Patterns of Assimilate Production and Translocation in Muskmelon (Cucumis melo L.)

Abstract

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...

Divergent metabolome and proteome suggest functional independence of dual phloem transport systems in cucurbits

“…High rates of carbon mass flow in phloem are predicted from the fast growth of cucurbit vegetative and fruit tissues (25). However, no previous reports showed high RFO content in cucurbit phloem exudates, for example (15,37). The total sugar content of around 1 M in FP is consistent with other species, and with measurements in cucurbit leaf cells (17), but is in sharp contrast to the low sugar content of the extrafascicular exudate.…”

The Arabidopsis Nitrate Transporter NRT1.7, Expressed in Phloem, Is Responsible for Source-to-Sink Remobilization of Nitrate

“…Using different time periods for assimilation and export could ensure that the older leaves still get sufficient nitrate for assimilation to meet their own nitrogen demand, and only excess nitrate is transported out. For most plants, the amount of sucrose exported from the source leaf during the night is similar to or slightly lower than the amount exported during the day (Pate et al, 1979;Mitchell et al, 1992). The sucrose exported during the night comes from hydrolysis of starch stored during the day.…”

Analysis of the systemic colonization of cucumber plants by Cucumber green mottle mosaic virus

“…Virus and assimilate phloem transport involves three steps: phloem loading in source organs, movement through the transport phloem and unloading in sink organs (Nelson & Van Bel, 1998;Oparka & Turgeon, 1999;Thompson & Schulz, 1999). In cucurbits, assimilate loading is presumably symplastic and the ICs of minor veins play a major role in the synthesis and loading of the transported oligosaccharides (Gross & Pharr, 1982;Haritatos et al, 1996;Holthaus & Schmitz, 1991;Mitchell et al, 1992;Turgeon, 1996). Similarly, it has been shown that CMV accumulates in the ICs of minor veins in inoculated cucumber cotyledons (Thompson & García-Arenal, 1998).…”