Biology Reference
In-Depth Information
dynamic nectar transformation. This intimate relationship between amy-
loplasts and nectary cell vacuoles before nectar secretion has only been
reported in this case. Although characterized by amyloplasts very similar to
those of
C. sativus
, the allied species
Cucurbita pepo
does not show this
close association throughout nectary development and nectar secretion (Nepi
et al., 1996a). This dissimilarity could be related to differences in the rate of
nectar secretion.
Starch hydrolysis products may be transformed in the vacuole by specific
enzymes, such as invertase. Peng et al. (2004) also demonstrated that pre-
nectar transport in
C. sativus
follows the apoplastic route. ATPase activity in
nectary parenchyma cells is required for transport of pre-nectar from secre-
tory cells to intercellular spaces and also for secretion of nectar on the
surface of the nectary. The authors found ATPase activity not only in the
plasmalemma of secreting cells, but also in vesicle membranes in intercellu-
lar spaces during nectar secretion.
The presence of invertase activity in nectar is still debated. According to
early studies reported by Baker and Baker (1983b and references therein)
invertase occurs in the nectar of
Tilia
(Malvaceae) and other species. Inver-
tase is not found in nectar from the extrafloral nectaries of
R. communis
(Nichol and Hall, 1988). Pate et al. (1985) noted that diluted extrafloral nec-
tar of
Vigna unguiculata
(Fabaceae) contained inverted sucrose whereas
undiluted nectar did not. This finding suggests that nectar contains freely
soluble invertase, the activity of which is inhibited osmotically at high sugar
levels, or that invertase is associated with nectariferous cells and cell debris
in nectar, being leached from these materials when nectar is diluted.
It is clear that we are far from understanding the coordinated activity of
all parts of the nectary. The studies cited above are restricted to a few species
and approach the topic from either an ultrastructural or a physiological per-
spective, which are never combined; sometimes they only consider part of
the nectary. It is probable that there is no general model of nectary func-
tion—a certain grade of variability can be expected on the basis of nectary
structure and ultrastructure and nectar production rate and composition.
3
DYNAMICS OF NECTAR PRODUCTION
It is generally accepted that the dynamics of nectar production coevolved
with the requirements of plant pollinators. For example, flowers pollinated
by diurnally active animals produce nectar and expose it during the day.
