Biology Reference
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through which the nectar is secreted) is positively correlated with nectar vol-
ume (Dafni et al., 1988; Petanidou et al., 2000). On the other hand, the
number of nectarostomata does not appear to play a significant role in con-
trolling nectar volume (Petanidou et al., 2000), which is in agreement with
other continental (Teuber et al., 1980; Davis & Gunning, 1991), but not tropi-
cal species (Galetto, 1995; but see Galetto & Bernardello, 2004). Among all
nectary attributes only the stomatal size affects nectar concentration and this
is a negative relationship (Petanidou et al., 2000). Species with small necta-
rostomatal openings secrete more concentrated nectars, at least within the
Lamiaceae (Petanidou et al., 2000). It is unknown whether this is a peculiar-
ity of Mediterranean nectaries, or a general characteristic of all Lamiaceae,
or even broader groups.
5
NECTAR AND THE POLLINATOR INTERFACE
5.1
Relating consumers to deep-flower nectars
Tubular, deep, and closed flowers can protect nectar from nectar thieves and
unwanted insects, such as short-tongued visitors who will have limited ac-
cess (Baker & Baker, 1983; Dafni, 1991; McCall & Primack, 1992; Menzel
& Shmida, 1993; Potts et al., 2001). The presence of numerous hairs and
stamens in the flowers of several Mediterranean species, such as those
within the genera
Cistus
and
Capparis
, may have a role similar to long co-
rollas in restricting air movement and excluding insects (Petanidou & Ellis,
1996; Petanidou, 2005). In this respect, the presence of honey leaves or
honey pockets (i.e., petal scales where nectar is accumulated) in bowl-
shaped flowers in some Mediterranean genera is probably related to a similar
nectar-protective function (e.g.,
Fritillaria, Nigella, Ranunculus
).
5.2
Nectar sugars and pollinators
High-sucrose nectars prevail in the Mediterranean, not only at the commu-
nity level and during the major flowering season (spring), but also during the
harshest season, i.e., summer (Petanidou, 2005). In addition to the reasons
mentioned on page 346, the propensity of sucrose-rich nectar species to flower
in spring-summer, versus hexose-rich species to flower in winter can be
explained on the basis of:
•
Co-evolution of plants with insects. By containing easy-to-digest mono-
saccharides (Nicolson, 1998), high-hexose nectars are more adapted to
consumption by an extensive array of mainly non-specialized pollinators
