Assimilate Transport

Movement of substances through the phloem

Chapter 30

Translocation• movement of

substances in phloem

• in leaf• sugars move

from chloroplasts to phloem

• forming the assimilate stream

Movement of assimilate stream in phloem• source to sink movement

• source – area exporting assimilates• sink – assimilate importing area

• What are examples?• Do these change during the life of a

plant?

Sour-sink examples

• seed manufacture• seed germination• seedling development• vegetative growth

• young vs. old• reproductive growth

Sugar Transport in Leaves – evidence via 14C

14CO2 exposure for 35 minutes; 14C incorporated in sugars, confined to sieve tubes

Aphid stylet

• content of phloem assimilate• 10-25% dry matter

• 90% of dry matter is sugar, mainly sucrose

• sieve-tube sap moves at 100cm/hour

Pressure-flow hypothesis

• osmotically generated pressure flow• on source end

• assimilates transported into sieve tube - phloem loading

• as sucrose enters, what happens to water?

• water increase raises turgor pressure

• on sink end• sucrose is unloaded

(removed) from sieve tube – phloem unloading

• What happens to water?• thus assimiliate is moved

from source to sink by bulk flow

Assimililate movement requires energy