Continuous monitoring of steady-state carbon dioxide exchange rates in mature muskmelon (Cucumis melo L.) leaves showed diurnal patterns of photosynthesis and respiration that were translated into distinct patterns of accumulation and phloem export of soluble sugars and amino acids. Leaf soluble sugar patterns in general followed the pattern of photosynthetic activity observed in the leaf, whereas starch accumulated steadily throughout the light period. Sugar and starch levels declined through the dark phase. Phloem exudate analysis revealed that diurnal levels of the major transport sugars (stachyose and sucrose) in the phloem did not appear to correlate directly with the photosynthetic activity of the leaf but instead were inversely correlated with leaf starch accumulation and degradation. The amino acid pool in leaf tissues remained constant throughout the diurnal period; however, the relative contribution of individual amino acids to the total pool varied with the diurnal photosynthetic and respiratory activity of the leaf. In contrast, the phloem sap amino acid pool size was substantially larger in the light than in the dark, a result primarily due to enhanced export of glutamine, glutamate, and citrulline during the light period. The results indicate that the sugar and amino acid composition of cucurbit phloem sap is not constant but varies throughout the diurnal cycle in response to the metabolic activities of the source leaf.Rates of assimilate production in source leaves are determined both by biochemical regulation of key biosynthetic and degradative enzymes and by regulation of compartmentalization events that distribute assimilates between storage (chloroplast, vacuole) and transport (phloem) compartments (3,22). Export of photosynthetically fixed carbon from a source leaf is, therefore, dependent on the functioning of many complex metabolic events that control the production of phloem-mobile assimilates such as sucrose and delivery of these solutes to the phloem (3,6,18). In some plants, a direct correlation can be found between sucrose levels in the leaf and the rate of carbon export from the leaf, suggesting that the rate of phloem transport is controlled directly by the rate of sucrose synthesis (3,6,18 levels have no correlation with rates of export, suggesting possibly that it may not be the synthesis of sucrose but rather its delivery to the phloem system that is important in determining export rates (3,22). This may indicate a role for vacuolar transport processes in the control of export (3).At present, virtually all of our current information concerning biochemical regulation of carbon partitioning comes from plants that translocate sucrose exclusively (18). Recently, a renewed interest has been shown in carbon partitioning in those species that, in addition to sucrose, also synthesize and export the raffinose oligosaccharide, stachyose. The biochemistry of carbon partitioning between phloem-mobile and storage metabolites is far more complicated in these plants because of the additi...
A major protein in detergent extracts of skeletal muscle appears at 38,000 daltons in electrophoretic separations. Previous investigations have provided indirect evidence that a 38-kD skeletal muscle protein is membrane associated, and this inference has served as the basis for speculations on 38-kD protein function. In the present study, affinity purified, polyclonal antisera against 38-kD protein from skeletal muscle are produced for immunolocalization and biochemical assays. Immunoblots of two-dimensional electrophoretic separations show that this protein is heterogenously charged at pI approximately 6.4. This 38-kD protein is not extracted from muscle in low ionic strength or high ionic strength buffers, in isotonic buffers from pH 4 to pH 8 or in buffers containing 5 mM EGTA. The 38-kD protein is extracted, however, by isotonic, pH 7.0 buffer containing 1.0% Triton-X. Light microscope observations using indirect immunofluorescence of anti-38-kD labeled tissue show the protein distributed in a reticular pattern within cross-sectional muscle but not at the cell surface. Longitudinal sections show the protein concentrated in periodic, transverse bands. Purified fractions of muscle plasma membrane analyzed by immunoblotting contain 38-kD protein. Immunoblots using anti-38 kD show that this protein is present in all vertebrate skeletal muscle examined, however, the protein is present only in cardiac muscle that contains transverse tubules. The antibody does not recognize aldolase, another 38-kD striated muscle protein.
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