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Add to Cart Add to Cart. Add to Wishlist Add to Wishlist. Primary effects such as reduced water potential and cellular dehydration directly alter the physical and biochemical properties of cells, which then lead to secondary effects. These secondary effects, such as reduced metabolic activity, ion cytotoxicity, and the production of reactive oxygen species, initiate and accelerate the disruption of cellular integrity, and may lead ultimately to cell death.
Different abiotic factors may cause similar primary physiological effects because they affect the same cellular processes. Secondary physiological effects caused by different abiotic imbalances may overlap substantially. It is evident that imbalances in many abiotic factors reduce cell proliferation, photosynthesis, membrane integrity, and protein stability, and induce production of reactive oxygen species ROS , oxidative damage, and cell death.
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As photoautotrophs, plants are dependent upon — and exquisitely adapted to — visible light for the maintenance of a positive carbon balance through photosynthesis. Higher energy wavelengths of electromagnetic radiation, especially in the ultraviolet range, can inhibit cellular processes by damaging membranes, proteins, and nucleic acids. However, even in the visible range, irradiances far above the light saturation point of photosynthesis cause high light stress, which can disrupt chloroplast structure and reduce photosynthetic rates, a process known as photoinhibition. Photoinhibition by high light leads to the production of destructive forms of oxygen.
Excess light excitation arriving at the PSII reaction center can lead to its inactivation by the direct damage of the D1 protein. The oxidative stress generated by excessive ROS destroys cellular and metabolic functions and leads to cell death.
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Outside of this range, varying amounts of damage occur, depending on the magnitude and duration of the temperature fluctuation. In this section we will discuss three types of temperature stress: high temperatures, low temperatures above freezing, and temperatures below freezing. However, nongrowing cells or dehydrated tissues e. However, high leaf temperatures combined with minimal evaporative cooling causes heat stress. Increases in leaf temperature during the day can be more pronounced in plants experiencing drought and high irradiance from direct sunlight. Temperature stress can result in damaged membranes and enzymes.
Plant membranes consist of a lipid bilayer interspersed with proteins and sterols, and any abiotic factor that alters membrane properties can disrupt cellular processes. High temperatures cause an increase in the fluidity of membrane lipids and a decrease in the strength of hydrogen bonds and electrostatic interactions between polar groups of proteins within the aqueous phase of the membrane. High temperatures thus modify membrane composition and structure, and can cause leakage of ions.
High tempeatures can also lead to a loss of the three-dimensional structure required for correct function of enzymes or structural cellular components, thereby leading to loss of proper enzyme structure and activity. Misfolded proteins often aggregate and precipitate, creating serious problems within the cell. Photosynthesis and respiration are both inhibited by temperature stress. Typically, photosynthetic rates are inhibited by high temperatures to a greater extent than respiratory rates. Although chloroplast enzymes such as rubisco, rubisco activase, NADP-G3P dehydrogenase, and PEP carboxylase become unstable at high temperatures, the temperatures at which these enzymes began to denature and lose activity are distinctly higher than the temperatures at which photosynthetic rates begin to decline.
This would indicate that the early stages of heat injury to photosynthesis are more directly related to changes in membrane properties and to uncoupling of the energy transfer mechanisms in chloroplasts. This imbalance between photosynthesis and respiration is one of the main reasons for the deleterious effects of high temperatures. On an individual plant, leaves growing in the shade have a lower temperature compensation point than leaves that are exposed to the sun and heat. Reduced photosynthate production may also result from stress-induced stomatal closure, reduction in leaf canopy area, and regulation of assimilate partitioning.
Freezing temperatures cause ice crystal formation and dehydration. Freezing temperatures result in intra- and extracellular ice crystal formation. Intracellular ice formation physically shears membranes and organelles. Extracellular ice crystals, which usually form before the cell contents freeze, may not cause immediate physical damage to cells, but they do cause cellular dehydration.
Consequently, water moves from the symplast to the apoplast, resulting in cellular dehydration. Cells that are already dehydrated, such as those in seeds and pollen, are relatively less affected by ice crystal formation. Ice usually forms first within the intercellular spaces and in the xylem vessels, along which the ice can quickly propagate.
This ice formation is not lethal to hardy plants, and the tissue recovers fully if warmed. However, when plants are exposed to freezing temperatures for an extended period, the growth of extracellular ice crystals leads to physical destruction of membranes and excessive dehydration. Imbalances in the mineral content of soils can affect plant fitness either indirectly, by affecting plant nutritional status or water uptake, or directly, through toxic effects on plant cells.
Soil mineral content can result in plant stress in various ways.
Several anomalies associated with the elemental composition of soils can result in plant stress, including high concentrations of salts e. The term salinity is used to describe excessive accumulation of salt in the soil solution. Salinity stress has two components: nonspecific osmotic stress that causes water deficits, and specific ion effects resulting from the accumulation of toxic ions, which disturb nutrient acquisition and result in cytotoxicity.
Salt-tolerant plants genetically adapted to salinity are termed halophytes , while less salt-tolerant plants that are not adapted to salinity are termed glycophytes.
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Soil salinity occurs naturally and as the result of improper water management practices. In natural environments, there are many causes of salinity. Terrestrial plants encounter high salinity close to the seashore and in estuaries where seawater and freshwater mix or replace each other with the tides. The movement of seawater upstream into rivers can be substantial, depending on the strength of the tidal surge.
Plant acclimation to environmental stress: a critical appraisal
Far inland, natural seepage from geologic marine deposits can wash salt into adjoining areas. Evaporation and transpiration remove pure water as vapor from the soil, concentrating the salts in the soil solution. Soil salinity is also increased when water droplets from the ocean disperse over land and evaporate. Human activities also contribute to soil salinization. Improper water management practices associated with intensive agriculture can cause substantial salinization of croplands. In many areas of the world, salinity threatens the production of staple foods.
Irrigation water in semiarid and arid regions is often saline. Only halophytes, the most salt-tolerant plants, can tolerate high levels of salts. Glycophytic crops cannot be grown with saline irrigation water. Salt incursion into the soil solution causes water deficits in leaves and inhibits plant growth and metabolism. High concentrations of salt cause protein denaturation and membrane destabilization by reducing the hydration of these macromolecules. One way plants can adapt to extreme environmental conditions is through modification of their life cycles.
For example, annual desert plants have short life cycles: they complete them during the periods when water is available, and are dormant as seeds during dry periods.
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Deciduous trees of the temperate zone shed their leaves before the winter so that sensitive leaf tissue is not damaged by cold temperatures. During less predictable stressful events e. For example, plants that can grow and flower over an extended period indeterminate growth are often more tolerant to erratic environmental extremes than plants that develop preset numbers of leaves and flower over only very short periods determinate growth. Phenotypic changes in leaf structure and behavior are important stress responses.
Because of their roles in photosynthesis, leaves or their equivalent are crucial to the survival of a plant. To function, leaves must be exposed to sunlight and air, but this also makes them particularly vulnerable to environmental extremes. Plants have thus evolved various mechanisms that enable them to avoid or mitigate the effects of abiotic extremes to leaves.
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