2.4
Influence of Light on Secondary Metabolites
Light is an important physical factor that can affect the growth and metabolite
production [46]. Light is a natural elicitor for many secondary metabolites such as
gingerol and zingiberene production in Zingiber officinale cultures, foliar tannins,
and a number of phenolic glycosides [47]. High light irradiation has seen to induce
anthocyanin production in cell suspension cultures of Perilla frutescens, apples, and
light-colored sweet cherry [46, 62]. The effect of light was also evidenced from
the digitoxin accumulation in Digitalis purpurea L., enhanced ginsenoside contents
in American ginseng plants, and increased artemisinin production in hairy root
cultures of Artemisia annua [62]. Light not only has stimulatory effect on the
formation of secondary metabolites, including flavonoid and anthocyanins, but
also influences the secretion mechanism of secondary products [47, 49, 62]. It
has been reported that photoperiod also affect the secondary metabolite content in
some plants such as Hypericum perforatum, in which maximal production of
metabolites occurs at flowering stage. Similarly, blue light has stimulatory effect
on the vanillin content in Vanilla planifolia [46].
Among various physical variables, ultraviolet (UV) irradiation was considered to
be the major inducer of secondary metabolites in several plant species including
peanut, rice, maize, and basil [51, 52]. UV-B has been seen to increase flavonoids in
barley, several Passiflora species [50]. UV-B significantly increased the quercetin
concentration in F. esculentum [51] and kaempferol in Populus trichocarpa leaves
[52]. In rice, UV-tolerant cultivar accumulated more C-glycosylflavones compared
to susceptible cultivar. Increased UV-B exposure stimulates the total phenolic
content as phenolics offer UV protection in plants. Similar to UV-B, UV-C irradiation also is shown to be the stimulus for phenylpropanoid pathway-derived compounds and flavonoid synthesis [52].
2.5
Influence of Heavy Metal on Secondary Metabolites
Heavy metal contamination in soil, air, and water may alter the chemical and
metabolite composition of plants leading to poor production and quality [15]. Metals
such as aluminum, cadmium, lanthanum, nickel, europium, and silver are known
to influence secondary metabolite production due to either inactivation or stimulation of enzymes involved in their production [36, 37]. For instance, metal ions in
the growth medium regulate the anthocyanin biosynthesis by inhibiting activity of
PAL [15]. Cu
2+ and Cd
2+ have been shown to induce secondary metabolites such as
shikonin, digitalin, and betalains [63]. However, combined Cd and Cu treatment
reduced the production of total phenolics, flavonoids, saponin, and overall medicinal
properties due to the inhibition of PAL activity in G. procumbens [15, 63]. Increases
in heavy metal-induced secondary metabolite biosynthesis also result from increased
synthesis of precursors [64]. At times heavy metal-induced stress activates the
transcription of the genes encoding the secondary metabolites synthesis which
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
603
Influence of Light on Secondary Metabolites
Light is an important physical factor that can affect the growth and metabolite
production [46]. Light is a natural elicitor for many secondary metabolites such as
gingerol and zingiberene production in Zingiber officinale cultures, foliar tannins,
and a number of phenolic glycosides [47]. High light irradiation has seen to induce
anthocyanin production in cell suspension cultures of Perilla frutescens, apples, and
light-colored sweet cherry [46, 62]. The effect of light was also evidenced from
the digitoxin accumulation in Digitalis purpurea L., enhanced ginsenoside contents
in American ginseng plants, and increased artemisinin production in hairy root
cultures of Artemisia annua [62]. Light not only has stimulatory effect on the
formation of secondary metabolites, including flavonoid and anthocyanins, but
also influences the secretion mechanism of secondary products [47, 49, 62]. It
has been reported that photoperiod also affect the secondary metabolite content in
some plants such as Hypericum perforatum, in which maximal production of
metabolites occurs at flowering stage. Similarly, blue light has stimulatory effect
on the vanillin content in Vanilla planifolia [46].
Among various physical variables, ultraviolet (UV) irradiation was considered to
be the major inducer of secondary metabolites in several plant species including
peanut, rice, maize, and basil [51, 52]. UV-B has been seen to increase flavonoids in
barley, several Passiflora species [50]. UV-B significantly increased the quercetin
concentration in F. esculentum [51] and kaempferol in Populus trichocarpa leaves
[52]. In rice, UV-tolerant cultivar accumulated more C-glycosylflavones compared
to susceptible cultivar. Increased UV-B exposure stimulates the total phenolic
content as phenolics offer UV protection in plants. Similar to UV-B, UV-C irradiation also is shown to be the stimulus for phenylpropanoid pathway-derived compounds and flavonoid synthesis [52].
2.5
Influence of Heavy Metal on Secondary Metabolites
Heavy metal contamination in soil, air, and water may alter the chemical and
metabolite composition of plants leading to poor production and quality [15]. Metals
such as aluminum, cadmium, lanthanum, nickel, europium, and silver are known
to influence secondary metabolite production due to either inactivation or stimulation of enzymes involved in their production [36, 37]. For instance, metal ions in
the growth medium regulate the anthocyanin biosynthesis by inhibiting activity of
PAL [15]. Cu
2+ and Cd
2+ have been shown to induce secondary metabolites such as
shikonin, digitalin, and betalains [63]. However, combined Cd and Cu treatment
reduced the production of total phenolics, flavonoids, saponin, and overall medicinal
properties due to the inhibition of PAL activity in G. procumbens [15, 63]. Increases
in heavy metal-induced secondary metabolite biosynthesis also result from increased
synthesis of precursors [64]. At times heavy metal-induced stress activates the
transcription of the genes encoding the secondary metabolites synthesis which
25 Nitric Oxide as a Signal in Inducing Secondary Metabolites During Plant Stress
603