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