Biology Reference
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nectar. According to this model the sucrose-only route may take place in the
apoplast through which sucrose passes unaltered, while the symplastic route
involves active transport and sucrose hydrolysis. The proposed model of
nectar secretion for
Ricinus
is not applicable in those cases where an
apoplastic barrier between the parenchyma and the epidermis conveys the
pre-nectar from the apoplast into the symplast.
Zhu et al. (1997) proposed a model for the nectary function of
Arabidop-
sis thaliana
(Brassicaceae), a species with a caducous floral nectary,
characterized by the development of “densely stained cells”. These cells are
quite different from normal nectariferous cells because they show signs of
degeneration long before secretion. “Densely stained cells” in nectariferous
parenchyma appear before secretion and gradually increase in the early sec-
retory stage, reaching a maximum with very few normal cells at the heavy
secretion stage. “Densely stained cells” of nectaries of
A. thaliana
are there-
fore suggested to function as a transferring tissue, despite the absence of wall
ingrowths. Although some vesicles are also observed in nectary parenchyma
cells during secretion, according to Zhu et al. (1997) it is not clear what kind
of substance is actually transported in these vesicles so that their role in nec-
tar secretion is obscure. According to the proposed model, pre-nectar is
transferred from sieve elements to nectary parenchyma cells by plasmodes-
mata and multivesicular structures, where it presumably accumulates and is
stored as a few starch grains in chloroplasts. The starch is then degraded in
the chloroplasts and is modified in nectary parenchyma cells. Next, the nec-
tar is transported by plasmodesmata or occasionally by vesicles, finally
gathering in the “densely stained cells”, which connect with modified sto-
mata forming a “corridor” for transferring nectar. The development of
“densely stained cells” is reported to be a process of programmed cell death
(PCD) leading to loss of function of the semipermeable plasmalemma, so
that nectar is easily and quickly transported to the top of the nectary (Zhu &
Hu, 2002).
Peng et al. (2004) observed a peculiar pattern of amyloplast degeneration
in the nectary parenchyma of
Cucumis sativus
(Cucurbitaceae) from 3 days
before anthesis to the day of anthesis. During this period the starch grains
become irregular in shape, indicating starch hydrolysis. Amyloplast mem-
branes stretch towards the tonoplast of the closest vacuole. The whole
amyloplast then intrudes into the vacuole and the electron density of the de-
generating amyloplasts changes from high to low (Peng et al., 2004). On the
day of anthesis, the starch grains and amyloplasts disappear totally, while
vacuole volume increases significantly. From these observations the authors
conclude that a combined amyloplast-vacuole complex acts as the centre of
