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causing the formation of very thin cell walls in the nectary parenchyma. It
has, however, also been suggested that they may simply be excretory prod-
ucts and discourage herbivory by invertebrates (Davies, 1999).
Protein bodies, finely granular and irregular in shape, have been found in
the vacuoles of
Maxillaria coccinea
(Orchidaceae)
(Stpiczyńska et al., 2003)
and very similar structures in the floral nectary of
Passiflora
(Durkee et al.,
1981; Durkee, 1982). Their role is unclear and requires further investigation.
They cannot be precursors of the protein component of nectar because of
their presence in secreting cells.
The floral nectary parenchyma and epidermal cells of most plants remain
intact throughout secretion (merocrine secretion). In a few cases, secretion of
nectar implies cell death (holocrine secretion) as reported for the floral nec-
taries of
Turnera ulmifolia
(Elias et al., 1975),
Helleborus foetidus
and
H.
bocconei
(Vesprini et al., 1999), and
Glycine max
(Horner et al., 2003). A
widespread degenerative process occurs in
T. ulmifolia
and
G. max
, but only
cell-by-cell in
Helleborus
, probably involving spatial reorganization of
secreting cells (Vesprini et al., 1999). This different pattern of cell degenera-
tion is probably related to the very different duration of nectar secretion:
short in
Turnera ulmifolia
and
Glycine max
(a few hours and 24 h, respec-
tively) and long in
Helleborus
(about 20 days). The long duration of nectar
secretion in
Helleborus
is not compatible with a rapid and massive degenera-
tion of secreting cells.
Horner et al. (2003) reported that in
Glycine max
, before the nectary pa-
renchyma and epidermal cells undergo programmed cell death, they produce
compounds of unidentified chemical composition that engorge their central
vacuole which has an apparently “discontinuous tonoplast”.
The fate of the nectary parenchyma after secretion may have different
patterns when nectar secretion does not cause cell death. The nectary tissue
may
•
Be involved in nectar reabsorption (Nepi et al., 1996)
•
Differentiate into another tissue (parenchyma tissue, as in the case of septal
nectaries of certain monocots; see “Gynopleural (septal) nectaries” on page
154)
•
Degenerate (Fig. 8)