Translocation in Plants

TRANSLOCATION IN PLANTS 

  • Plants derive carbon and the majority of their oxygen from atmospheric CO2. 
  • Plants absorb essential minerals from the soil to meet specific nutritional needs. 
  • Water uptake from the soil provides plants with hydrogen, contributing to their overall nutritional balance. 
  • Mineral Ion Uptake: 

- Not all minerals can be passively absorbed by roots due to specific factors. 

- Minerals exist in the soil as charged particles (ions) that cannot freely move across cell membranes.

 - Soil minerals typically have lower concentration than in the root, necessitating active absorption. 

- Active absorption into epidermal cell cytoplasm is required, demanding energy in the form of ATP. 

- Active ion uptake contributes to the water potential gradient in roots, facilitating water absorption by osmosis. 

- Some ions passively move into epidermal cells. 

- Ions are absorbed by both passive and active transport mechanisms. 

- Specific proteins in root hair cell membranes actively pump ions from soil into epidermal cell cytoplasm. 

- Endodermal cells regulate solute movement with embedded transport proteins, controlling types and quantities reaching the xylem.

 - Due to suberin layer, root endodermis actively transports ions in one direction only, influencing nutrient flow.

  • Translocation Mechanism: 

- After reaching the xylem through active or passive uptake, mineral ions are transported throughout the plant via the transpiration stream. 

- Growing regions like apical and lateral meristems, young leaves, developing flowers, fruits, seeds, and storage organs act as major sinks for mineral elements. 

- Mineral ions are unloaded at fine vein endings through diffusion and active uptake by cells. 

- Older, senescing parts frequently remobilize minerals, with dying leaves exporting their content to younger leaves before falling. 

- Elements like phosphorus, sulfur, nitrogen, and potassium are readily mobilized, while structural components like calcium are not remobilized. 

- Xylem exudates analysis reveals nitrogen traveling as both inorganic ions and in organic forms such as amino acids. Phosphorus and sulfur are also carried in small amounts as organic compounds. 

- Exchange of materials occurs between xylem and phloem, challenging the traditional belief that xylem transports only inorganic nutrients and phloem transports only organic materials. 

  • Phloem Transport Dynamics: 

- Phloem, a vascular tissue, transports primarily sucrose, serving as the conduit for food movement from source to sink within the plant. 

- Generally, leaves act as the source, synthesizing food (sucrose), while the destination or sink is the part of the plant that needs or stores the food. This relationship can be reversed based on seasonal changes or the plant's requirements. 

- Sugar stored in roots, usually a sink, can be mobilized as a source of food in early spring when buds act as sinks, requiring energy for growth and photosynthetic apparatus development. 

- The direction of movement in the phloem can be upwards or downwards, making it bi-directional. This is in contrast to xylem, which exhibits unidirectional movement (upwards). 

- Unlike the one-way flow of water in transpiration, phloem sap can transport food in any required direction, provided there is a source of sugar and a sink capable of using, storing, or removing the sugar. 

- Phloem sap consists mainly of water and sucrose, but it also contains other sugars, hormones, and amino acids that contribute to the overall transport or translocation process through the phloem.

 

 

 

  • Pressure Flow Hypothesis: 

- The accepted mechanism for sugar translocation from source to sink is the pressure flow hypothesis. 

- Glucose produced at the source (via photosynthesis) is converted to sucrose. 

- Sucrose is actively transported into companion cells and then into living phloem sieve tube cells at the source. 

- Loading at the source creates a hypertonic condition in the phloem, causing water from adjacent xylem to move into the phloem via osmosis.

 - Osmotic pressure buildup in the phloem sap results in movement towards areas of lower pressure, facilitating mass movement in the phloem.

 - At the sink, active transport moves sucrose out of the phloem, where it is utilized for energy, starch, or cellulose production. As sugars are removed, osmotic pressure decreases, leading to water movement out of the phloem. 

- Phloem tissue comprises sieve tube cells forming long columns with sieve plates containing holes. Cytoplasmic strands pass through these holes, forming continuous filaments. 

- Increasing hydrostatic pressure in the phloem sieve tube initiates pressure flow, facilitating sap movement through the phloem. 

- Girdling, the removal of a ring of bark up to the phloem layer, demonstrates that phloem is responsible for food translocation. The experiment shows unidirectional transport towards the roots. 

- Absence of downward food movement results in swelling above the girdled portion after a few weeks, confirming the role of phloem in one-way food transport.