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