Introductory Crop Physiology

Mechanism of tolerance of stress in plant

Tolerance of stress in plants

Mechanism of tolerance of plant to stress


• In general two types of stress resistance are recognized and these are avoidance and tolerance. In the avoidance an internal environment is created within the plants so that, its cells are not put under stress. In case of tolerance, plant has the capacity to withstand stress. In some species both the characters may be present.
• Plants experience usually drought which is one of the commonest stress. Plants have developed several mechanisms to tide over this stress such as development of thick cuticle, sunken stomata, formation of seeds with low water contents, completion of life cycle in short duration by the desert plants are some of the measures adopted by plants against drought.

• In some plants water is retained in enough quantities, or the leaves are reduced to scales.
• Dehydration leads to loss of water molecules and thus proteins are disrupted.
• Water molecules have several functions to perform and one of these is to help to keep complex fluids in a stable configuration.
• Water loss leading to high concentration of cell sap and intercellular fluid causes a greater decrease in the water potential of the fluid, this cause stress on the protoplasm and change in the cell pH also be there.
• Most of the biochemical processes are adversely affected because of the water imbalance.


On the basis of plant's response to available water, the plants are usually classified into three categories


• 1. Hydrophytes: the plants growing at a place where water is always available e.g., in a ponds.
• 2. Xerophytes: the plants growing at places such as deserts, where water is scarce at most of the time.
• 3. Mesophytes: the plants growing at places where water availability is intermediate.




• Plants have developed several mechanisms to tolerate drought conditions. One of these is the presence of hydrophilic substances in the protoplasm like high molecular weight protein, some carbohydrates (eg alginic acid), low molecular compound like polyhydric alcohols acts as hydrophilic compounds.
• These attributes are very common in sea weeds which are subjected to high and low tides. Sugars are usually accumulated in drought conditions in such plants, since their presence in solution directly lowers the water potential of cell sap. This helps the plants to retain and conserve water and save the protoplasm from desiccation.
• Xerophytes in the desert are actually exposed to wide range of water potentials.
• Plants such as palms that grow at an oasis where, their roots reaches the water table or other plants such as mesquit (Prosopis glandosa) and alfalfa (Medicago sativa) that have roots that extend as much as 50 m down to the water table never experience extremely negative water potential. They are called as water spenders. They certainly avoid the drought. Of course seeds plants must be able to use the available soil water while they are extending their roots to the water table.
• So called desert ephemerals (is one marked by short life cycles, usually six to eight weeks. The word ephemeral means transitory or quickly fading.) are annual plants that escape the drought by existing only as dormant seeds during the dry season. When enough rainfall to wet the soil to a considerable depth, these seeds often germinates, perhaps in response to the leaching away of germination inhibitors.
• Many of these plants grow to maturity and set at least one seed per plant before all soil moisture has been exhausted. They are well suited to dry region and thus are xerophytes in the true sense of the word, yet their active and metabolizing protoplasm is never exposed to extremely negative water potential and is not drought hardy.
• Succulent species such as cacti, century plant (Agave americana) and various other CAM plants are water saver. They resist the drought by storing water in their succulent tissue. Enough water is stored and its rate of loose is so extremely low (because of an exceptionally thick cuticles and sotmatal closure during the day times) that they can exist for long periods without added moisture.
• Succulents are drought avoidance but not truly drought tolerant. The water potential in this tissue is often -1.0 MPa. Some of the succulents especially the cacti have extensive shallow root system, which is capable of absorbing surface moisture after a storm. Of course the moisture is then stored in these succulent tissue.
• Many other desert plants also have several adaptations that reduce water loss, although they don’t actually store water in succulent tissue. The adaptations are small leaf blades, increases heat transfer by convection, lowering leaf temperature and thus reducing transpiration.
  
• Other adaptation that apparently reduces transpiration includes sunken stomata, shedding of leaves during dry periods and have heavy pubescences on leaf surface. Although these modification may reduce the water lose, they never completely prevent it and are by themselves insufficient to protection against extreme drought.







Sunken stomata of spruce needle conifer

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