Biology Reference
In-Depth Information
appeared to perform optimally under temperate conditions where neither tem-
perature nor solar irradiance appeared to limit nectar secretion (Petanidou &
Smets, 1996).
3.4
Water stress
Because water is suspected to be a permanent limiting factor for nectar se-
cretion in the Mediterranean, it is probably logical to assume that the highest
nectar yields occur in the years of highest precipitation. Although there are
no hard data, there is some support for this hypothesis from observations
made in the wild on
Capparis spinosa
,
Thymus capitatus
,
Prasium majus
,
Satureja thymbra
,
Asphodelus aestivus
, and
Ornithogalum exscapum
(Petanidou & Smets, 1996; Petanidou et al., 1996; Petanidou, 1999).
To address the question of nectar secretion under water stress experimen-
tally, Petanidou et al. (1999) studied the effect of irrigation on nectar
secretion in three Lamiaceae species typical of
phrygana
(
Satureja thymbra,
Stachys cretica,
and
Thymus capitatus
). Experimenting on potted plants taken
from the wild, along with control measurements carried out on naturally
growing non-irrigated potted plants, they found that after treatment only
T. capitatus
produced higher nectar volumes and total sugars per flower.
Nectar yield in
S. thymbra
did not change with irrigation, whereas
S. cretica
showed dissimilar trends depending on the irrigation time within the flower-
ing period. The authors concluded that irrigation may promote nectar
secretion only in flowering periods that are unfavourable for growing, e.g.,
in summer. During such periods available water resources are probably allo-
cated solely to nectar secretion—which may add up to considerable
quantities—rather than to vegetative growth and excessive flower produc-
tion, as may occur during spring.
Under typical Mediterranean conditions high-sucrose nectars predomi-
nate over high-hexose nectars, which implies that the former have been
selected for (Petanidou, 2005). An explanation may be given by the overrid-
ing effect of drought, the most ecophysiologically effective constraint in the
region. High-hexose nectars consume more water than high-sucrose nectars
for the same amount (weight) of sugars (Nicolson, 1998, 2002). Therefore,
by having high-sucrose nectars, Mediterranean plants avoid excessive water
loss from hundreds of ephemeral flowers. In addition, by having high-sucrose
nectars hidden within deep flowers, plants avoid nectar loss through evapo-
ration during the hot and dry period of the year (Petanidou, 2005).
