are thought to repel some nectar robbers and sometimes potential pollinators
[38]. Flower parts have a strong effect on the fitness of a plant and, thus, are
protected by relatively high amounts of defensive secondary metabolites. Plants’
secondary metabolites are known to be present in good amount in leaves, but some
reports suggest that flowers have more amounts of secondary metabolites in comparison with leaves. Thus, allocation of secondary metabolites should be directed to
the most valuable tissues, hence flowers. Similarly, 1,2-saturated pyrrolizidine
alkaloids (T-and iso-phalaenopsine) that are produced in every tissue of Phalaenopsis orchid hybrids are the highest in the pollinia [39]. Chemical differences between
nectar, pollen, and flowers indicate that plants can regulate these compounds in
specific tissues [40, 41] as can be seen in Delphinium sp. where its nectar, anthers,
corollas, stems, and pollen contain similar alkaloids differing only in their concentration, suggesting a similar origin [42].
Pollinators have the ability to find nectar-rich flower and avoid the toxic nectars.
It is generally seen that they become proficient in avoiding toxic nectar by their past
experiences [43]. There are some examples, like moths learning to avoid quinine by
recognizing its odor in food [44] and bumble bees learning to avoid alkaloids
gelsemine or quinine [45]. Toxicity of nectar is generally associated with the
secondary metabolite concentrations and its combination ratio with the carbohydrate, but still there are some secondary metabolites like cyanogenic glycoside
amygdalin, which are non-detectable by the honey bees even in the sucrose solutions
[46, 47]. Besides all these examples, it is interesting to know that some alkaloids
may optimize pollination service without being beneficial to the pollinator. As
shown by Wright and co-workers [48], caffeine in food affects the perpetuation of
bees returning to the food source.
The presence of secondary metabolites in nectar, apart from helping in deterring
the robbers and herbivores, has other advantages as well. Secondary metabolites
prevent the nectar spoilage from microbes [49] and increase the resistance in
pollinators against parasites and pathogens [50]. Besides the presence of secondary
metabolites in fruits and seeds helps in seed and fruit dispersal by attracting birds and
animals. There are many reports showing induced plant responses to herbivory as a
process of resistance offered by plants. These inductions effect the plant and
pollinator interactions as well. In a study on wild tomato, Solanum peruvianum,
herbivore-induced emission of volatile organic compounds alters pollinator behavior
and consequentially affects plant fitness [51]. Leaf herbivory by Manduca sexta
induced alkaloids in the nectar of Nicotiana tobaccum [52], and leaf damage in
N. sylvestris increased nicotine concentrations in N. sylvestris flowers [53]. It is still
unclear that the presence of secondary metabolites in nectar and flowers, and the
pollinators acquired adaptations toward SMs has been co-evolved? Some studies
suggest that pollinators evolved in response to the secondary metabolites. Furthermore, some studies suggest that pollinators impose selection pressure on plants,
especially on floral traits, so it is rational to say that they might have co-evolved in
some cases [54].
Angiosperm parasitic weeds Orobanche (~28 spp.) and Striga (~100 spp.) pose a
serious threat to crop plants, while Cuscuta is a problem of both trees and crop
1 Co-evolution of Secondary Metabolites During Biological Competition for. . .
9
[38]. Flower parts have a strong effect on the fitness of a plant and, thus, are
protected by relatively high amounts of defensive secondary metabolites. Plants’
secondary metabolites are known to be present in good amount in leaves, but some
reports suggest that flowers have more amounts of secondary metabolites in comparison with leaves. Thus, allocation of secondary metabolites should be directed to
the most valuable tissues, hence flowers. Similarly, 1,2-saturated pyrrolizidine
alkaloids (T-and iso-phalaenopsine) that are produced in every tissue of Phalaenopsis orchid hybrids are the highest in the pollinia [39]. Chemical differences between
nectar, pollen, and flowers indicate that plants can regulate these compounds in
specific tissues [40, 41] as can be seen in Delphinium sp. where its nectar, anthers,
corollas, stems, and pollen contain similar alkaloids differing only in their concentration, suggesting a similar origin [42].
Pollinators have the ability to find nectar-rich flower and avoid the toxic nectars.
It is generally seen that they become proficient in avoiding toxic nectar by their past
experiences [43]. There are some examples, like moths learning to avoid quinine by
recognizing its odor in food [44] and bumble bees learning to avoid alkaloids
gelsemine or quinine [45]. Toxicity of nectar is generally associated with the
secondary metabolite concentrations and its combination ratio with the carbohydrate, but still there are some secondary metabolites like cyanogenic glycoside
amygdalin, which are non-detectable by the honey bees even in the sucrose solutions
[46, 47]. Besides all these examples, it is interesting to know that some alkaloids
may optimize pollination service without being beneficial to the pollinator. As
shown by Wright and co-workers [48], caffeine in food affects the perpetuation of
bees returning to the food source.
The presence of secondary metabolites in nectar, apart from helping in deterring
the robbers and herbivores, has other advantages as well. Secondary metabolites
prevent the nectar spoilage from microbes [49] and increase the resistance in
pollinators against parasites and pathogens [50]. Besides the presence of secondary
metabolites in fruits and seeds helps in seed and fruit dispersal by attracting birds and
animals. There are many reports showing induced plant responses to herbivory as a
process of resistance offered by plants. These inductions effect the plant and
pollinator interactions as well. In a study on wild tomato, Solanum peruvianum,
herbivore-induced emission of volatile organic compounds alters pollinator behavior
and consequentially affects plant fitness [51]. Leaf herbivory by Manduca sexta
induced alkaloids in the nectar of Nicotiana tobaccum [52], and leaf damage in
N. sylvestris increased nicotine concentrations in N. sylvestris flowers [53]. It is still
unclear that the presence of secondary metabolites in nectar and flowers, and the
pollinators acquired adaptations toward SMs has been co-evolved? Some studies
suggest that pollinators evolved in response to the secondary metabolites. Furthermore, some studies suggest that pollinators impose selection pressure on plants,
especially on floral traits, so it is rational to say that they might have co-evolved in
some cases [54].
Angiosperm parasitic weeds Orobanche (~28 spp.) and Striga (~100 spp.) pose a
serious threat to crop plants, while Cuscuta is a problem of both trees and crop
1 Co-evolution of Secondary Metabolites During Biological Competition for. . .
9