Biology Reference
In-Depth Information
Figure 2.
Semi-diagramatic drawing representing the possible pathway of phloem sap and
pre-nectar from the sieve element (se) to the stomata (st):
continuous line
represents the
apoplastic route, and the
dotted
one the symplastic route. Phloem sap is transferred from sieve
element (se) to the phloem parenchyma (php) by wall ingrowths of companion cells (cc). It is
then transferred as pre-nectar through the subnectary parenchyma (snp) and subsequently
transformed into nectar in the nectary parenchyma (np).
•
Granulocrine secretion is transport of a sugar solution into vesicles derived
from dilated cisternae of ER or from dictyosomes that fuse with the plas-
malemma, releasing nectar into the wall area.
According to Fahn (2000), pre-nectar flows from the phloem endings,
through the nectariferous cells, to the secretory cells which exude the nectar
externally. Fahn also reports different transport mechanisms for pre-nectar
and nectar. Transport of pre-nectar mainly takes place by the symplastic
route, whereas nectar secretion occurs by active molecular transport through
membranes (eccrine secretion) or by transport via vesicles whose mem-
branes fuse with the plasmalemma (granulocrine secretion). These two
distinct mechanisms of pre-nectar and nectar transport are not confirmed by
other studies, in which eccrine or granulocrine pre-nectar transport are postu-
lated to occur in parenchyma and secretory cells (Wist & Davis, 2006).
Nor can it be excluded that symplastic and apoplastic transport of pre-nectar
