were fully hydrated. After excision, plants were immediately sealed in plastic bags and transported to the laboratory. Tissue samples were collected and analyzed on the basis of internode number. The reference point for determining internode number was the TVD leaf (12); the youngest leaf whose blade is fully exposed and not enclosed in the sheaths of older leaves. The first internode was the node below the TVD leaf sheath attachment, and additional internodes were numbered consecutively down the stalk. A 2-year-old sugarcane stalk grown under the conditions described above typically contained approximately 50 internodes. In this study, internodes 2, 10, 20, 30, and 40 were analyzed.Internode tissue sections were prepared for analysis inside a humidified chamber to reduce water loss. The stalk was cut in half at least 1 cm from the node and the epidermis and any pith tissue was removed with a knife, because the fiber content is higher and sucrose concentrations lower in these tissues (3,6

“…Access to the phloem for experimental manipulation of the unloading process has been gained in but few systems, most notably sugar cane stalks (6) and Cuscuta-parasitized stems of Viciafaba (16). While much has been learned of the kinetics of photosynthate import, anatomy of the tissues involved, and uptake of assimilates by sink consumer cells (4,5,15), little is known of the mechanisms controlling the exit of solutes from the phloem sieve tubes.…”

mentioning

confidence: 99%

An In Vivo Technique for the Study of Phloem Unloading in Seed Coats of Developing Soybean Seeds

Thorne

Rainbird²

1983

A technique has been developed which permits mechanistic studies of phloem unloading in developing seeds of soybean (Glyrune mar 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 diethylsdibestrol, 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 to 8 hours at rates of 0.5 to 1.0 micromoles per hour, providing a valid, convenient in viva technique for studies of phloem unloading and seed growth mechanisms.The study of solute exit from the phloem ('unloading') in sink tissue has been severely hampered by the lack of convenient, valid techniques. Access to the phloem for experimental manipulation of the unloading process has been gained in but few systems, most notably sugar cane stalks (6) and Cuscuta-parasitized stems of Viciafaba (16). While much has been learned of the kinetics of photosynthate import, anatomy of the tissues involved, and uptake of assimilates by sink consumer cells (4,5,15), little is known of the mechanisms controlling the exit of solutes from the phloem sieve tubes.The phloem retains its solutes with minimal radial exchange;sites of unloading are notable exceptions. A popular hypothesis is that the sieve tube/companion cell/phloem parenchyma complex counters leakage by continuously reloading assimilates along the entire vascular length, and that sinks permit unloading by locally inhibiting the reloading mechanism (5, 10). This model is consistent with the data suggesting passive efflux from the phloem of developmentally immature sinks incapable of reloading, and for storage sinks where rapid hydrolysis or compartmentation prevents reloading of sucrose (4, 5, 15). However, other studies have provided evidence that, under some conditions, sinks directly effect solute exit from the phloem complex in a controlled, energydependent manner (13, 16). Given the diversity of sink tissues ' Visiting Scientist.which exist, it is probable that more than one unloading mechanism operates. This paper characterizes a novel technique for studying phloem unloading and maternal/embryo transfer of photosynthate in legume fruit. MATERIALS AND METHODSSoybean (Glycine max cv Clark) were grown as previously described (14) except that some plants were grown symbiotically by inoculating the seed at sowing with an appropriate strain of Rhizobium Japonicum and deleting nitrogen from the nutrient solution. Plants were utilized for experiments at various...