Biology Reference
In-Depth Information
African orchids
A. verdickii
(Koopowitz & Marchant, 1998) and
M. venosum
(Luyt & Johnson, 2002), the nectar from unpollinated flowers is not reab-
sorbed and after anthesis it is probably lost.
Using radio-labelled sucrose, Shuel (1961) demonstrated that part of the
stigma exudate is derived from reabsorbed nectar and, conversely, sugar re-
absorbed from the stigma exudate may appear in nectar. This suggests a
general recycling of substances within the flower.
Nectar sugar reabsorption also plays an important ecological function,
being involved in nectar homeostatic mechanisms (Galetto et al., 1994;
Nicolson, 1995), the molecular basis of which is far from clear (Castellanos
et al., 2002). Nectaries are supposed to have a “sugar sensing” mechanism
for regulating nectar concentration. Sugar secretion may occur passively on
the basis of a concentration gradient, while regulation of concentration in the
apoplast could be achieved by sucrose hydrolysis and/or sugar reabsorption
(Castellanos et al., 2002). It was hypothesized that reabsorption could be a
response to modifications of cell turgor which in turn respond rapidly to
changes in osmolality (Castellanos et al., 2002 and references therein).
The nectar homeostatic mechanism enables regulation of nectar volume,
concentration, and thus viscosity, by reducing the effect of water loss due to
evaporation. Since nectar composition and concentration are adapted to the
type of animal visitor (Baker & Baker, 1983a), the nectar homeostatic
mechanism may be important to ensure visits by the most efficient pollina-
tor. Reabsorption of sugars reduces viscosity, which may facilitate nectar
probing, as in
Penstemon gentianoides
(Scrophulariaceae; Cruden et al.,
1983) and bird-pollinated flowers (Baker, 1975; Nicolson, 1995; Nicolson &
Nepi, 2005). The nectar homeostatic mechanism is presumed to be more
pronounced in plants with a long period of nectar presentation because they
are presumably exposed to variations in weather conditions and pollinator
visits that affect nectar characteristics. The homeostatic mechanism is in any
case not very precise. A regulation system that compensates precisely for
variations in nectar volume and concentration owing to pollinator activity,
temperature, evaporation, water stress, light stress, etc., would presumably
be metabolically expensive (Castellanos et al., 2002).
One would expect that plants living in dry habitats could compensate for
water evaporation with solute reabsorption, thus maintaining a constant nec-
tar concentration. This was demonstrated for
Grevillea robusta
(Proteaceae;
Nicolson, 1995), but in
A. castanea
constant nectar concentration was
