Ascent of Sap

Ascent of sap 

  • Ascent of sap refers to the upward movement of water through the plant, particularly from the roots to the aerial parts. 
  • Xylem, the specialized vascular tissue, is responsible for the upward transport of water, minerals, and some organic nitrogen and hormones. 
  • Vessels and tracheids within the xylem facilitate the bulk flow of water. 
  • Roots are the primary sites for water absorption in plants. 
  • Root hairs, present at the tips of roots, play a crucial role in water and mineral absorption. 
  • Root hairs, extensions of root epidermal cells, significantly increase the surface area for absorption. 
  • Water and minerals are absorbed by root hairs through diffusion. 
  • Water Movement in Roots:

- Apoplast Pathway: Water moves through the system of adjacent cell walls (apoplast), excluding the casparian strips of the endodermis. This pathway involves mass flow and relies on water's adhesive and cohesive properties. 

- Symplast Pathway: Water travels through interconnected protoplasts via plasmodesmata, involving cytoplasmic strands. Movement is relatively slower, occurring down a potential gradient.

 

 

 

  • Apoplastic movement is dominant due to loosely packed cortical cells. 
  • Casparian strip in the endodermis prevents water movement through the apoplast, directing it through the symplast. 
  • Water enters xylem vessels and tracheids, which are non-living conduits and part of the apoplast.

 

 

 

  • Symbiotic association of fungus with the root system, enhancing water and mineral absorption through a vast fungal surface area. Mutualistic exchange involves minerals and water provided by the fungus and sugars by the plant roots. 
  • Some plants, like Pinus, depend on mycorrhizae for germination and establishment. 
  • Driving Forces for Ascent of Sap:

- Root Pressure: Pressure exerted by water entering the roots creates a force propelling water upwards. 

- Capillarity: Adhesive and cohesive properties of water enable capillary action, aiding the rise of water through narrow tubes like xylem vessels. 

- Transpiration Pull: Water loss through transpiration creates tension, resulting in a negative pressure that pulls water upwards.

 

Transpiration pull

  • Water ascends through the xylem at impressive rates, reaching up to 15 meters per hour. 
  • Unlike animals with a circulatory system, plants lack a heart, yet they efficiently transport water. 
  • The mechanism behind water movement—whether it is 'pushed' or 'pulled'—has long intrigued researchers. 
  • Transpiration from leaves is identified as the primary force propelling water upwards. 
  • Cohesion-Tension-Transpiration Pull Model:

- The prevailing model for water transport, emphasizing the cohesive and tensional properties of water coupled with transpiration. 

- The cohesion between water molecules and tension generated by transpiration collaboratively contribute to the pull.