aestivum, vincristine in Catharanthus roseus, and polyamines in Helianthus
annuus are some examples of salinity-induced regulation of secondary metabolites
[24, 57, 58]. Similarly a positive correlation between proline accumulation and
salinity tolerance has been reported in tomato [29]. Salt-induced ABA is also a
player in decreasing photosynthesis due to stomatal closure and plant growth
inhibition under salt stress. Furthermore, higher salt concentration in growth
media resulted in accumulation of high levels of terpenoids, phytoalexins, and
zealexins, while lower salt concentration substantially induced the content of
kauralexins in maize roots [56].
2.3
Influence of Drought on Secondary Metabolites
Drought stress is the major abiotic stress that can impact food production across
the world [59]. Active accumulation of compatible solutes and osmoprotectants is
the most common drought-induced metabolic adjustment in majority of the plants
[32, 60]. Besides the osmotic adjustment, reprogramming of plant metabolism also
occurs in drought-stressed plants leading to multiple other changes in plant
secondary chemistry [30, 33]. Drought is known to induce an increase in secondary metabolites such as phenols, saponin, anthocyanin, and flavonoids in several
plant species [30, 34, 35]. For instance, moderate water deficit enhanced
saikosaponins in Bupleurum chinense and salvianolic acid in Salvia miltiorrhiza
roots [31]. The content of glycine betaine and the total alkaloids in C. roseus
plants increased due to drought in comparison with the unstressed control plants
[61]. Drought-induced changes in secondary metabolite composition, including
elevated tocopherol and carotenoid contents, have been associated with
improve photoinhibition tolerance in several plants. Hence, the plants or plant
tissues with anthocyanin or flavonoids are protected from drought [25].
Drought stress is also known to change the ratio of chlorophyll “a” and “b” and
carotenoids [33–35].
Drought stress also increased the production of rosmarinic, ursolic, and oleanolic
acid in Prunella vulgaris and betulinic acid content in Hypericum brasiliense [34,
35 ]. Similarly, the accumulation of alkaloids including narkotine, morphine, and
codeine in P. somniferum was significantly increased due to drought. Drought effects
can be associated with drought severity and vary for different compound classes. For
instance, the total inulin percentage in Cichorium intybus roots increased by mild
drought stress, whereas severe drought stress decreased inulin yield [32]. Another
interesting study suggested more complex and differential regulation of secondary
metabolites synthesized via shikimate and isoprenoid synthesis pathways in eucalypts [62]. In these plants drought had no effect on isoprenoids, monoterpenes, and
sesquiterpenes, while condensed tannins were enhanced, and concentrations of
macrocarpals decreased due to drought [57, 62]. Thus, all these studies underscore
the need of more focused studies on secondary chemistry under various ecotypes and
drought severities [59].
602
P. Santisree et al.
annuus are some examples of salinity-induced regulation of secondary metabolites
[24, 57, 58]. Similarly a positive correlation between proline accumulation and
salinity tolerance has been reported in tomato [29]. Salt-induced ABA is also a
player in decreasing photosynthesis due to stomatal closure and plant growth
inhibition under salt stress. Furthermore, higher salt concentration in growth
media resulted in accumulation of high levels of terpenoids, phytoalexins, and
zealexins, while lower salt concentration substantially induced the content of
kauralexins in maize roots [56].
2.3
Influence of Drought on Secondary Metabolites
Drought stress is the major abiotic stress that can impact food production across
the world [59]. Active accumulation of compatible solutes and osmoprotectants is
the most common drought-induced metabolic adjustment in majority of the plants
[32, 60]. Besides the osmotic adjustment, reprogramming of plant metabolism also
occurs in drought-stressed plants leading to multiple other changes in plant
secondary chemistry [30, 33]. Drought is known to induce an increase in secondary metabolites such as phenols, saponin, anthocyanin, and flavonoids in several
plant species [30, 34, 35]. For instance, moderate water deficit enhanced
saikosaponins in Bupleurum chinense and salvianolic acid in Salvia miltiorrhiza
roots [31]. The content of glycine betaine and the total alkaloids in C. roseus
plants increased due to drought in comparison with the unstressed control plants
[61]. Drought-induced changes in secondary metabolite composition, including
elevated tocopherol and carotenoid contents, have been associated with
improve photoinhibition tolerance in several plants. Hence, the plants or plant
tissues with anthocyanin or flavonoids are protected from drought [25].
Drought stress is also known to change the ratio of chlorophyll “a” and “b” and
carotenoids [33–35].
Drought stress also increased the production of rosmarinic, ursolic, and oleanolic
acid in Prunella vulgaris and betulinic acid content in Hypericum brasiliense [34,
35 ]. Similarly, the accumulation of alkaloids including narkotine, morphine, and
codeine in P. somniferum was significantly increased due to drought. Drought effects
can be associated with drought severity and vary for different compound classes. For
instance, the total inulin percentage in Cichorium intybus roots increased by mild
drought stress, whereas severe drought stress decreased inulin yield [32]. Another
interesting study suggested more complex and differential regulation of secondary
metabolites synthesized via shikimate and isoprenoid synthesis pathways in eucalypts [62]. In these plants drought had no effect on isoprenoids, monoterpenes, and
sesquiterpenes, while condensed tannins were enhanced, and concentrations of
macrocarpals decreased due to drought [57, 62]. Thus, all these studies underscore
the need of more focused studies on secondary chemistry under various ecotypes and
drought severities [59].
602
P. Santisree et al.