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take place simultaneously (Wergin et al., 1975; Davis et al., 1986, 1988;
Stpiczyńska, 1995; Stpiczyńska et al., 2003c; Wist & Davis, 2006).
Pre-nectar transport follows the symplastic path in nectaries with
trichomes as secretory structures. The apoplastic route of pre-nectar is im-
peded by lignification or complete cutinization of the radial walls of the stalk
cell, i.e., the second cell of the hair (Fahn, 1979b; Sawidis et al., 1987a;
Davis et al., 1988). In
Hibiscus rosa-sinensis
(Malvaceae) the basal cells of
the trichomes (i.e., the first cells of the trichomes), situated at the level of
epidermal cells, have a greater number of plasmodesmata, implying a role in
the collection and conveyance of pre-nectar from nectary parenchyma cells
towards secretory hairs (Sawidis et al., 1987b). Great density of plasmodes-
mata also occurs in the walls of stalk cells, which, besides providing a
barrier to apoplastic transport, favour symplastic flow of pre-nectar. After
entering the secreting hairs, pre-nectar flows from cell to cell through plas-
modesmata to reach the tip cell (Sawidis et al., 1987b). Here nectar
accumulates in spaces between the cell membrane and the cuticle prior to its
“pulsed” release (Sawidis et al., 1987b).
In their study based on freeze-substituted material from the nectary of
Abutilon
(Malvaceae), Robards and Stark (1988) postulated an apoplastic
route of pre-nectar transport at all levels along the hairs: there seemed to be
an open extracytoplasmic space outside all cells of the secreting hair. They
observed “secretory reticulum” (SR) in cells of secreting hairs. This complex
internal membrane system is sometimes also observed in chemically fixed
material. The SR is very abundant and its total surface area may be 23 × 103
µm
2
in a single trichome (Robards & Stark, 1988). The authors did not study
SR ontogeny, but saw it as similar to ER. They did not observe fusion sites
of SR cisternae with cell plasma membranes. With these limitations they
proposed a model of nectar secretion through
Abutilon
hairs:
It is envisaged that pre-nectar moves into the symplast of the hair via the
numerous plasmodesmata in the transverse walls of the stalk cells. In
each of the hair cells some of the pre-nectar is loaded from the cytoplasm
into the SR. It is at this stage that a filtration effect takes place, so defin-
ing the chemical composition of the secreted product. The sucrose is
partially hydrolyzed to glucose and fructose (
Abutilon
nectar contains all
three sugars) but evidence is not yet available as to whether this takes
place at the membrane or within the cavity of the SR. As loading into the
SR continues, a hydrostatic pressure builds up until, ultimately, a minute
pulse of nectar is forced into the freely permeable apoplastic space
bounded by the plasmalemma to the interior and the cuticle to the exte-
rior. The continuing build up of pressure within this compartment
