nectar produced (less than 1 μL to few ml proportional to the nectary parenchyma
volume).
Sugars, proteins, and free amino acids are the three major components of floral
nectar among which sugars are the most dominant [49]. Nectar is highly variable at
any taxonomic scale indicating that plant phylogeny can be a stronger determinant of
nectar composition [50]. But, pollinator type can also shape the composition of
nectar because different pollinators show preferences for solutions of different
viscosity and/or sugar composition [51]. In general, insect pollinated flowers, like
in cherry, produce relatively concentrated nectar.
Secondary metabolites and volatile compounds in flower nectar are appearing in
low level. Compounds belonging to the secondary metabolism are either synthesized
in the nectaries themselves or can also be derived directly from the phloem. They can
have a range of effects on pollinator preference and performance, from fully negative
to positive. More often, these compounds are usually regarded as “toxic compounds”
and are involved in antimicrobial defensive functions, protection from nectar robbers, and pollinator attraction [29, 52]. On the other hand, secondary metabolites can
significantly stimulate bees to feed, while indirectly, pollinators can have benefits
from them by reducing gut pathogen loads [53, 54].
The phenolic compounds in nectar have several roles in attraction and/or repelling honey bees (phenolic compounds can give an astringent taste, thus inhibiting
herbivores) in nutrition of pollinators, in oxidation prevention of other nectar substances, and in providing an aggregate value to honey commercialization by the
certification of the botanical origin [29, 55]. In some cases, nectar constituents may
also help defend the flower against invaders, which allows flower to promote out
crossing and achieve its ultimate goal, and that is to set a fruit/seed [52]. Besides
polyphenolic compounds, some amount of abscisic acid (ABA) can be found. The
role of this plant hormone is the protection of plants in conditions of environmental
stress, especially in reducing the penetration of UV-B ultraviolet radiation [56].
Also, jasmonic acid, its precursors and its derivatives, have been identified as
a hormone that affects the secretion of floral nectar and defense responses [57, 58].
Volatile compounds, important cues that help insects locate flowers, mediate plant
response to pathogen infection, plant-parasitoid signaling in response to herbivory,
and plant-pollinator communication during flowering. Most of the floral fragrance
compounds are terpenoids (most common monoterpenes), simple aromatics, amines,
and hydrocarbons [59].
Amino acids are contributing to the taste of nectar and are important source of
nutrients for animals, especially for those that are exclusively dependent on nectar
for their nutrition, such as butterflies [60]. As it was stated in [34], phenylalanine is
the most abounded one in nectar (which generally has a strong phagostimulatory
effect on honeybees), followed by tyrosine, threonine, histidine, and aspartic acid.
Also, it seems that some amino acids, like asparagines, are avoided by all guilds and
bee families, while glycine-threonine, H-serine, serine, β-alanine, valine, leucine are
bypassed by most bee families. Besides, some level of non-protein amino acids can
be detected in nectar. Those compounds can be toxic and found in seeds which serve
as deterrents to insect feeding. However, β-alanine, ornithine, homoserine, and γ760
M. F. Akšić et al.
volume).
Sugars, proteins, and free amino acids are the three major components of floral
nectar among which sugars are the most dominant [49]. Nectar is highly variable at
any taxonomic scale indicating that plant phylogeny can be a stronger determinant of
nectar composition [50]. But, pollinator type can also shape the composition of
nectar because different pollinators show preferences for solutions of different
viscosity and/or sugar composition [51]. In general, insect pollinated flowers, like
in cherry, produce relatively concentrated nectar.
Secondary metabolites and volatile compounds in flower nectar are appearing in
low level. Compounds belonging to the secondary metabolism are either synthesized
in the nectaries themselves or can also be derived directly from the phloem. They can
have a range of effects on pollinator preference and performance, from fully negative
to positive. More often, these compounds are usually regarded as “toxic compounds”
and are involved in antimicrobial defensive functions, protection from nectar robbers, and pollinator attraction [29, 52]. On the other hand, secondary metabolites can
significantly stimulate bees to feed, while indirectly, pollinators can have benefits
from them by reducing gut pathogen loads [53, 54].
The phenolic compounds in nectar have several roles in attraction and/or repelling honey bees (phenolic compounds can give an astringent taste, thus inhibiting
herbivores) in nutrition of pollinators, in oxidation prevention of other nectar substances, and in providing an aggregate value to honey commercialization by the
certification of the botanical origin [29, 55]. In some cases, nectar constituents may
also help defend the flower against invaders, which allows flower to promote out
crossing and achieve its ultimate goal, and that is to set a fruit/seed [52]. Besides
polyphenolic compounds, some amount of abscisic acid (ABA) can be found. The
role of this plant hormone is the protection of plants in conditions of environmental
stress, especially in reducing the penetration of UV-B ultraviolet radiation [56].
Also, jasmonic acid, its precursors and its derivatives, have been identified as
a hormone that affects the secretion of floral nectar and defense responses [57, 58].
Volatile compounds, important cues that help insects locate flowers, mediate plant
response to pathogen infection, plant-parasitoid signaling in response to herbivory,
and plant-pollinator communication during flowering. Most of the floral fragrance
compounds are terpenoids (most common monoterpenes), simple aromatics, amines,
and hydrocarbons [59].
Amino acids are contributing to the taste of nectar and are important source of
nutrients for animals, especially for those that are exclusively dependent on nectar
for their nutrition, such as butterflies [60]. As it was stated in [34], phenylalanine is
the most abounded one in nectar (which generally has a strong phagostimulatory
effect on honeybees), followed by tyrosine, threonine, histidine, and aspartic acid.
Also, it seems that some amino acids, like asparagines, are avoided by all guilds and
bee families, while glycine-threonine, H-serine, serine, β-alanine, valine, leucine are
bypassed by most bee families. Besides, some level of non-protein amino acids can
be detected in nectar. Those compounds can be toxic and found in seeds which serve
as deterrents to insect feeding. However, β-alanine, ornithine, homoserine, and γ760
M. F. Akšić et al.