Biology Reference
In-Depth Information
Instead of pore closure by guard-cell movements, closure of the modified
stomata of the floral nectary may occur exclusively by occlusion in some
species (Fig. 6). The occluding material is of uncertain nature (Gaffal et al.,
1998). It cannot be excluded that nectar may crystallize in the stomatal aper-
ture. It has been hypothesized that occlusion of the stomatal pores may be a
mechanism to seal off potential entry sites for pathogens (Davis, 1997;
Razem & Davis, 1999). Micro-organisms have been found in the stomatal
apertures (Gaffal et al., 1998) and the nectary has been recognized as the
primary site of infection by
Erwinia amylovora
, the agent of fire blight dis-
ease in
Malus
and
Pyrus
(Buban et al., 2003).
2.2
Nectary parenchyma
The nectary parenchyma is generally composed of a few to several layers of
small, isodiametric cells, generally with thin walls, dense granular cyto-
plasm, small vacuoles, and relatively large nuclei. Even if there are different
types of nectaries and they have a non-uniform structure they always belong
to the class of secreting cells. Owing to their secretory activity, all these kind
of cells have extra copies of DNA realized by means of multinucleate cells,
polyploid nuclei, or polytenic chromosomes (D'Amato, 1984). Nevertheless,
even in ultrastructural studies little attention has been paid to the nuclei of
nectary secreting cells and multinucleate cells were never observed.
These peculiar cytological characteristics mean that the nectary paren-
chyma can very often be distinguished easily from the ground parenchyma.
Unusually collenchymatous cells with thick walls were observed in the nec-
tary of
Maxillaria coccinea
(Orchidaceae) (Stpiczyńska et al., 2003).
Vacuole size in nectary parenchyma cells varies according to the stage of
nectary development: small vacuoles are present in the pre-secretory phase,
and may increase in volume at the time of secretion, but generally a sharp
increase in vacuole volume takes place after secretion. The cytoplasm is
usually rich in ribosomes and mitochondria. These organelles generally in-
crease in number at the moment of secretion, indicating increased energy
requirements for nectar production. Intercellular spaces are present and in-
crease at the time of secretion.
It is not uncommon to find cells undergoing cell division in actively se-
creting nectaries (Gaffal et al., 1998; Nepi et al., 1996). Continued cell
division and the lack of subsequent cell extension in small-celled nectarifer-
ous tissue are more or less comparable to meristematic tissue (Gaffal et al.,
1998 and references therein). This implies that nectary parenchyma cells
