Osmotic Adjustment (OA)
Definition
- Osmotic adjustment is the active accumulation of solutes (osmolytes) in plant cells in response to water deficit (drought, salinity, or high temperature stress).
- This accumulation lowers the osmotic potential (Ψs) of cells, allowing them to maintain turgor pressure (Ψp) even under stress.
In simple words: Plants load solutes into cells → Ψs becomes more negative → water is retained → cell turgor is maintained.
Mechanism
- Under stress, plants synthesize or accumulate compatible solutes (non-toxic even at high concentrations).
- Examples:
- Proline
- Glycine betaine
- Mannitol, sorbitol
- Sugars (sucrose, trehalose)
- Inorganic ions (K⁺, Cl⁻, Ca²⁺)
- These solutes lower Ψs without interfering with metabolism, thereby facilitating water uptake from drier soils.
Significance in Plants
- (i) Maintenance of Turgor Pressure; Ensures cell expansion, stomatal opening, and growth even under drought/salt stress. Prevents wilting.
- (ii) Sustained Photosynthesis; Guard cells with OA can maintain partial stomatal opening → allows CO₂ uptake while reducing excessive water loss.
- (iii) Improved Water Uptake; By lowering Ψs, roots maintain a gradient (soil Ψw > root Ψw) → continuous water absorption under stress.
- (iv) Drought and Salinity Tolerance; Helps plants survive longer periods of stress. OA is considered a physiological marker for drought resistance in crop breeding.
- (v) Yield Stability; Crops with better OA show higher yields under stress conditions (sorghum, wheat, barley, pearl millet).
- (vi) Protection of Enzymes and Membranes; Compatible solutes (e.g., proline) act as osmoprotectants → stabilize proteins, membranes, and enzymes under dehydration.
Key Exam Points
- Osmotic adjustment = accumulation of solutes to lower Ψs.
- Maintains turgor and physiological activity during water stress.
- Compatible solutes: proline, glycine betaine, sugars, polyols, K⁺.
- Strong correlation between OA and drought tolerance.
- OA is a selection criterion in crop improvement programs.
