aminobutyric acid (GABA) are also accumulated in nectar but are non-toxic [49].
According to the mineral analysis of nectar ion composition, concentration of K
+ is
the highest, following by Na
+
. Some levels of Ca
2+ and Mg
2+ have been also
detected. According to [61], potassium and sodium chloride deter honey bees.
Proteins/enzymes, or so called “nectarin” in floral nectar includes invertase, transglucosidase, transfructosidase, phosphatase, tyrosinase, alliinase, nectarin, I-superoxide dismutase, and others, playing important role in hydrolysis of sucrose,
polymerization of glucose and fructose molecules, possibly defense and many
more. Trace amounts of lipids, organic acids, iridoid glycosides (catalpol), vitamins,
alkaloids (anabasine, gelsemine, nicotine, and caffeine), terpenoids (thymol),
glucosinolates, and cardenolides can be found in nectar too.
Recently, a gene that encodes an apoplastic invertase of Arabidopsis has been
discovered. This gene represents the first gene whose function is required for floral
nectar secretion [62].
Chemical screening is usually done by standard chromatographic techniques
hyphenated to spectral methods. New technologies and advanced techniques conquer difficulties in analyzing small fluid volumes, enabling more detail identification
and quantification of nectar components.
Most of the individual studies on nectaries, nectar, and nectar consumers were
included in a comprehensive book review [63]. Cherry nectar properties and chemistry were not examined to a great extent, and just a few papers discussing the
composition of sour cherry floral nectar were published so far. As for the sweet
cherry, no available data could be found. Therefore, presented results on cherry
nectar included in this chapter rely on just a few published papers [46, 64] where
nectar sugar profiles of sour cherry cultivars were reported. Most of the data on
phenolics were drawn from the study carried out on “Oblačinska” sour cherry clones
[64].
4.1
Nectar Carbohydrate Profile
Nectar carbohydrate profile is prevailed by three sugars, the disaccharide sucrose and
its monosaccharide units, fructose, and glucose. Nectar components are believed to
originate from phloem sap that is enzymatically processed and transformed within
nectaries [65]. Since the phloem sap contains mostly sucrose, chemical reactions
must occur to produce glucose and fructose in the nectar. The relative amounts of
each are determined by hydrolyses of sucrose catalyzed by transglucosidases and
transfructosidases localized in the nectaries which occur before or during nectar
secretion [66].
The total sugar concentration in floral nectar can range from 5% (w/v) to 80% (w/
v) [67] and may differ among individuals, populations, cultivars, or subspecies of the
same species [38, 39, 68, 69]. Also, amounts and relative concentrations of the major
constituents, glucose, fructose, and sucrose may vary among species from almost all
sucrose to all hexose. According to [61], sucrose, maltose, glucose, fructose, trehalose, and melezitose are sweet for bees; while lactose, melibiose, raffinose, xylose,
29 Sugar and Polyphenolic Diversity in Floral Nectar of Cherry
761
According to the mineral analysis of nectar ion composition, concentration of K
+ is
the highest, following by Na
+
. Some levels of Ca
2+ and Mg
2+ have been also
detected. According to [61], potassium and sodium chloride deter honey bees.
Proteins/enzymes, or so called “nectarin” in floral nectar includes invertase, transglucosidase, transfructosidase, phosphatase, tyrosinase, alliinase, nectarin, I-superoxide dismutase, and others, playing important role in hydrolysis of sucrose,
polymerization of glucose and fructose molecules, possibly defense and many
more. Trace amounts of lipids, organic acids, iridoid glycosides (catalpol), vitamins,
alkaloids (anabasine, gelsemine, nicotine, and caffeine), terpenoids (thymol),
glucosinolates, and cardenolides can be found in nectar too.
Recently, a gene that encodes an apoplastic invertase of Arabidopsis has been
discovered. This gene represents the first gene whose function is required for floral
nectar secretion [62].
Chemical screening is usually done by standard chromatographic techniques
hyphenated to spectral methods. New technologies and advanced techniques conquer difficulties in analyzing small fluid volumes, enabling more detail identification
and quantification of nectar components.
Most of the individual studies on nectaries, nectar, and nectar consumers were
included in a comprehensive book review [63]. Cherry nectar properties and chemistry were not examined to a great extent, and just a few papers discussing the
composition of sour cherry floral nectar were published so far. As for the sweet
cherry, no available data could be found. Therefore, presented results on cherry
nectar included in this chapter rely on just a few published papers [46, 64] where
nectar sugar profiles of sour cherry cultivars were reported. Most of the data on
phenolics were drawn from the study carried out on “Oblačinska” sour cherry clones
[64].
4.1
Nectar Carbohydrate Profile
Nectar carbohydrate profile is prevailed by three sugars, the disaccharide sucrose and
its monosaccharide units, fructose, and glucose. Nectar components are believed to
originate from phloem sap that is enzymatically processed and transformed within
nectaries [65]. Since the phloem sap contains mostly sucrose, chemical reactions
must occur to produce glucose and fructose in the nectar. The relative amounts of
each are determined by hydrolyses of sucrose catalyzed by transglucosidases and
transfructosidases localized in the nectaries which occur before or during nectar
secretion [66].
The total sugar concentration in floral nectar can range from 5% (w/v) to 80% (w/
v) [67] and may differ among individuals, populations, cultivars, or subspecies of the
same species [38, 39, 68, 69]. Also, amounts and relative concentrations of the major
constituents, glucose, fructose, and sucrose may vary among species from almost all
sucrose to all hexose. According to [61], sucrose, maltose, glucose, fructose, trehalose, and melezitose are sweet for bees; while lactose, melibiose, raffinose, xylose,
29 Sugar and Polyphenolic Diversity in Floral Nectar of Cherry
761