Question

Explain sap flow using pressure flow hypothesis. how does it explain sap flow in the spring

Answer 1

The pressre flow hypothesis was put forward by ernst munch to explain the flow of sap in the phloem.According to this hypothesis,the sugars are synthesized in the leaves.The sugar which is left over after the cellular respiration is converted into sucrose and is transported actively to the companion cells of the veins of leaves.This sucrose then reaches the sieve tube elements and this causes a rise in the sucrose concentration in them.This increase in concentration drives the water from the adjecent xylem into the phloem.This increases the hydrostatic pressure in the phloem.this increase in osmotic pressure will drive the contents towards region of low osmotic pressure-the sink.The solutes are offloaded actively and this causes the water to get reabsorbed into the xylem,lowering the hydrostatic pressure.

In the spring,the storage parts acts as the sink.This causes the active loading of the sugars into the phloem from the earlier sink like roots and storage organs.This causes the water from xylem to flow into the phloem,increasing the hydrostatic pressure.The other regions of the plant will have a lesser hydrostatic pressure compared to the phloem and this drives the sap to the other growing regions.The sugars are actively unloaded and the pressure returns back to normal along with the return of the water to xylem