Biology Reference
In-Depth Information
attractiveness of nectar to pollinators and could therefore play a significant
role in the pollination process. Phenolic compounds, for instance, may posi-
tively contribute to the taste of nectar at very low concentrations (Baker,
1977), while at other times—especially in higher quantities—they may repel
honeybees (Adler, 2000a; Hagler & Buchmann, 1993).
In the Mediterranean, several secondary compounds have been identified
in the nectars of plants associated mostly with honey-making. Such com-
pounds include grayanotoxins (in the nectar of
Rhododendron luteum
; Buys,
2000), flavonoids (e.g., kaempferol, in the nectar of rosemary
Rosmarinus offi-
cinalis
; Ferreres et al., 1998), and glycosides (e.g., amygdalin, a cyanoglyco-
side found in the nectar of the almond
Amygdalus communis
; London-Shafir
et al., 2003; and arbutin in the nectar of the strawberry tree
Arbutus unedo
;
Pryce-Jones, 1944). The presence of such substances makes nectar either toxic
(e.g., in
Rhododendron luteum
and
Amygdalus communis
) or at least repel-
lent to some visitors. The evolutionary significance of such toxic nectars
remains, to a major extent, unknown.
2.1.5
Nectar viscosity
Another characteristic of Mediterranean nectars that might be related to the
presence of secondary compounds is viscosity. Nectar viscosity is mainly
related to sugar concentration, which is high in the region (Kearns & Inouye,
1993; Petanidou & Smets, 1995). It may also result from rapid evaporation of
the exposed nectars of many species—especially those with open flowers,
e.g.,
Urginea maritima
,
Thapsia garganica
,
Euphorbia acanthothamnos
, and
Ruta graveolens
(Dafni & Dukas, 1986; Petanidou & Smets, 1995). Yet, the
viscosity of nectar may also be due to the presence of pectic substances as a
result of post-secretory hydrolytic phenomena (Saeed et al., 1975); the pres-
ence of polysaccharides may also contribute to high nectar viscosity (Josens
& Farina, 2001; Dafni et al., 2005).
An interesting case of nectar viscosity has been detected in the nectar of
the
phrygana
species
Phlomis fruticosa
(Petanidou, 1991; Petanidou & Smets,
1995). Repeated observations over time showed that two types of flowers
appeared in a patchy distribution on the same and over several individual
plants: one with viscous and another with non-viscous nectar (Petanidou,
unpublished data). Interestingly, these flower types did not differ in sugar
concentration measured by HPLC analysis, but flowers with viscous nectars
had a significantly higher sucrose/hexose ratio and higher total amino acid
content (Table 2), implying that viscosity was caused by proteolytic phe-
nomena resulting in an amino acid excess in these nectars. A more focused
