Plant Growth And Survival During Moderate And Severe Salinity Stress

Excess salinity in the soil solution to which plant roots are exposed can be derived from geochemical sources, sea water infiltration of coastal ground waters, sea water salts in wind and rain, excess fertilizer application to soils or supplementary irrigation with salt-containing irrigation waters (the water evaporates leaving salts to accumulate in the soil). Excessive accumulation of salts in the rhizosphere can lead to growth inhibition, leaf necrosis, accelerated onset of senescence, wilting and death. Different physiological mechanisms can be involved. An osmotic mechanism involves the build-up of salts in the rhizosphere or in the small volume of fluids in the apoplastic cell wall compartments. This leads to more negative water potentials which can decrease or even reverse the inwardly directed water potential gradients responsible for water uptake, turgor maintenance and cell expansion (see Eq. (1)). Decreases in water uptake can be gradually reversed, in the case of cells by cytoplasmic accumulation of additional solutes through the process of osmotic adjustment (Evlagon et al., 1990; Munns and Tester, 2008; Neumann et al., 1988) or, in the case of root water uptake, by the development of more negative xylem water potentials. However, growth of maize seedling roots and leaves (but not of bean or rice leaves cf. Lu and Neumann, 1999; Neumann et al., 1988), may continue to be reduced, by parallel salinity-induced reductions in the physical extensibility of the expanding cell walls, even after full osmotic adjustment (cf. Neumann, 1993; Neumann et al., 1994). Interestingly, reduced cell wall extensibility (sometimes termed 'wall stiffening') induced by salinity in growing maize leaves can also be induced by exposure of maize seedling roots to osmotic stress alone; thus, toxic effects of sodium or other ions are not necessarily involved. Instead, the regulatory involvement of root to leaf hydraulic signals in initiating decreases in wall extensibility of growing tissues (and stomatal closure) has been postulated (e. g., Chazen and Neumann, 1994; Chazen et al., 1995; Christmann et al., 2007). Excessive salinity in the rhizosphere can also cause hydraulic limitations to leaf growth by inducing regulated decreases in root hydraulic conductivity (Azaizeh and Steudle, 1991; Chazen et al., 1995; Evlagon et al., 1990; Lu and Neumann, 1999). Thus, ongoing research into genomic and epigenetic changes that regulate either cell wall mechanics or plant hydraulics may provide useful approaches for potentially limiting the plant growth inhibition that occurs during water and salinity stress.

Another mechanism involved in adverse plant-responses to salinity is associated with excessive, concentration-dependent uptake of salts into affected cells. This can lead to toxic symptoms associated with ionic and hormonal imbalances (Munns and Tester, 2008). The toxicity associated with excess salt accumulation in the cytoplasm has also been mechanistically related to increased generation of reactive oxygen species (ROS). Increased levels of ROS, aside of potential signaling roles, can have damaging effects on essential cellular components such as membranes, proteins and nucleic acids (see reviews by Halliwell and Gutteridge, 1989; Miller et al., 2010). Endogenous or salt-induced increases in levels of antioxidant enzymes and their associated substrates may more or less successfully mitigate the potentially adverse effects of excessive ROS accumulation. For example, a clear correlation was revealed between relatively high endogenous levels of anti-oxidant activity (enzymes and substrates) in the leaves and roots of a salt-resistant wild tomato (Lycopersicon penellii) and lower levels in a much less salt-resistant cultivated tomato (Shalata and Tal, 1998; Shalata et al., 2001). Shalata and Neumann (2001) showed that an exogenous supply of ascorbic acid, a water soluble antioxidant substrate found in plants and also known as vitamin C in humans, could mitigate increases in lipid peroxidation caused by highly stressful root exposures of whole, transpiring, tomato seedlings to 300 mM NaCl for up to 9 h. The 9-h salt treatment rapidly and uniformly induced a complete wilting of the shoots, and in the absence of supplementary ascorbic acid, the 9-h treatment was 100% lethal. However, supplementary supplies of ascorbic acid via the roots facilitated a remarkable recovery from wilting and a continuation of apparently normal growth in circa 50% of the wilted seedlings following a return to non-saline root media. This remedial effect was not obtained when roots were supplied equivalent concentrations of other small organic molecules without equivalent antioxidant activity. Subsequently, several studies have provided additional evidence for a potentially protective role of increased levels of either endogenous or exogenous ascorbic acid, in the apoplast as well as the cytoplasm of salinised plants (cf. Athar et al., 2008; Hemavathi et al., 2009; Huang et al., 2005; Yamamoto et al., 2005). However, the practical utility of chemical treatments with exogenous antioxidants, or of using antioxidant traits (among others) in breeding for salinity tolerance, remains to be demonstrated under variable field conditions.

Most attempts at improving the resistance of crop plants to salinity stress have rightly focused in recent years on genomic approaches aimed at discovering and utilising genes associated with two vital physiological parameters, that is, salt exclusion and growth maintenance (see reviews by Munns, 1993; Neumann, 1997). The introduction into agricultural practice of genetically engineered crop varieties can, however, be a very prolonged and difficult process even when compared with the time required to introduce new varieties produced by more traditional breeding approaches (cf. Potrykus, 2010; Yang et al., 2010). Fortunately, conventional breeding for enhanced ability to maintain growth and exclude sodium ions can now be accelerated by use of genetic marker technology and appears to be resulting in more salt-resistant varieties of major crop species such as maize, rice and wheat (Schubert et al., 2009; personal communications by Rana Munns, CSIRO, Australia and Glen Gregorio, IRRI, Philippines). The process of breeding more resistant varieties may be further accelerated by the recent introduction of industrialised processes for mass characterisation and selection of individuals showing desirable phenotypic traits resulting from phenotypic plasticity in populations of isogenic lines subjected to different environmental challenges (Munns and Tester, 2008; Nicotra and Davidson, 2010). Moreover, the realisation that inheritable epigenetic changes, as well as genomic changes are involved in evolutionary selection processes may have important new implications for plant breeding. For example, Hauben et al. (2009) give impressive examples of the potential benefits associated with the exploitation of apparently stable epigenetic variability in breeding programs. They showed that respiratory energy-use efficiency is an important epigenetically regulated factor in determining seed yield in canola (Brassica napus). Thus, individual plants in an isogenic canola population and their self-fertilised progenies were recursively selected for respiration intensity and populations with distinct physiological and agronomical characteristics, including increased stress resistance, were isolated. Most importantly, this apparently simple approach appeared to facilitate further improvements in the already high yield potentials of existing commercial hybrids.

0 0

Post a comment