2.1
Influence of Temperature Stress on Secondary Metabolites
Both low and high temperatures effect the metabolic process in plants. Plants often
face challenges with high and low temperatures. While high temperatures induce
premature leaf senescence and reduce membrane integrity, the rate of photosynthesis and biomass production in plants and low temperature leads to osmotic
injury, desiccation, oxidative stresses, etc. [27, 38, 39, 41, 43]. In order to
maximize their temperature tolerance, plant species adjust the metabolism to either
increase or decrease the secondary metabolites. Low temperature induces the
synthesis of several types of cryoprotectant compounds including nitrogenous
compounds like proline, glycine and betaine; sugar alcohols like sorbitol, ribitol,
and inositol; soluble sugars like saccharose, raffinose, stachyose, and trehalose;
and low molecular weight to maintain the osmotic balance [53]. Leaves of wheat
and alfalfa accumulate putrescine and spermidine when exposed to a low-temperature stress. Temperature stress also modulates alkaloid and phenolic compound
production in several plant species [40, 41, 45]. For instance, cold acclimation in
apple tree was reported to be associated with a marked increase in the accumulation of chlorogenic acid. In several plants, the enhanced phenolic production
in turn results in the cell wall lignification or suberinization [54]. Similarly,
higher levels of phenolic acid, anthocyanin, flavones, and antioxidant capacities
were observed in strawberry, sugarcane, and lettuce when grown under elevated
temperatures [54]. In addition, the total phenol level and especially the geneistin
levels were observed highest after cold temperature treatment in soybean roots
[55]. However, the low temperature reduced the accumulation of alkaloids
such as morphine and benzylisoquinoline in Papaver somniferum [44, 45]. The
anthocyanin and flavonoid biosynthesis was also promoted by low temperature in
Zea mays seedlings and leaves of A. thaliana, Petunia hybrid, and Rosa hybrid
[55, 56]. It was further supported by the observed increase in the transcript
accumulation of phenylpropanoid pathway genes including phenylalanine ammonia-lyase and chalcone synthase in Arabidopsis. Conversely, anthocyanin and
carotenoid accumulation was reduced by high temperature in several species
including Vitis vinifera and in Brassicaceae. This inhibition was partly
attributed to the pigment degradation and reduced gene transcription under
elevated temperature [56].
2.2
Influence of Salt on Secondary Metabolites
Salt stress is a global problem limiting agricultural production throughout the
world [56]. Salt stress lead to cellular dehydration, ionic and osmotic stress in
plants that subsequently results in accumulation or decrease of specific secondary
metabolites. Salt stress is known to either induce or reduce the production of
secondary metabolites to maximize the tolerance in plants [21, 23, 29]. Increased
production of anthocyanins in Vitis vinifera cultures, polyphenols in Aegiceras
corniculatum, tropane alkaloid in Datura innoxia, glycine betaine in Triticum
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
601
Influence of Temperature Stress on Secondary Metabolites
Both low and high temperatures effect the metabolic process in plants. Plants often
face challenges with high and low temperatures. While high temperatures induce
premature leaf senescence and reduce membrane integrity, the rate of photosynthesis and biomass production in plants and low temperature leads to osmotic
injury, desiccation, oxidative stresses, etc. [27, 38, 39, 41, 43]. In order to
maximize their temperature tolerance, plant species adjust the metabolism to either
increase or decrease the secondary metabolites. Low temperature induces the
synthesis of several types of cryoprotectant compounds including nitrogenous
compounds like proline, glycine and betaine; sugar alcohols like sorbitol, ribitol,
and inositol; soluble sugars like saccharose, raffinose, stachyose, and trehalose;
and low molecular weight to maintain the osmotic balance [53]. Leaves of wheat
and alfalfa accumulate putrescine and spermidine when exposed to a low-temperature stress. Temperature stress also modulates alkaloid and phenolic compound
production in several plant species [40, 41, 45]. For instance, cold acclimation in
apple tree was reported to be associated with a marked increase in the accumulation of chlorogenic acid. In several plants, the enhanced phenolic production
in turn results in the cell wall lignification or suberinization [54]. Similarly,
higher levels of phenolic acid, anthocyanin, flavones, and antioxidant capacities
were observed in strawberry, sugarcane, and lettuce when grown under elevated
temperatures [54]. In addition, the total phenol level and especially the geneistin
levels were observed highest after cold temperature treatment in soybean roots
[55]. However, the low temperature reduced the accumulation of alkaloids
such as morphine and benzylisoquinoline in Papaver somniferum [44, 45]. The
anthocyanin and flavonoid biosynthesis was also promoted by low temperature in
Zea mays seedlings and leaves of A. thaliana, Petunia hybrid, and Rosa hybrid
[55, 56]. It was further supported by the observed increase in the transcript
accumulation of phenylpropanoid pathway genes including phenylalanine ammonia-lyase and chalcone synthase in Arabidopsis. Conversely, anthocyanin and
carotenoid accumulation was reduced by high temperature in several species
including Vitis vinifera and in Brassicaceae. This inhibition was partly
attributed to the pigment degradation and reduced gene transcription under
elevated temperature [56].
2.2
Influence of Salt on Secondary Metabolites
Salt stress is a global problem limiting agricultural production throughout the
world [56]. Salt stress lead to cellular dehydration, ionic and osmotic stress in
plants that subsequently results in accumulation or decrease of specific secondary
metabolites. Salt stress is known to either induce or reduce the production of
secondary metabolites to maximize the tolerance in plants [21, 23, 29]. Increased
production of anthocyanins in Vitis vinifera cultures, polyphenols in Aegiceras
corniculatum, tropane alkaloid in Datura innoxia, glycine betaine in Triticum
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
601