Biology Reference
In-Depth Information
3
FACTORS SHAPING NECTAR SECRETION
AND OTHER CHARACTERISTICS
The volume and concentration of nectar secreted by a flower depend on the
following three factors: (i) ambient humidity and temperature (Corbet et al.,
1979), (ii) selective reabsorption of solutes or water (Búrquez & Corbet,
1991; Nicolson, 1995), and possibly (iii) changes in the concentration at
which nectar is secreted (Corbet, 2003). This means that nectar volume and
concentration depend both on external conditions (climate, weather) and
plant characteristics (e.g., related to structural and physiological attributes of
plants and flowers). I discuss these factors in more detail below.
In a series of studies, Petanidou and Vokou (1990, 1993) and Petanidou
and Smets (1995) argued that the severe water deficit and very high tempera-
tures characterizing the Mediterranean summer and spring may have
detrimental effects on nectar secretion rates and volumes (see also Herrera,
1985). As nectar secretion evidently continues even under extremely harsh
conditions, one question is: to what extent are Mediterranean plants adapted
to secrete nectar under unfavourable conditions, i.e., in high temperatures or
low humidity?
3.1
Temperature
The nectar secretion rate increases with temperature, with an optimum de-
pendent on the species in question (Fahn, 1949; Shuel, 1952; Beutler, 1953b;
Huber, 1956; Corbet, 1990; Jakobsen & Kristjánsson, 1994). Petanidou and
Smets (1996) hypothesized that because Mediterranean plants are adapted to
high temperatures their optimal nectar secretion takes place at higher tem-
peratures than that of temperate plants. In other words, relatively high
temperatures could induce nectar secretion in Mediterranean plants. They
tested their hypothesis on thyme
Thymus capitatus
, a typical
phrygana
plant,
flowering under controlled temperature and humidity. Interestingly, nectar
secretion in thyme flowers increased with temperature up to 38ºC as long as
plants were not water-stressed or light-limited. The optimal temperature for
nectar secretion was found to be 32.5ºC, much higher than optimal tempera-
tures known for temperate species, e.g.,
Oenothera biennis
(optimal tem-
perature 24ºC),
Borago officinalis
(23.5ºC), and
Trifolium repens
(10ºC and
18ºC) (Shuel, 1952; Huber, 1956; Jakobsen & Kristjánsson, 1994). Petanidou
and Smets (1996) also observed that in the open and under temperate
summer conditions (i.e., low temperatures and solar irradiance) nectar secre-
tion in thyme depended more on changes in light levels than on temperature.
The authors concluded that temperature stress may stimulate nectar secretion
