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Transcript
Translocation
•
•
•
•
Occurs in Phloem
Bidirectional
Composition
Mechanism of
translocation
• Munch Pressure flow
hypothesis
• Phloem loading and
unloading
• Source-sink
relationships
Phloem Structure
•
•
•
•
•
•
•
Sieve tube members:
Angiosperms; Sieve cells:
Gymnosperms
Companion cells
Sieve tube
members/companion cells
derived from same mother
cell
Both living at maturity
Sieve tube members lose
organelles at maturity
contain cytoplasm
Sieve plate with pores
P-protein
Primary
phloem
2ndary
Phloem
Phloem
Electron Micrograph
Companion cell
Sieve tube elements
Composition of phloem
• Girdling experiments
• analysis of aphid
exudate
• composition
• sugars
• amino acids
(nitrogen):ureides,
glutamic acid,
glutamine
• organic acids
• hormones
Composition
Sugars
• Types of sugars
• sucrose main
transport sugar: nonreducing sugar
• some species:
raffinose, stachyose
transported
• sugar alcohols:
sorbitol, mannitol
Nitrogen compounds
• Transport from the
root
• Amino acids
• uriedes
Mechanism of translocation
• Munch Pressure flow
hypothesis
• Sugar loaded into
phloem
• increase in solute
potential
• leads to increase in
water uptake from
xylem
• builds up hydrostatic
pressure
• unloaded at sink
Source-sink relationships
• Bidirectional: up and
down plant
• is it in same cell, not
clear
• rate 30-150 cm/hr,
slower than water
Source to Sink
• Proximity of source to
sink is critical
• sinks may change
during life cycle
• young leaves net
import
• as mature become
exporter
• roots, shoots, tubers,
fruits are strong sink
Phloem Loading/unloading
•
•
•
•
Active process
evidence from respiratory inhibitors
Loading: apoplast or symplast
apoplast retrieve leaky sucrose?
Sugar transporters
•Sugar- H+ co-transport
symplast via
plasmodesmata:polymer trapping
Summary of loading
Unloading at sink
• symplast young leaves
• apoplast seeds/grain
• acid invertase
Loading/unloading