Biology Reference
In-Depth Information
ultimately reaches the level where pores in the cuticle over the tip cell
become patent and release a pulse of nectar to the exterior (Robards &
Stark, 1988).
This model combined the classical symplastic transport of nectar in sec-
retory hairs with a new system of apoplastic transport where the SR is
primarily involved.
A detailed plausible hypothesis for the function of the nectary as a whole
is only available in sporadic cases of parenchymatous nectaries.
According to Fahn (1988), hydrolysis of sucrose in the nectary cells
maintains a sucrose concentration gradient that could cause a passive flow of
sucrose from sieve elements to nectary secreting cells. This model may also
explain the preferential flow of pre-nectar towards secretory cells rather than
neighbouring cells; it is, however, not applicable to nectaries where sucrose
is the dominant sugar secreted.
A somewhat similar model with new hypotheses was formulated by
Nichol and Hall (1988). Excised nectaries of various species cultivated
in vitro
continue to secrete nectar for a period if sugars are supplied (Matile,
1956; Findlay et al., 1982; Bieleski & Redgwell, 1980; Nichol & Hall,
1988). This experimental approach is important to understand the function of
nectaries, especially their status as autonomous organs with respect to other
parts of the flower. Nichol and Hall (1988) studied nectar secretion in ex-
cised extrafloral nectaries of
Ricinus communis
, finding that sucrose,
glucose, and fructose can each sustain secretion of nectar, unlike other sug-
ars, and that the nectar secreted in each experiment contained all three
sugars. These results indicate the presence of sucrose synthase and sucrose
invertase activity in the nectary parenchyma. On the other hand, the phloem
vessels of
in situ
extrafloral nectaries of
R. communis
exclusively translocate
sucrose (Baker et al., 1978).
Nichol and Hall (1988) demonstrated that hydrolysis of sucrose occurs as
a final step of secretion during transport across the membrane of the tono-
plast of secreting cells, and no invertase activity was found in the nectar.
They also questioned why nectar consistently contains sucrose as well as
hydrolysis products. They postulated two transport pathways in
R. communis
extrafloral nectaries. One involves sucrose hydrolysis to the monosaccharides
glucose and fructose; it is energy-dependent (ATPase activity was found in
the secretory epidermis) and inhibited by anaerobic conditions. The second
pathway only involves sucrose; it does not require energy because it is not
inhibited by anaerobic conditions and contributes to the sucrose component of
