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Biological Sciences

Phloem Transport Chapter 10Phloem Transporty Sources area of excess supply aka photosynthate ex mature leafs and storage organ in their exporting phase some binanual plants store sugar in their roots die then grow again using the stored sugar if theyre in exporting phase y Sinks area which is in need of photosynthate for growthstorage ex roots tuber immature fruitsleaves and non photosynthetic organs y Photosynthate 1Allocation uses mechanisms inside of source cells to allocate resources to astorage bgrowthmetabolism constant need ctransport to sinks if immediate needs are met C turns into sucrose and transported by phloem 2 Partitioning uses mechanisms throughout plant to allocate photosynthate to sinks such as agrowing parts shootingrooting stem bstorage organs creproductive tissues harvestable part of the plant farmers what it to have high yield y Mechanisms to improve yield of commercial plant parts harvestable section such as fruits or seeds 1 Increase photosynthetic rate little success 2 Increase harvest index altering partitioning of resources to sinks that are considered harvestablebiomas of the plant that is harvestabley Sieve tube elements SEy Live cells lose nuclei tonoplast golgi ribosomes or cytoskeleton but maintain PM plastids and SER aka only essentials They cannot perform protein synthesisunderpressure vs Xylem which are dead underpressurehave no organelles y perform long distance transport between different parts of the plant arranged in files y companion cells help regulate movement they have dense cytoplasm as they must contain enough stuff to sustain the SE which lack the organelles to sustain themselves They have a lot cytoplasmic continuity with SE y thick primary cell wall with sieve plates at the ends pores where PM is continous no 2ndary wall y damage eg herbivory causes pressureleak of photosynthate prevented by 1 PProtein shortterm clogs the sieve plates 2 Callosecallose synthesis in PM longterm complex carbs that permanently block the sieve plate y More is known and SE in angiosperms its unknown how transport works in gymnosperms becomes it seems that the SE are not continuous with each other y Companion cells y Dense cytoplasm with a lot of mitochondria to carry out metabolism eg proteinATP synthesis required by the SE y They also produce pproteins which move into SE when required y Transfer of photosynthate from source to SE 3 in mature leaves 1 Ordinary companion cells plasmodesmata with SE only 2 Transfer cells plasmodesmata with SEcell wall ingrowths extra PM surface area with other cells 3 Intermediary cells plasmodesmata with SE and other cells y These differences distinguish apoplastic and Symplastic loading y Translocation patterns y Sourcesink isnt exclusively updown y Particular sources preferentially supply particular sinks depending on 1 Proximity upper mature leaves supple immature leaves vs lower mature leaves supply roots 2 Development phase vegetative growth phase favours shootroot apices sinks vs reproductive growth which favours flowerfruit sinks changes are due to changes in phloem connections 3 Vascular connections more direct connectionsmore likely the connection is used y The patterns are flexible you can use radioisotopes to monitor routes and changes y Their exist competition between sinks if you eliminate 1 sink another one gets more photosynthate Ex put radioactive C14 in mature leave sucrose and watch were it does it usually goes to nearby immature leaves but if you cut the close leaves off it will go to the farther immature leavesy Molecules transportedy Mainly sucrose high 0309M y Non reducing sugarssugar alcohol
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