Chapter 11.  Translocation and Distribution of Photoassimilates

• The big ideas:
• phloem is the tissue responsible for photosynthate.
• phloem structure.
• the mechanism of phloem transport (Munch pressure flow model).
• loading, unloading, flow.
• distribution of photosynthate (=photoassimilate, text) among competing sinks
• evidence that phloem is the tissue:
• girdling experiments
• radioactive tracer experiments
• aphid stylets
• analysis of exudates: sugars (sucrose and oligosaccharides), proteins, amino acids, ions, hormones
• phloem structure
• 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 plates
• have protoplasm but lose tonoplast, ribosomes, etc. during development. Retain plasmalemma (plasmodesmata provide cell to cell symplastic continuity to companion cells and ultimately to source/sink cells)
• 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 11.6, 11.7
• 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 11.8)
• 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.