Biology Reference
In-Depth Information
lose their thylakoid structure and starch grains increase in size a few days
before anthesis (Zer & Fahn, 1992; Fahn & Shimony, 2001). In other cases,
proplastids differentiate into amyloplasts and store great amounts of starch in
many large grains per plastid before nectar secretion begins (Durkee et al.,
1981; Figueiredo & Pais, 1992; Pais & Figueiredo, 1994; Nepi et al., 1996)
(Figs. 9 and 10). In
Passiflora biflora, Rosmarinus officinalis
, and
Cucurbita
pepo
(Durkee et al., 1981; Zer & Fahn, 1992; Nepi et al., 1996), nectary pa-
renchyma proplastids start to accumulate starch derived from the
photosynthesis of other floral parts during pre-anthesis (Pacini et al., 2003
and references therein). In these three species, starch also accumulates in the
epidermis, though the number of grains per plastid is fewer than in paren-
chyma cells. Amyloplasts in the nectar-producing parenchyma are generally
almost devoid of stroma and packed with starch (Fig. 10) (Nepi et al., 1996).
They also contain many starch grains per amyloplast; this increases starch
surface area, facilitating and speeding hydrolysis during nectar production.
The type of plastids and presence of starch are heterogeneous features of
orchid floral nectaries (Table 1 and Fig. 9). Plastids may have an undifferen-
tiated appearance and contain osmiophilic bodies (
Gymnadenia conopsea
and
Platanthera chlorantha)
; they may have thylakoid-like membranes that re-
semble choloroplasts (
Hexisea imbricata, Maxillaria coccinea, Platanthera
bifolia),
or they may be amyloplasts
(Limodorum abortivum).
Undifferentiated plastids and chloroplasts may or may not store starch in
the pre-secretory phase.
No starch grains were observed in plastids of the
nectary cells of the orchids
Gymnadenia conopsea
and
Maxillaria coccinea
(Table 1 and Fig. 9), however starch has been found in plastids of other
orchids, such as
Hexisea imbricata, Platanthera bifolia,
and
Platanthera
chlorantha.
The quantity of starch in plastids peaks in mature buds and decreases
with the onset of nectar production. Thus many authors infer that hydrolysis
of starch in the parenchyma contributes directly to nectar carbohydrate con-
tent (Rachmilevitz & Fahn, 1973; Durkee et al., 1981; Zer & Fahn, 1992; Nepi
et al., 1996; Pacini et al., 2003; Peng et al., 2004). The sugars derived by starch
breakdown can also be used to produce energy for the process of secretion.
The general pattern of starch decreasing at the moment of nectar secretion
was not found in
Trifolium pratense
(Fabaceae) and
Ecballium elaterium
(Cucurbitaceae). In red clover, starch grains in plastids were actually more
numerous and larger in florets at the end of nectar production (Eriksson,
1977). In
Ecballium elaterium,
plastids have well-differentiated thylakoids
and grana in the early stage of nectary development; they store starch, reaching
