Translocation and Root Pressure

What is a source?

Where sugars are loaded into the phloem

Photosynthetic tissue

Storage organs unloading stored solutes

What is a sink?

Where solutes are unloaded and used.

Parts developing food stores

Growing roots/ active processes in roots

Draw labelled phloem diagram and describe its features

Features:

• living - have a plasma membrane

• very limited organelles - some cytoplasm, nucleus degenerates

• cell walls - develop into sieve end plates, plasmodesmata enlarge = sieve pores so adjacent cells are connected through large perforations

• companion cells - high metabolic rate/ lots of ribosomes/mitochondria

• columns of cells

• contains phloem sap = solution of sucrose and other compounds to be transported

• no mitochondria in sieve tube elements

Mass flow hypothesis (long answer Q)

Root Pressure

Reasons for transpiration to not occur

• High atmospheric humidity 

• Night conditions - plants close stomata. This may allow xylem to become air filled.

• When the leaves have fallen, the xylem often become air filled and need to be refilled when the new leaves have grown.

Root pressure mechanism

The movement of mineral ions makes the xylem hypertonic and so water moves across from the root tissue by osmosis. This raises the pressure in the vessels causing the sap to move upwards. 

Note: root pressure is not dependent on atmospheric pressure so there is no limit to the height to which the xylem sap can rise.