Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 35

treatment are common among the three species, and only one-third of responsive metabolites
in L. creticus are not shared with the glycophytes. Interestingly, the changes in the pool sizes
of these metabolites are only marginal [97]. A few changes in the metabolic profile are
extremophile-specific, but most salt-elicited changes in metabolism are similar. Other studies
in glycophytes under salt stress indicate that organic acids and intermediates of the citric acid
cycle tend to decrease [98]. Also in genus Lotus , model species ( L. japonicus , L. filicaulis , and L.
burttii ) and cultivated species ( L. corniculatus , L. glaber , and L. uliginosus ) exhibit consistent
negative correlation in the Cl - levels in the shoots and tolerance to salinity, but metabolic
profiles diverge amongst genotypes; asparagine levels are higher in the more tolerant geno‐
types. These results support the conclusion that Cl - exclusion from the shoots represents a key
physiological mechanism for salt tolerance in legumes; moreover, an increased level of the
osmoprotectant asparagine is typical [99]. In L. japonicus , which has a robust metabolic
response to salt stress, levels of proline and serine, polyolsononitol and pinitol, and myo-
inositol increase [75].
All these studies demonstrate that the metabolic plant response to salinity stress is variable
depending on the genus and species and even the cultivar under consideration. Differential
metabolic rearrangements are in intimate correlation with genetic backgrounds. Furthermore,
the plant physiology in salt stress with time proceeds through a complex metabolic response
including different systematic mechanisms and changes. Inside a salt-stressed plant as a
biological unit, different tissues respond differentially and in some cases the responses are
even contrasting. From comparative ionomics studies, it is evident also that under salinity
stress, differential homeostasis of ions as Cl - , Na + , and K + is correlated with distinct nutritional
changes in extremophile and glycophyte species, even inside the same genus. Noticeable
differences exist between plant species in the way they react to surpass the osmotic pressure
imposed by high soil salt content through mechanisms such as tolerance, efficiency in salt
exclusion, changes in nutrient homeostasis, and osmotic adjustment. From the aforementioned
studies, metabolic markers in the response to high salinity in plants include glycine betaine,
sucrose, asparagine, GABA, malic acid, aspartic acid, and trans -aconitic acid. In legumes,
increases in levels of the amino acids asparagine, proline, and serine are notable as are increases
in polyolsononitol, pinitol, and myo-inositol [75].
8. Plant metabolomics and oxidative stress
An increase in intracellular levels of ROS is a common consequence of adverse growth
conditions. An imbalance between ROS synthesis and scavenging is caused in a manner
independent of the nature of the stress; it is induced by both biotic and abiotic types of stress.
Toxic concentrations of ROS cause severe damage to protein structures, inhibit the activity of
multiple enzymes of important metabolic pathways, and result in oxidation of macromolecules
including lipids and DNA. All these adverse events compromise cellular integrity and may
lead to cell death [100, 101]. Normal cellular metabolic activity also results in ROS generation
under regular growth conditions. Thus, cells sense uncontrolled elevation of ROS and use them
as a signaling mechanism to activate protective responses [102]. In this context plants have