Morphological & Physiological Drought Tolerance Mechanisms of Plants
- Morphological mechanisms: Escape, Avoidance, Phenotypic plasticity
1.1 Water Savers
1.2 Water spenders
1.3 Phenotypic flexibility
- Plant growth is greatly affected by water deficit. At a morphological level, the shoot and root are the most affected and both are the key components of plant adaptation to drought.
- Plants generally limit the number and area of leaves in response to drought stress just to cut down the water budget at the cost of yield loss.
- Since roots are the only source to acquire water from soil, the root growth, its density, proliferation and size are key responses of plants to drought stress.
- Plants bearing small leaves are typical of xeric environments. Such plants withstand drought very well, although their growth rate and biomass are relatively low
- Leaf pubescence is a xeromorphic trait that helps protect the leaves from excessive heat load. Hairy leaves have reduced leaf temperatures and transpiration.
- To summarize, plants may escape drought stress by cutting short their growth duration, and avoid the stress with the maintenance of high tissue water potential either by reducing water loss from plants or improved water uptake, or both.
- Some plants may reduce their surface area either by leaf shedding or production of smaller leaves.
2. Physiological mechanisms
2.1 Cell and tissue water conservation
- Osmotic adjustment allows the cell to decrease osmotic potential and, as a consequence, increases the gradient for water influx and maintenance of turgor.
- Improved tissue water status may be achieved through osmotic adjustment and/or changes in cell wall elasticity. This is essential for maintaining physiological activity for extended periods of drought.
- Wild melon—drought tolerance—–high accumulation of citrulline, glutamate, arginine in leaves
- It has been identified that among various mechanisms, osmotic adjustment, abscisic acid and induction of dehydrins may confer tolerance against drought injuries by maintaining high tissue water potential
- With the accumulation of solutes in cytoplasm, the osmotic potential of the cell is lowered, which attracts water into the cell and helps with turgor maintenance., thereby minimizing the harmful effects of drought
- Osmotic adjustment is an important trait in delaying dehydrative damage in water-limited environments by continued maintenance of cell turgor and physiological processes .
- The osmotic adjustment also facilitates a better translocation of pre-anthesis carbohydrate partitioning during grain filling, while high turgor maintenance leads to higher photosynthetic rate and growth
2.2 Antioxidant defense
- Apart from CAT, POD & peroxidins, four other enzymes play important role in “Ascorbate glutathione cycle” under drought stress, for scavenging superoxide radicals & H2O2
- Stroma of chloroplast
2.3 Cell membrane stability
- Biological membranes are the first target of many abiotic stresses. It is generally accepted that the maintenance of integrity and stability of membranes under water stress is a major component of drought tolerance in plants.
- Cell membrane stability, reciprocal to cell membrane injury, is a physiological index widely used for the evaluation of drought tolerance
- CMS is directly propotional to drought tolerance
- The cell membrane disruption under stress increase the viscosity of cytoplasmic components that further increases the chances of molecular interactions that can cause protein denaturation and membrane fusion
- A broad range of compounds can provide protection against molecular interactions e.g. pro, GB, carnitine, mannitol, sorbitol, polyols, trehalose, sucrose, oligosaccharide