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 14 C 14 CO 2 exposure for 35 minutes; 14 C 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