Nectar Production in Abutilon II. Submicroscopic Structure of the Nectary

Abstract

Ab8tractThe nectary hair cells of Abutilon are particularly rich in mitochondria and endoplasmic reticulum. The vacuoles are initially small but enlarge and coalesce during nectar production when invaginations of the plasmalemma are often present at the tips and at the cross-walls of the hairs. The presence of these invaginations suggests that nectar may be transported through the nectary by a process resembling pinocytosis and exocytosis. Entry of nectar into the hair is restricted to a route through the protoplast of the stalk cell by a cutinized sheath in the outer walls of this cell.Pretreatment of the nectaries in anaerobic conditions or low temperature results in changes in the structure of the endoplasmic reticulum.

The ecological significance of toxic nectar

“…There are several possible pathways by which nectar components can move from vascular tissue to nectaries and then be transported outward (reviewed in Fahn 1988). Four possible pathways for the flow of ''pre-nectar'' from phloem endings through the parenchymatous cells of the nectaries and into secretory cells have been suggested: (1) via the apoplast, (2) via exocytosis and endocytosis (Findlay and Mercer 1971), (3) via molecular transport across the plasmalemma and passage through cell walls, or (4) via plasmodesmata. Fahn (1988) reviewed studies of nectary ultrastructure and concluded that the transport of pre-nectar is mainly through the symplast rather than apoplast (but see Genc 1996).…”

Nectar Production in Abutilon IV. Wate'r and Solute Relations

“…However, neither of the above proposed sites for an active transport step gains any support from the electron-microscope studies (Findlay and Mercer 1971b) since there is no particular accumulation of mitochondria or other organelles either at the hair tips or at the base of the hairs, but the mitochondria and endoplasmic reticulum are abundant throughout the hair cells. Nevertheless, it appears that although the active transport actually may be widespread in the hair its physiological effect (perhaps because of the construction of the hair) can be predicted from model 3(i).…”

“…Ziegler (1965) has considered such a model. There is some evidence from electron micrographs (Findlay and Mercer 1971b) for a process of this type in Abutilon.…”

“…In Abutilon, the stalk cell at the base of the hairs could be the site for this active transport as here movement of substances appears to be restricted to a route through the cell protoplast because the lateral walls of these cells are modified (Findlay and Mercer 1971b). As the sugar concentration in the hair increases, the water potential decreases and water will flow into the hair.…”

Vesicle-Mediated Solute Transport between the Vacuole and the Plasma Membrane

“…Evidence for a direct vesicle-mediated transport system between the vacuole and the plasmalemma is less apparent from cytological studies in sugarsecreting nectary cells, although in a few instances claims of such have appeared (Findlay and Mercer, 1971). Instead of a direct transport route of tonoplast vesicles from the vacuole to the plasmalemma, a more complex vesicle-mediated system is envisioned when one integrates the extensive cytological and anatomical observations of secreting nectaries with our current knowledge of Suc accumulation into intracellular compartments (Leigh, 1997).…”