Biology Reference
In-Depth Information
maintain the potential of cell regeneration, at least in some species such as
Digitalis purpurea
(Gaffal et al., 1998),
Cucurbita pepo
(Nepi et al., 1996),
and
Helleborus
sp. (Vesprini et al., 1999).
The structure and ultrastructure of nectary parenchyma appears to depend
mainly on two features: the mechanism of pre-nectar transport (through the
apoplast or symplast) and the source of nectar carbohydrates (starch reserves
or direct photosynthesis). The term pre-nectar refers to substances trans-
ported into nectary tissue to be transformed into nectar by the nectary
parenchyma or epidermal cells.
On the basis of numerous plasmodesmata between the cells, Fahn
(1979b) proposed the symplast as the main path of pre-nectar transport into
the parenchyma cells of
Lonicera japonica
, but evidence is also available for
pre-nectar transport via the apoplast (Davis et al., 1988; Peng et al., 2004).
The two mechanisms may possibly take place simultaneously (Wergin et al.,
1975; Davis et al., 1986; Davis et al., 1988; Stpiczyńska, 1995; Stpiczyńska
et al., 2003; Wist & Davis, 2006). Plasmodesmata are generally found
between nectary parenchyma and subnectary parenchyma cells; their fine
structure in nectaries has been reviewed by Eleftheriou (1990).
Nectar secretion, i.e., the transfer of nectar outside the protoplast of pa-
renchyma cells, may be granulocrine or eccrine. Eccrine secretion involves
transport of individual molecules across the secretory cell membrane. In
granulocrine secretion molecules are grouped and transported in ER- or dic-
tyosome-derived vesicles that fuse with the plasmalemma and release the
molecules outside the nectary cells (Fahn, 1988). When granulocrine secre-
tion occurs, parenchyma cells are rich in ER cisternae, dictyosomes, and
vesicles (Rachmilevitz & Fahn, 1973; Fahn, 1987b; Arumugasamy et al.,
1990b) and an increase in the number of these organelles indicates imminent
nectar secretion. Robards and Stark (1988) demonstrated an extensive “sec-
retory reticulum”, i.e., an internal membrane system closely associated with
the plasmalemma, within the secretory trichomes in the nectary of
Abutilon
.
On the other hand, when ER cisternae and Golgi vescicles are rare and
their number remains almost unchanged during flower development, eccrine
secretion is likely (Elias et al., 1975; Eriksson 1977; Nepi et al., 1996;
Razem & Davis, 1999; Stpiczyńska et al., 2003).
Different pre-nectar transport mechanisms have been documented in the
same family (Eriksson, 1977; Davis et al., 1988; Nepi et al., 1996; Peng
et al., 2004), in flowers of the same species (Meyberg & Kristen, 1981) and
