Biology Reference
In-Depth Information
1983; Witt et al., 1999; Nepi et al., 2001). In
Mandevilla pentlandiana
(Apo-
cynaceae) most nectar is produced during the flower bud stage (Torres &
Galetto, 1998). Slow producers are generally early initiators and their nectar
is commonly protected by a thick corolla or calyx to prevent predation or
evaporation. In contrast, fast producers are late initiators and offer less pro-
tection to the nectar (Cruden et al., 1983).
Nectar secretion can be continuous over the flower lifespan until senes-
cence or may cease at certain times. Such differences in nectar production
dynamics were recently found in six
Ipomoea
(Convolvulaceae) species
(Galetto & Bernardello, 2004). Nectar production dynamics could, however,
not clearly be related to the pollinator guild and it was suggested that struc-
tural constraints may play a major role in the determination of nectar traits.
Cessation of nectar secretion may take place at two different times: when
a maximum is reached or during a period of pollinator inactivity (Cruden
et al., 1983 and references therein). In the first case nectar removal may or may
not induce resumption of secretion. Secretion does not resume after nectar
removal in species where a single pollinator visit is sufficient to maximize
seed set. Cruden et al. (1983) pointed out that if nectar secretion resumes
after its removal, measuring the nectar of bagged flowers may lead to a gross
underestimation of the amount of nectar that can be produced by flowers
during the day.
Cessation of nectar secretion may also occur between the two sexual
phases of dichogamous plants, as demonstrated in
Carum carvi
(Apiaceae).
Nectar is produced in the male and female phases but there is no nectar
secretion in the “neutral” phase between them: the male and female phases are
both followed by nectar reabsorption (Langenberger & Davies, 2002).
When nectar secretion relies on the accumulation of starch, the quantity
of sugar produced is already determined before secretion by the quantity of
starch accumulated in nectary parenchyma: the nectary produces a fixed
amount of sugars. In these types of nectaries, nectar removal has no effect on
total sugar secretion, as in the two cucurbits
Cucurbita maxima
(Ashworth &
Galetto, 2002) and
C. pepo
(Nepi et al., 2001). On the other hand, when
nectar secretion is derived directly from photosynthesis, nectar removal may
increase or decrease total nectar secretion or it may have no effect (Galetto
& Bernardello, 1992, 1993, 1995; Bernardello et al., 1994; Galetto et al.,
1994, 1995, 2000; Rivera et al., 1996; Torres & Galetto, 1998; Vesprini &
Galetto, 2000).
