RESPONSE TO STRESS
Stress
Stress can be defined as any environmental factor that can have an adverse effect on a
Plant’s growth, reproduction, and survival. Plants can respond to stress in several ways they can escape the effects of stress by completing their growth during less stressful periods or they may suffer injury if the stress is present and they cannot cope. Plants can cope with environmental stress through a combination of development and physiological responses.
Response to water stress
On bright, warm, dry day, a plant may be stressed by a water deficiency because it is losing water by transpiration faster than the water can be restored by uptake from the soil. This continuous drought can stress plants for weeks or month. Severe water defict, may kill a plant.
But plants have control systems that enable them to cope with less extreme water deficts. Many of a plants responses to water defict help the plant conserve water by reducing the rate of transpiration. Water defict can stimulates increased synthesis and release of abscisic acid from mesophyll cells in the leaf, and this hormone helps keep stomata closed by acting on guard cell membranes and causes guard cell to loose turgor.
Leaves respond to water defict in
several other ways, because cell expansion is a turgor- dependent process, a water defict will inhibit the growth (expansion) of yong leaves. This response minimizes the transcriptional loss of water by slowing the increase in leaf surface.
When the leaves of many grasses and other
plants wilt from a water defict, they roll into a shape that reduces transcription by exposing less leaf surface to the sun. While all of these responses of leaves help the plant conserve water, they also reduces photosynthesis. This is one reason a drought diminished crop yield
Response To Oxygen Stress
An overwatered house plant may suffocate because the soil lacks the air spaces that provide oxygen for cellular respiration in the roots. Some plants are structurally adapted to very wet habitats. For example, the submerged roots of trees called mangroves, which inhabit coastal marshes, are continuous with aerial roots that provide access to oxygen. Experimentally it was shown that oxygen deprivation stimulates the production of hormone ethylene, which causes some of the cells in the roots cortex to age and die.
Enzymatic destruction of the cell walls
creates air tubes that function as ‘snorkes’ providing oxygen to the submerged roots.
Response To Salt Stress An excess of sodium chloride or other salts in the soil threatens plants for two reasons.
First, by lowering the water potential of the soil solution, salt can cause a water defict in plants even though the soil has plenty of water. This is because in an environment with a water potential more negative than that of the root tissue, roots will lose water rather than absorb it. The second problem with saline soil is that sodium and certain other ions are toxic to plants when their concentration is relatively high.
The selectively permeable membranes of root cells impede the uptake of most harmful ions, but this only aggravates the problem of acquiring water from soil that is rich in solutes. Many plants can respond to moderate soil salanity by producing
compatible solutes, organic compounds that keeps the water potential of cells more negative than that of the soil solution without admitting toxic quantities of salt.
However, most plants cannot survive salt stress for very long. The exception are halophytes, salt tolerant plants with
special adaptations such as salt glands, which pump salts out of the plant across the leaf epidermis.
Response to Heat Stress
Exccesive heat can harm and eventually kill a plant by denaturing its enzymes and damaging its metabolism in other ways. One function of transpiration is evaporative cooling. On a warm day, for example, the temperature of a leaf is below air temperature which favours transpiration
and causes water deficiency in many plants; the closing of stomata in response to this stress conserves water but sacrifices evaporative cooling. This dilemma is one of the reasons that very hot, dry days take such a toll on most plants. Many plants response to this stress by synthesizing relatively large quantities of special proteins called heat shock proteins. Some
heat shock proteins are identical to chaperon proteins, which function in unstressed cells as temporary scaffolds that other proteins fold into their functional conformations.
Response to Cold Stress
One problem plants face when the temperature of the environment falls is a change in the fluidity of cell membranes. When a membrane cools below a critical point, it loses its fluidity as the lipids become locked into crystalline structures. This alters solute transport across the
membrane and also adversely affects the functions of membrane proteins. Plants respond to cold stress by altering the lipid composition of their membranes.
For example, membrane lipids increase in their proportion of unsaturated fatty acids, which have shapes that help keep
membranes fluid at lower temperature by impeding crystal formation. Freezing is a more severe version of cold stress. At freezing temperatures, ice crystals begin to form in most plants. If ice crystals are confined to cell walls and Intercellular spaces, the plant will probably survive.
However, if ice crystals begin to form within protoplasts, the sharp crystal perforate membranes and organelles, killing the cells. However, plants like oaks, maple, rhododendrons native to regions where winters are cold have special adaptations that enable to cope with
freezing stress. For example, changes in the solute composition of live cells allow the cytosol to cool below 0 degree Celsius without crystal formation Although ice crystals may form in the cell walls.
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