Chapter 6.  Allocation, Translocation and Partitioning of Photoassimilates

• The processes:
• Photoassimilate must be allocated appropriately so that sucrose for transport is produced, starch for storage is produced, and RuBP is regenerated.
• Translocation takes place in the phloem, transporting sugars from sources to sinks.
• Photoassimilates must be appropriately portioned among various sinks, e.g., meristems, fruits, shoots, roots.
• Allocation
• Sucrose and starch synthesis are competing processes.
• Sucrose is synthesized in the cytosol, starch in the chloroplast (Figure 6.2).
• Balance between processes is regulated by the triose-P/P translocator (antiporter) and regulation of enzymes catalyzing hexose-P synthesis.
• Translocation in the phloem: evidence that phloem is the tissue:
• girdling experiments (Figure 6.5)
• radioactive tracer experiments (Figure 6.6)
• aphid stylets
• analysis of exudates: sugars (sucrose and oligosaccharides), proteins, amino acids, ions, hormones
• Phloem structure (Figure 6.8)
• A constituent of vascular bundles ("veins") in leaves and stele in stem and roots
• Primary and secondary phloem.
• Unlike xylem, phloem cells are alive, with protoplasm - transport mechanisms must take this into account
• Sieve element are joined end-to-end to form sieve tubes)
• Sieve areas and sieve plates
• Have protoplasm but lose tonoplast, ribosomes, etc. During development. Retain plasmalemma. Plasmodesmata provide cell to cell symplastic continuity between sieve elements, to companion cells and ultimately to source/sink cells. Figure 1.29 shows structure of plasmodesmata.
• Companion cells
• Retain full complement of cytoplasmic organelles.
• Sieve element + companion cell = se-cc complex (text), develop from same mother cell
• P protein and callose - plug, isolate damaged sieve elements.
• Sources and sinks
• Sources export photosynthate, sinks receive it
• Source/sink identity changes developmentally:
• Young leaves are sinks, mature leaves are sources.
• Roots, fruits, meristems are generally sinks.
• Storage organs switch sink/source identity.
• Mechanism of translocation (Munch pressure flow model): see Figure 6.10, 6.11.
• At source:
• Sucrose is diffuses from mesophyll to companion cells
• Loaded (ATP dependent process) from companion cells to sieve elements
• Result is decrease in osmotic potential, water flows in from xylem from xylem, increase in turgor pressure in phloem
• At sink
• Sucrose is unloaded from sieve elements to companion cells (ATP dependent process)
• Result is increase in osmotic potential, water flows out to xylem, decrease in turgor pressure in phloem.
• Consequently, turgor pressure is high at source, low at sink, water and solutes move along pressure gradient from source to sink.
• Phloem loading (see Figure 6.12, 6. 13)
• Via the apoplast
• Sugar diffuses into cell wall, actively transported (cotransport, symport) across plasmalemma of se-cc
• Via the symplast
• There is symplastic continuity from mesophyll to sieve element via plasmodesmata.
• Movement is via diffusion, the "polymer trap" mechanism may prevent back flow.
• Phloem unloading
• Essentially loading in reverse, can occur apoplastically and symplastically
• Partitioning of assimilate among sinks
• Three factors: proximity, vascular connections, sink strength
• Sink strength = sink size x sink activity
• Sink source communication:
• Turgor pressure?
• Hormones?
• Sink demand can affect photosynthetic rate of source leaves.