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Since plants are not mobile, they have to endure various abiotic stresses under both natural and agricultural conditions. Among these, drought with its accompanying high temperatures, is gaining more importance due to global warming, which is mainly caused by the greenhouse effect.

The likely increase in the evapotranspiration potential caused by an increase in air and soil temperature brings forth other abiotic stresses such as salinity, high temperature and increased ultraviolet (UV-B) radiation. Abiotic stresses are major constraints on worldwide crop production and can account for up to 70% of the yield loss in crop plants. Salinity alone affects productivity on about 80 million hectares of global arable land.

Abiotic stresses such as drought, salinity, high temperatures, hypoxia conditions, nutrient deficiency, increased UV-B radiation, herbicides, etc., account for more crop productivity losses than any other factor. Twelve percent of total crop yields are lost due to pathogen infection, which is equivalent to 900 million tons worldwide annually. Some of the abiotic stresses affecting plant growth and metabolism are summarized in this paper.


DROUGHT

Especially in arid and semi-arid regions of the world, water is the major limiting factor for plant growth. As an early response to water deficit, leaf area is decreased and the growth of young leaves is inhibited.

Drought stress causes stomata to lose turgor and close to minimize transpiration. Cell division is normally less sensitive, but cell expansion is very sensitive to water deficit. Drought stimulates the production of abscisic acid (ABA) that causes leaves to drop.

Growth of shallow roots is inhibited and seedling growth is reduced under water deficient conditions. Water stress usually affects both stomatal conductance and photosynthetic activity as a result of which superoxide radicals are produced and photo-oxidation of chlorophyll (leaf bleaching symptoms) and thereby severe inhibition of photosynthesis occur. Drought also reduces enzyme activity e.g.rubisco, acid invertase, nitrate reductase, nitrite reductase etc.

Adaptive responses of plants against drought include cell wall hardening, reduced plant size and growth rate, ABA production and thus arrested growth, stomatal closure and reproduction failure and accumulation of compatible solutes by the cells to provide osmotic adjustment.


SALINITY

While drought and salinity, the major constraints restricting growth and development of higher plants, are quite different environmental conditions, they are frequently discussed together because they cause similar physiological problems for plants. In both stresses, water tends to be lost from plant cells.

In addition, salt stress involves both osmotic stress and ionic stress, resulting from high concentrations of potentially toxic salt ions within plant cells. Plants protect themselves from salt stress by excluding toxic ions from the leaves, sequestering them into the vacuoles for maintenance of turgor and synthesizing compounds that aid in keeping water inside the cell, all of which also occur under osmotic stress conditions (Hasegawa et al., 2000).

These nontoxic compounds increase the osmotic potential of the cell and allow normal metabolic processes to continue. Such compounds include mannitol (a sugar alcohol), polyos, sugars, glycinebetaine, sorbitol and proline, which do not intrude upon normal metabolic processes (Hayashi and Murata, 1998) and provide osmotic adjustment by generating a more negative water potential, thereby helping to maintain water movement into the leaf and consequently, leaf turgor.


HIGH TEMPERATURE

Excessive heat caused by high leaf temperature and water deficit can denature enzymes and disrupt metabolism. Evaporation through leaves may lower the temperature of leaves 3-10°C below the ambient temperature.

In oleander (Nerium oloeander), acclimation to high temperatures is related to a greater degree of saturation of fatty acids in membrane lipids, which makes the membranes less fluid (Raison et al., 1982). Above certain temperature (e.g. 40°C in most temperate plants), plants begin to synthesize large quantities of special proteins called ‘heat shock proteins’. It is suspected that these heat shock proteins like chaperone proteins, help to prevent the denaturing of enzymes by creating a scaffold around the enzyme.


LOW TEMPERATURE

Damage to plant cells occurs when the water in the cell walls and intercellular spaces freezes. Plants respond to cold stress by altering the lipid composition of plasma membranes, e.g. more unsaturated fatty acids are incorporated into the membrane to maintain fluidity. The lower the water potential in these areas the more water leaves the cells, resulting in an increase in the concentration of solutes and lowering the freezing point of the cytosol.

Plants in cold regions increase the concentration of sugars in their leaves before winter. Sugars are tolerated in higher concentrations than many ionic salts. Chilling damage can be minimized if plants are first hardened (acclimated) by gradual exposure to cool, but noninjurious, temperatures.

During acclimation to cool temperatures the activity of desaturase enzymes increases and the proportion of unsaturated lipids rises (Palta et al., 1993).

Desaturation of fatty acids thus provides some protection against damage from chilling.


FLOODING

In flooded soils, the air spaces are filled with water and therefore lack sufficient oxygen to sustain the life of roots. Oxygen deprivation causes the production of ethylene, which causes the cells in the root cortex to undergo apoptosis. This creates air tubes that allow oxygen to reach the flooded roots.

Oxygen shortage in roots, like water deficit or high concentrations of salt, can stimulate ABA production and movement of ABA to leaves, resulting in defoliation.


OXIDATIVE STRESS

The role of reactive oxygen species (ROS) such as superoxide (O₂¯), hydrogen peroxide (H2O2) hydroxyl radical (·OH) and singlet oxygen (¹O₂) during biotic and abiotic stress responses has been well documented. In spite of the fact that ROS is detrimental to plants, causing lipid peroxidation, enzyme inactivation, and oxidative damage to DNA, it is a key event causing the induction of systemic acquired resistance (SAR) in the hypersensitive reaction (HR).

Under salt or drought stress, stomatal closure limits CO2 fixation and NADP+ regeneration by the Calvin cycle in the chloroplasts thus enhancing ROS formation. The primary source of H₂O₂ in the chloroplasts is thought to be the Mehler Reaction (Heldt, 1997):



In the presence of metal ions such as iron (Fe⁺², Fe⁺³), superoxide and hydrogen peroxide may react via Haber-Weiss reaction to produce hydroxyl radical, which is highly toxic to biological molecules (Bowler et al., 1992):



To control the level of ROS and to protect cells against a variety of external stimuli, including biotic and abiotic stresses, plants have a set of defense mechanisms. These mechanisms involve multicomponent response systems such as induction of defense genes, induction of systemic acquired resistance, production of pathogenesis related (PR) proteins, accumulation of stress metabolites, enhancement in the activity of enzymes scavenging ROS (superoxide dismutase, catalase, peroxidases, ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase) and a network of low molecular mass antioxidants (ascorbate, glutathione, phenolic compounds, tocopherols).

Reinforcement of cell wall and cuticle are other mechanisms of defense (Breusegem et al., 2001; De Gara et al., 2003). There are many studies showing that under abiotic stress conditions, which induce an overproduction of ROS in cells, plant resistance is achieved by increasing the activity of ROS-detoxifying enzymes or the biosynthesis or regeneration of antioxidant metabolites (Cakmak et al., 1993; Scandalios 1993; Acar et al., 2001; Bor et al., 2003). Therefore, it seems that alteration in the expression/activity of ROSdetoxifying enzymes could also be a key step in the activation of phytopathogen defense.

Reactive oxygen species are thought to be an important element of the HR regulation scheme, together with other secondary messengers like salicylic acid (SA), ethylene, jasmonic acid (JA) and nitric oxide (NO). Some of these events contribute to the limitation of pathogen spread within the infected tissue, while other events, e.g. ion fluxes or changes in protein phosphorylation state, additionally serve as a signal initiating transcription-dependent part of the local response, as well as the starting point for the transduction of systemic signals to distant parts of the plant (Talarczyk and Hennig, 2001).

The development of SAR is associated with the expression of a series of genes, including those encoding pathogenesis-related (PR) proteins and phytoalexins, accumulation of ROS, rapid alteration in cell walls and enhanced activity of various defense related enzymes such as peroxidases, chitinases and ß(1,3) glucanases.


The plant activators

When pathogens attack plants, they respond by producing signal molecules including SA, systemin, JA and ethylene and the coordinate-expression of a set of genes, many of which encode pathogenesisrelated proteins (PR). This acquired resistance (SAR) often results in an enhanced capacity for defensegene activation not only at the site of initial infection, but also in distal non-inoculated tissues, thereby protecting the whole plant from secondary pathogen attack.

Plant activators are agents that can effectively ‘switch on’ these plant defense mechanisms before the plant is under attack. Once activated, a plant can naturally protect itself against a broad spectrum of pathogens. In addition, these plant pathways enhance plant vigor and stress tolerance, and increase crop yield and quality by increasing nutrient uptake and photosynthesis within the plant.


NATURALLY INDUCED SAR VERSUS ACTIVATED SAR

In naturally induced SAR the disease continues to develop before SAR becomes fully effective, with the first damage occurring under field conditions.

Natural SAR only occurs sporadically and not uniformly across the field. With the application of a Plant Activator, the plant’s defense is activated before the expected onset of disease. The Plant Activator induces SAR in the whole field and not only individual plants, offering a long-lasting protection against a variety of pathogens in different crops.

References

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