Download - Translocation - WOU
Translocation• Occurs in Phloem • Bidirectional • Composition• Mechanism of
translocation• Munch Pressure flow
hypothesis • Phloem loading and
unloading• Source-sink
relationships
Phloem Structure
2ndaryPhloem
Primaryphloem
• 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
Phloem
Electron Micrograph
Sieve tube elements
Companion cell
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: non-reducing sugar
• some species: raffinose, stachyosetransported
• 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