Plants are continuously affected by a variety of environmental factors. Whereas biotic environmental factors are other organisms such as sym-bionts, parasites, pathogens, herbivores, and competitors, abiotic factors include parameters and resources which determine plant growth like temperature, relative humidity, light, availability of water, mineral nutrients, and CO2 , as well as wind, ionizing radiation, or pollutants (Schulze et al. 2002). The effect each abiotic factor has on the plant depends on its quantity or intensity. For optimal growth, the plant requires a certain quantity of each abiotic environmental factor. Any deviation from such optimal external conditions, that is, an excess or deficit in the chemical or physical environment, is regarded as abiotic stress and adversely affects plant growth, development, and/or productivity (Bray et al. 2000). Abiotic stress factors include, for example, extreme temperatures (heat, cold, and freezing), too high or too low irradiation, water logging, drought, inadequate mineral nutrients in the soil, and excessive soil salinity. As especially drought and salt stress are becoming more and more serious threats to agriculture and the natural status of the environment, this chapter will focus on these stress factors. They are recurring features of nearly all the world's climatic regions since various critical environmental threats with global implications have linkages to water crises (Gleick 1994, 1998, 2000). These threats are collaterally catalyzed by global climate change and population growth.
The latest scientific data confirm that the earth's climate is rapidly changing. Due to rising concentrations of CO2 and other atmospheric trace gases, global temperatures have increased by about 1°C over the course of the last century, and will likely rise even more rapidly in coming decades (IPCC 2007). Scientists predict that temperatures could rise by another 3-9°C by the end of the century with far-reaching effects. Increased drought and salinization of arable land are expected to have devastating global effects (Wang et al. 2003b) . Abiotic stress is already the primary reason of crop loss worldwide, reducing average yields for most major crop plants by more than 50% (Bray et al. 2000; Wang et al. 2003b). It will soon become even more severe as desertification will further increase and the current amount of annual loss of arable area may double by the end of the century because of global warming (Evans 2005; Vinocur and Altman 2005) . Simultaneously, rapid population growth increasingly generates pressure on existing cultivated land and other resources (Ericson et al. 1999) . Population migration to those arid and semiarid areas increases the problems of water shortage and worsens the situation of land degradation in the destination, and in turn causes severe problems of poverty, social instability, and population health threats (Moench 2002). Water scarcity and desertification could critically undermine efforts for sustainable development, introducing new threats to human health, ecosystems, and national economies of various countries. Therefore, solutions to these problems are desperately needed, such as the improvement of salt and drought tolerance of crops, which in turn requires a detailed knowledge about salt and drought tolerance mechanisms in plants.
The viability of plants in both dry and saline habitats depends on their ability to cope with (I) water deficit due to a low water potential of the soil and (II) restriction of CO2 uptake. Plants growing on saline soils are additionally confronted with (III) ion toxicity and nutrient imbalance.
Water deficit (I) causes detrimental changes in cellular components because the biologically active conformation and thus the correct functioning of proteins and biomembranes depends on an intact hydration shell. As a consequence, severe osmotic stress can lead to an impairment of amino acid synthesis, protein metabolism, the dark reaction of photosynthesis or respiration and can cause the breakdown of the osmotic system of the cell (Larcher 2001; Schulze et al. 2002). Water deficit can be counteracted by compatible solutes, organic compounds which are highly soluble and do not interfere with cellular metabolism. They serve as a means for osmotic adjustment and also function as chaperons by attaching to proteins and membranes, thus preventing their denatur-ation. This protective function of compatible solutes can also alleviate ion specific effects of salt stress caused by ion toxicity and ion imbalance such as the precipitation of proteins due to changes in charge or the destruction of membranes caused by alterations of the membrane potential.
Regarding the restriction of CO2 uptake (II), the negative effects of osmotic stress described earlier force plants to minimize water loss; growth depends on the ability to find the best tradeoff between a low transpiration and a high net photo-synthetic rate (Koyro 2006). However, various plant species show a clearly reduced assimilation rate under osmotic stress conditions due to sto-matal closure (Huchzermeyer and Koyro 2005). A consequence can be an excessive production of reactive oxygen species (ROS) which are highly destructive to lipids, nucleic acids, and proteins (Kant et al. 2006; Turkan and Demiral 2009; Geissler et al. 2010). However, generated ROS can be scavenged by the antioxidative system which includes nonenzymatic antioxidants and antioxidative enzymes (Blokhina et al. 2003).
Ion toxicity (III) on saline habitats is caused by ion specific effects on membranes and proteins: On the one hand, changes of the ionic milieu lead to alterations of the membrane potential and thus to a destruction of biomembranes (Schulze et al. 2002). On the other hand, the hydration and charge of proteins are negatively influenced, so that their precipitation is promoted, but their activity is reduced (Kreeb 1996). These effects of salt stress can be alleviated by the protective chaperone function of compatible solutes, similarly as explained above for osmotic stress.
When looking at drought and salt tolerance of plants in the face of global climate change, another important aspect should be considered: Compared to salinity and drought, elevated atmospheric CO2 concentrations have contrary effects on plants: They often improve photosynthesis while reducing stomatal resistance in C3 plants, thus increasing water use efficiency, but decreasing photorespiration and oxidative stress (Urban 2003; Kirschbaum 2004; Rogers et al. 2004). Furthermore, more energy can be provided for energy-dependent tolerance mechanisms such as the synthesis of compatible solutes and antioxi-dants. Therefore, the salt and drought tolerance and the productivity of these plants can be enhanced under elevated CO2 (Ball and Munns 1992; Wullschleger et al. 2002; Urban 2003), increasing their future suitability as crops. Against the background described earlier, this review uncovers how compatible solutes and antioxidants alleviate environmental stress, especially drought and salt stress, and the role elevated CO2 concentrations can play in this context.
2 Compatible Solutes Which Can Prevent Detrimental Changes Under Environmental Stress
Severe osmotic stress can cause detrimental changes in cellular components. The best characterized biochemical response of plant cells to osmotic stress is the accumulation of high concentrations of either organic ions or other low
Fig. 1.1 Chemical structure of some important compatible solutes in plants
Fig. 1.1 Chemical structure of some important compatible solutes in plants
molecular weight organic solutes termed compatible solutes. These compounds are highly soluble in water, electrically neutral in the physiological pH range, and noninhibitory to enzymes even at high concentrations, so that they do not interfere with essential metabolic (enzymatic) reactions (Rhodes et al. 2002). The structure of some important compatible solutes is shown in Fig. 1.1.
Organic solutes play a crucial role in higher plants grown under dry or saline conditions. However, their relative contribution varies among species, cultivars, and even between different compartments within the same plant (Ashraf and Harris 2004). A wide range of metabolites which can prevent these detrimental changes in cellular components have been identified, including mono-, di-, oligo-, and polysaccharides (glucose, fructose, sucrose, trehalose, raffinose, and fruc-tans), sugar alcohols (mannitol, glycerol, and methylated inositols), quaternary amino acid derivatives (Pro, GB, b-alaninebetaine and pro-linebetaine), tertiary amines (1,4,5,6-tetrahydro-2-mehyl-4-carboxyl pyrimidine), and sulfonium compounds (choline-O-sulphate, dimethylsul-phoniopropionate) (Flowers and Colmer 2008; Vinocur and Altman 2005) . The primary function of compatible solutes is to reduce water potential, to maintain turgescent cells, and to ensure balanced water relations (Wang et al. 2003a).
In addition, high concentration of compatible solutes exists primarily in the cytosol to balance the low water potentials achieved by high apo-plasmic and vacuolar Na+ and Cl- concentration (Turkan and Demiral 2009). Recent studies indicate that compatible osmolytes also protect sub-cellular structures and mitigate oxidative damage caused by free radicals produced in response to salt stress (Slama et al. 2008; Smirnoff and Cumbes 1989). In many halophytes, organic osmolytes such as Pro or GB accumulate at suitably high concentrations to create osmotic potentials even below 0.1 MPa. In contrast to halophytes, in many glycophytes the concentrations of compatible solutes do not seem to be high enough to generate sufficiently low osmotic potentials (Turkan and Demiral 2009) . This difference between halophytes and glycophytes can be used as an early indicator for salt resistance. Therefore, in the next chapters, the most important compatible solutes are described in detail.
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