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Plant Stress Physiology Introductory article David Rhodes, Purdue University, West Lafayette, Indiana, USA Article Contents Anna Nadolska-Orczyk, Plant Breeding and Acclimatization Institute,Radzikow, Blonie, Poland . Stress Factors, Their Influence on Plant Metabolism, and Tolerance or Resistance to Stress ‘Stress’ in plants can be defined as any external factor that negatively influences plant . Physiological Correlations between Stress growth, productivity, reproductive capacity or survival. This includes a wide range of Sensitivity/Resistance and Reactions of Different Plants to Stress factors which can be broadly divided into two main categories: abiotic or environmental . Interactions Between Different Stresses in stress factors, and biotic or biological stress factors. Signalling and Metabolism . Integrative Approaches to Understanding Stress Stress Factors, Their Influence on Plant Responses . Ecophysiological Adaptations – Examples Metabolism, and Tolerance or Resistance to Stress Na 1 and Cl 2 inhibition of photosynthesis and protein synthesis, and displacement of Ca2 1 from membranes, Stress factors resulting in altered membrane permeability and K 1 The abiotic stress factors that most commonly influence leakage). Water excesses, or flooding stress, cause anoxia plant performance include deficiencies or excesses of water or hypoxia in the root zone; waterlogging fills soil pores (drought and flooding), extremes of irradiance, excessively and spaces, preventing oxygen diffusion to the root. The low or high temperature, or deficiencies or excesses of failure of waterlogged roots to undergo normal aerobic several nutrients, including macro- and micronutrients, metabolism leads to inhibition of protein synthesis, and 1 2 2 2 inhibition of ion acquisition and transport. This, coupled high salinity (i.e. excesses of Na ,Cl and/or SO4 ), and extremes of soil pH. Abiotic stresses may also include with alterations of water permeability of the roots in mechanical stresses (e.g. wind, hail, mechanical impedance flooded soils, often leads to macronutrient deficiencies in of root growth in compacted soils, and wounding), and the shoot, and ironically, shoot water deficits in response to stresses associated with toxic, manmade chemicals, includ- flooding. ing gaseous pollutants (sulfur dioxide, nitrogen oxides, Because higher plants rely on water loss via transpira- ozone), heavy metals and xenobiotics (e.g. herbicides). tion for leaf cooling, water deficit stress can predispose Biotic stresses include a wide range of plant pathogens plants to greater injury when exposed to high tempera- (bacteria, fungi and viruses) and herbivorous animals. tures. Water deficits prevent evaporative cooling and Because of the enormous diversity of these stresses, this associated heat loss by causing closure of stomates, which article will focus only on the physiology of plant responses dramatically restricts water loss, carbon dioxide fixation, to the abiotic stresses. and hence growth. Chilling stress, and a number of other environmental stresses that disrupt membrane-bound electron transport systems, can predispose plants to high Interaction of stress factors photon fluxes as a result of damage to the photosynthetic machinery. When such stress-injured leaves are exposed to In many cases the abiotic stresses do not occur indepen- high light intensities, this leads to production of toxic dently, and thus the stress environment may involve a active oxygen species, including superoxide, hydrogen complex of interacting stress factors. For example, acid peroxide (H2O2), singlet oxygen and hydroxyl radical. stress may frequently be associated with aluminium Superoxide is formed by oxygen photoreduction by toxicity. Aluminium is abundant in many soils, and under photosystem I in the chloroplast. Superoxide is then 3 1 acidic conditions (pH 5 5.5) the phytotoxic species, Al , dismutated to H2O2. The hydroxyl radical can then form is solubilized to levels that inhibit plant growth. Both by the reaction of H2O2 with superoxide. Singlet oxygen salinity and freezing stresses can induce water deficits; it is forms by energy transfer from excited triplet state well established that a high concentration of dissolved chlorophyll to oxygen. The chloroplast is particularly solutes in the root zone lowers the soil water potential, susceptible to these active oxygen species, whose synthesis creating a situation similar to soil water deficits, and that is enhanced on exposure to excess excitation energy, slow freezing of plant tissue resulting in extracellular ice particularly when carbon fixation is impaired. These active formation will result in cellular dehydration. Consequently oxygen species denature proteins, damage nucleic acids there are some common osmotic stress features of water and cause lipid peroxidation. Injury from these active deficit, salinity and freezing stresses. However, salinity oxygen species is common with damage from pollutants stress involves both osmotic and specific ion effects (e.g. such as ozone, nitrogen oxides and sulfur dioxide, because ENCYCLOPEDIA OF LIFE SCIENCES © 2002, John Wiley & Sons, Ltd. www.els.net 1 Plant Stress Physiology they generate active oxygen and other free radicals, with Facultative/inducible adaptations ultraviolet B (UV-B) radiation stress, and with post-anoxic injury, which occurs when roots are exposed to oxygen Many plant species, when exposed to a sublethal stress after a period of oxygen deficiency (waterlogging). (such as a heat shock, or a period of chilling, or exposure to water deficits), acquire tolerance and/or resistance to a subsequent stress that would be lethal or severely dama- Stress resistance versus tolerance ging to nonpretreated plants. This phenomenon is known It is evident from the brief discussion above that stresses as ‘acclimation’ or ‘hardening’. Acclimation responses are result in cellular injury or damage which frequently result generally distinguished by their stress inducibility from the from impaired or perturbed metabolism. The production inherent, constitutively expressed tolerance or resistance of damaging active oxygen species as a consequence of mechanisms or ‘adaptations’ discussed above. However, impairments in the photosynthetic apparatus is an the terminology is often used loosely in the literature. In excellent example of this. Stress damage or injury is our opinion, metabolic acclimation responses can best be typically referred to as a ‘strain’. By analogy with regarded as ‘facultative metabolic adaptations’. Several engineering terminology, stress resistance 5 stress/strain. examples of facultative metabolic adaptations of plants to The smaller the strain incurred by a specific stress, the different stresses which are proposed to confer increased greater the stress ‘resistance’. The term stress ‘tolerance’ stress resistance/tolerance are summarized in Table 1. implies that the development of stress-induced strains can Asterisks denote that confirmatory evidence for the be tolerated and/or actively repaired during the stress or proposed function(s) have been obtained with mutants following stress relief. and/or transgenic plants. An example of a morphological acclimation to anaero- bic stress exhibited by certain plant species is the production of intracellular gas channels (aerenchyma) which improve aeration of the root. Aerenchyma forma- Physiological Correlations between tion accelerates the transfer of oxygen from the aerial Stress Sensitivity/Resistance and tissues to the oxygen-deficient tissues of the stem base and root; however, in some flooding-tolerant species aerench- Reactions of Different Plants to Stress yma formation is constitutive, occurring in the absence of oxygen deficiency. An example of a facultative metabolic Constitutive adaptations adaptation to anaerobic stress is the coordinated induction Many plants exhibit constitutively expressed traits that are of glycolytic enzymes and enzymes of lactate and ethanol recognized to confer resistance or tolerance to environ- fermentation which allow continued energy production mental stresses; these are frequently referred to as during anoxia or hypoxia (Table 1). Phosphate starvation ‘adaptations’. Examples of morphological adaptations to may lead to a host of facultative metabolic adaptations heat, light and water deficit stresses might include leaf including induction/derepression of phosphate transpor- hairiness and waxiness which influence light absorption ters and alterations of pathways of glycolytic carbon flow and hence heat balance. An example of a combined and mitochondrial respiration to circumvent the adeny- late- and inorganic phosphate (Pi) dependent reactions of anatomical and metabolic stress adaptation is C4 photo- synthesis, which reduces photorespiration, leading to a respiration, facilitating continued respiration in Pi de- higher temperature optimum for photosynthesis and prived plants (Table 1). Osmotic adjustment, the active accumulation of solutes in response to drought or salinity higher water use efficiency in comparison to C3 photo- stress, lowering solute potential, and facilitating main- synthesis. C4 photosynthesis requires spatial separation of distinct metabolic functions between discrete mesophyll tenance of turgor, is an example of a facultative metabolic and bundle-sheath cells of the leaf (see ‘Ecophysiological adaptation to drought and salinity stress (Table 1). The Adaptations – Examples’, below). Constitutive crassula- induction of heat-shock proteins, which may play roles as cean acid metabolism
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