Biology Reference
In-Depth Information
bigger than in parenchyma cells. Plastids of epidermal cells do not usually
store starch (Razem & Davis, 1999), except when a very high rate of nectar
secretion is required, as in Passiflora sp. (Durkee et al., 1981), Rosmarinus
officinalis (Zer & Fahn, 1992), and Cucurbita pepo (Nepi et al., 1996).
When nectar secretion does not occur through stomata, the epidermis it-
self is involved in the secretion process via epidermal secreting cells or
secreting trichomes. Though this has not yet been demonstrated, the two
manners of secretion are not mutually exclusive and may take place contem-
poraneously (Nepi et al., 2001). In certain plants, the outer walls of the
epidermal cells involved in nectar secretion have wall ingrowths (Schnepf &
Pross, 1976; Fahn, 1979a; Fahn & Benouaiche, 1979 and references therein;
Davis et al., 1988) and are regarded as transfer cells (Pate & Gunning, 1972).
Wall protuberances are thought to aid eccrine secretion of individual mole-
cules and are seldom found in systems where secretion by vesicles (granulo-
crine secretion) has been suggested (Kronestedt-Robards & Robards, 1991).
The anatomical differences in the structure of nectary epidermis concern:
cuticle structure and patterning, the presence/absence and structure of secret-
ing trichomes, the presence/absence of stomata (Table 1).
A continuous cuticle is generally present on the surface of the nectary
epidermis, although it may be thinner than on the areas adjacent to the nec-
tary (Gaffal et al., 1998 and references therein) or discontinuous as in septal
nectaries. A cuticular lining of intercellular spaces in the more peripheral
nectary parenchyma and the substomatal chamber has been reported in a
number of species (Rachmilevitz & Fahn, 1973; Davis et al., 1988; Maldonado
& Otegui, 1997; Razem & Davis, 1999 and references therein) and may en-
hance nectar movement once in the apoplast.
The patterning, thickness, and permeability of the nectary cuticle vary
widely. In the case of nectary trichomes, the cuticle covering the secreting
cell seems to be completely impermeable and the nectar accumulates in a
subcuticular space formed by separation of the cuticle from the epidermis
(Fig. 2). As secretion proceeds, the cuticle stretches and becomes very thin.
It has not been determined whether the nectar is released when the cuticle
breaks or whether thin areas of the stretched cuticle become permeable to
nectar.
Search WWH ::




Custom Search