Biology Reference
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Multicellular, linear trichomes as in the floral nectaries of Abutilon
(Kronestedt-Robards et al. 1986) and Hibiscus rosa-sinensis (Malvaceae)
(Sawidis et al., 1987a)
Multicellular, capitate trichomes as in the extrafloral nectaries of Vicia
faba (Davis et al., 1988) (Table 1)
The detailed ultrastructural development of nectary trichomes has been
investigated in Abutilon (Kronestedt-Robards et al., 1986) and in Hibiscus
(Sawidis et al., 1987a). The first event to take place is an outgrowth of epi-
dermal cells followed by periclinal division. Volume increase of epidermal
cells is accompanied by cell polarization, manifested by displacement of or-
ganelles towards the apical region.
The most specialized cells of pluricellular trichomes are the basal, stalk,
and tip cells. The basal cells (situated at the level of the other epidermal
cells) have a greater number of plasmodesmata than adjacent cells (Sawidis
et al., 1987b). After entering the secreting hairs, pre-nectar flows from cell to
cell through plasmodesmata (symplastic route) reaching the tip cell (Sawidis
et al., 1987b). The apoplastic route of pre-nectar is impeded by lignification
or complete cutinization of the lateral walls of the stalk cells (Fahn, 1979b;
Sawidis et al., 1987a; Davis et al., 1988). The tip cells have very elaborate
systems of ER, dictyosomes, and vesicles and they are thought to be in-
volved in granulocrine secretion (Fahn, 1979b; Kronestedt-Robards et al.,
1986; Sawidis et al., 1987b, 1989).
The floral nectary of Tropaeolum majus (Tropaeolaceae) has epidermal
hairs, but the main source of nectar is the nectary parenchyma and nectar is
exuded through the modified stomata (Rachmilevitz & Fahn, 1975).
2.1.2
Nectary-modified stomata
Nectar exudation through stomata appears to be the most common manner of
nectar release (Table 1, Bernardello, 2007). Nectar flow may be so high that
the stomatal aperture enlarged (Fig. 4). The nectary stomata may be located
on the surface of the nectary or in deep depressions (Fig. 5)
Stomata involved in nectar secretion have been described as “necta-
rostomata” (Smets & Cresens, 1988). They are considered to be “modified”
with respect to leaf stomata because they are not able to finely regulate their
aperture (Davis & Gunning, 1992, 1993). In actively secreting nectaries, the
stomata are raised slightly above the epidermis, while most stomata of not
yet secreting nectaries are open but not raised (Gaffal et al., 1998; Nepi
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