Biology Reference
In-Depth Information
8.1
Environmental variables
Enviromental parameters can affect nectar properties. Nectar, especially from
exposed nectaries, tends to reach a concentration in equilibrium with the RH
of the air (Corbet et al., 1979): low RHs tend to cause water evaporation and
concentrate nectar, while very high RHs tend to dilute nectar. However, the
nectar of unprotected flowers of several species does not reach the high con-
centration values that would be in equilibrium with low RH (Corbet et al.,
1979). At a given RH the rate at which evaporation elevates solute concen-
tration is inversely related to the size of the nectar drop (Corbet, 2003). The
effect of RH can be reduced by the following:
•
Morphological features of flowers that offer more protection to the nectar
(e.g., long corolla tubes, spurs).
•
Waterproofing lipid monolayers on the nectar surface, as hypothesized by
Corbet et al. (1979) in
E. vulgare.
•
Reabsorption of sugar (Nicolson, 1995).
•
Constant high-secretion rate of diluted nectar (Nicolson, 1995; Nicolson &
Nepi, 2005).
•
A combination of these factors—reabsorption of sugar and a very high se-
cretion rate of diluted nectar seem to be common strategies to maintain low
sugar concentration in dry habitats (Nicolson, 1995; Nicolson & Nepi,
2005).
Temperature is the environmental variable that is most often cited as re-
lated to nectar secretion rate. Temperature affects the rate of photosynthesis
that contributes, directly or indirectly, to nectar production (Burquéz &
Corbet, 1991, 1998). Nectar secretion decreases at low temperature in most
species but decreasing nectar production is also reported with increasing
temperature in
Ipomopsis longiflora
(Polemoniaceae; Freeman & Head,
1990) and
Trifolium repens
(Fabaceae; Jakobsen & Kristjansson, 1994). The
optimum range of temperature for nectar secretion is known in only a few
species (Jakobsen & Kristjansson, 1994 and references therein; Nicolson, 1995;
Petanidou & Smets, 1996 and references therein). In Mediterranean plants,
nectar secretion is adapted to higher temperatures (Petanidou & Smets, 1996;
Petanidou, 2007); e.g., optimal nectar secretion in
Thymus capitatus
Lamiaceae) is at 32.5°C. When this plant is grown at lower temperatures,
nectar production is a function of light intensity (Petanidou & Smets, 1996).
In natural conditions, the best nectar yields may occur in years with high
precipitation (Petanidou & Smets, 1996). Water availability has long been
invoked as a major factor in the regulation of nectar secretion rate (Wyatt
