Biology Reference
In-Depth Information
glance at the data showed that among all the amino acids and amino acid
compounds detected, GABA was the only amino acid with an extremely
high contribution in non-viscous versus viscous nectars (decreasing by 99% in
the latter). Other amino acids (valine, phenylalanine, methionine, tryptophan,
arginine, alanine + tyrosine) had much higher contributions in viscous versus
non-viscous nectars, with valine showing the highest increase in viscous nec-
tars (169%). Because GABA is an amino acid absolutely dependent on the
presence of common salt (NaCl) (Keynan & Kanner, 1988; Wolfersberger,
2000), its higher content in non-viscous nectars may indicate that these nec-
tars are additionally protected against an early breakdown by their higher
NaCl content.
The evolutionary significance of viscosity as a nectar characteristic is as
yet unknown, but it can be presumed that higher viscosity—if mostly due to
protein hydrolysis—assists in the preservation of nectar attributes by con-
tributing to the slowing down of disaccharide breakdown (cf. Table 2). In
this way the large and long-lasting flowers of
Phlomis fruticosa
may pre-
serve their high-sucrose nectar throughout anthesis while waiting for their
relatively infrequent pollination partners, viz. long-tongued specialist bees
(Petanidou, 1991; Petanidou et al., 1995). By limiting sucrose breakdown,
higher viscosity favours water economy in the plant, as no excessive water is
consumed to keep nectar concentrations stable in case of sucrose hydrolysis
(Nicolson, 1998; Petanidou, 2005). The presence of two different types of
nectar in the flowers of the same individual and within the same population
of
Phlomis fruticosa
, i.e., a non-viscous type protected by the presence of
GABA and NaCl, and a viscous type as a result of protein hydrolysis, high-
lights the importance of nectar preservation under the harsh Mediterranean
conditions, an issue that undoubtedly needs further investigation.
2.2
Issues of nectar quantity and quality
In general, nectar secretion (quantity)
at community level is lower in the
Mediterranean compared to other regions. Cruden et al. (1983) found an
average of 2.10 ± 0.67 µl nectar volume produced per flower of exclusively
bee-visited species (
n
= 12) in the southwestern United States. In tropical
systems, Opler (1983) distinguished between highly rewarding, large bee-
pollinated species producing 9.75 ± 4.350 µl of nectar (
n
= 19), and low
rewarding, small bee/wasp-pollinated species secreting only 0.63 ± 0.182 µl
(
n
= 14) of nectar per flower. In contrast, per flower nectar yield in the Medi-
terranean is always low: in a Spanish
garrigue
community, Herrera (1985)
could not ascertain the presence of nectar in 41% of the species studied
(
n
= 122), concluding that in total only 35% of the species could be considered
