respectively. As expected, fructose, glucose, and sucrose were found to be the major
constituents of all investigated “Oblačinska” sour cherry nectar samples [64]. Based
on the total sugar content found in nectar, certain clones have been singled out as the
most concentrated (up to 97.6 mg/mL), while the others had dilute nectars (23 mg/
mL of sugars). Averagely fructose, glucose, and sucrose amounted 36.8%, 28.9%,
and 30.9% of the total content of all carbohydrates, respectively, and this was in line
with the other results [47]. The ranking based on fructose content takes into
consideration human sensation of taste. Also, fructose in the concentration range
from 15 mg/mL to 60 mg/mL in 14 sour cherry cultivars was reported [41], while in
“Oblačinska” sour cherry nectars fructose content was up to 34 mg/mL [64].
On the basis of the sucrose/(glucose + fructose) quotient [20], the nectar of
“Újfehértói fürtös” belonged to the sucrose-rich group each year, like the majority
of sour cherry cultivars, whereas the secretory product of “Pándy 48” could be
classified into the sucrose-dominant category in one of the seasons [46]. According
to the proposed quotient, one “Oblačinska” sour cherry clone was hexose dominant
[S/(G + F) < 0.1], four clones were hexose rich [S/(G + F) = 0.1–0.49], while other
11 were sucrose rich [S/(G + F) = 0.5–0.99] [64]. These results are in accordance
with some previous results [46]. Proportions of sucrose over fructose and glucose
have been linked with different classes of pollinators and found to be important in
plant-mutualism interactions [75].
The sucrose-dominant nectar composition of 45 species belonging to tribe
Sinningieae (Gesneriaceae) was also documented [76]. Several authors have
suggested that sucrose-rich nectar is mostly found in flowers pollinated by insects
with long mouth parts, whereas hexose-rich nectar has been found in flowers
pollinated by short-tonged insects [20, 32, 77–79]. On contrary, an analysis in
Antirrhinum and Lycium has revealed constant sugar composition despite a large
variety of pollinators [78, 80].
According to [64], other carbohydrates, including the monosaccharides (arabinose and rhamnose), the disaccharides (maltose, isomaltose, trehalose, gentiobiose,
turanose), the trisaccharides (panose, melezitose, maltotriose, isomaltotriose), as
well as the sugar alcohols (glycerol, erythritol, arabitol, galactitol, and mannitol)
were present as minor constituents in sour cherry nectars. The presence of mannitol,
melezitose, panose, and maltotriose was not confirmed in some of the studied
nectars. Minor sugars identified in nectars of some flowers, such as arabinose,
galactose, mannose, gentiobiose, lactose, maltose, melibiose, trehalose, melezitose,
raffinose, and stachyose can be toxic to potential pollinators [68, 81–84].
Among minor constituents, isomaltose, maltose, and sorbitol were the dominant
in comparison to other components. For the sorbitol (a polyol with low molecular
weight, highly soluble, and non-reducing compounds), results were fully expected,
because this sugar alcohol is the main photosynthetic product and the primary
translocated carbohydrate in Rosaceae [85]. Although has no influence to insects’
preference, sorbitol is a frequent constituent of Mediterranean nectars [22]. Also, if it
can be found in the fruit, it improves the sweet taste and texture of the mesocarp [86].
But also, its accumulation is considered as an adaptive response of plants to drought,
salinity, or chilling stress [87].
29 Sugar and Polyphenolic Diversity in Floral Nectar of Cherry
763
constituents of all investigated “Oblačinska” sour cherry nectar samples [64]. Based
on the total sugar content found in nectar, certain clones have been singled out as the
most concentrated (up to 97.6 mg/mL), while the others had dilute nectars (23 mg/
mL of sugars). Averagely fructose, glucose, and sucrose amounted 36.8%, 28.9%,
and 30.9% of the total content of all carbohydrates, respectively, and this was in line
with the other results [47]. The ranking based on fructose content takes into
consideration human sensation of taste. Also, fructose in the concentration range
from 15 mg/mL to 60 mg/mL in 14 sour cherry cultivars was reported [41], while in
“Oblačinska” sour cherry nectars fructose content was up to 34 mg/mL [64].
On the basis of the sucrose/(glucose + fructose) quotient [20], the nectar of
“Újfehértói fürtös” belonged to the sucrose-rich group each year, like the majority
of sour cherry cultivars, whereas the secretory product of “Pándy 48” could be
classified into the sucrose-dominant category in one of the seasons [46]. According
to the proposed quotient, one “Oblačinska” sour cherry clone was hexose dominant
[S/(G + F) < 0.1], four clones were hexose rich [S/(G + F) = 0.1–0.49], while other
11 were sucrose rich [S/(G + F) = 0.5–0.99] [64]. These results are in accordance
with some previous results [46]. Proportions of sucrose over fructose and glucose
have been linked with different classes of pollinators and found to be important in
plant-mutualism interactions [75].
The sucrose-dominant nectar composition of 45 species belonging to tribe
Sinningieae (Gesneriaceae) was also documented [76]. Several authors have
suggested that sucrose-rich nectar is mostly found in flowers pollinated by insects
with long mouth parts, whereas hexose-rich nectar has been found in flowers
pollinated by short-tonged insects [20, 32, 77–79]. On contrary, an analysis in
Antirrhinum and Lycium has revealed constant sugar composition despite a large
variety of pollinators [78, 80].
According to [64], other carbohydrates, including the monosaccharides (arabinose and rhamnose), the disaccharides (maltose, isomaltose, trehalose, gentiobiose,
turanose), the trisaccharides (panose, melezitose, maltotriose, isomaltotriose), as
well as the sugar alcohols (glycerol, erythritol, arabitol, galactitol, and mannitol)
were present as minor constituents in sour cherry nectars. The presence of mannitol,
melezitose, panose, and maltotriose was not confirmed in some of the studied
nectars. Minor sugars identified in nectars of some flowers, such as arabinose,
galactose, mannose, gentiobiose, lactose, maltose, melibiose, trehalose, melezitose,
raffinose, and stachyose can be toxic to potential pollinators [68, 81–84].
Among minor constituents, isomaltose, maltose, and sorbitol were the dominant
in comparison to other components. For the sorbitol (a polyol with low molecular
weight, highly soluble, and non-reducing compounds), results were fully expected,
because this sugar alcohol is the main photosynthetic product and the primary
translocated carbohydrate in Rosaceae [85]. Although has no influence to insects’
preference, sorbitol is a frequent constituent of Mediterranean nectars [22]. Also, if it
can be found in the fruit, it improves the sweet taste and texture of the mesocarp [86].
But also, its accumulation is considered as an adaptive response of plants to drought,
salinity, or chilling stress [87].
29 Sugar and Polyphenolic Diversity in Floral Nectar of Cherry
763