Biology Reference
In-Depth Information
encountered crop, i.e., the standing crop encountered by pollinators foraging
systematically, is likely to exceed the mean standard crop measured by an
unselective ecologist who randomly samples flowers (Kadmon, 1992; Cor-
bet, 2003). Shmida and Kadmon (1991) pointed out that the mean nectar
standing crop per flower, the variance in nectar standing crop, and the scales
of variation contributing to the total variance, may all vary considerably at
different times during the day, so that a forager returning to the same patch
after a short time may encounter a completely different pattern of nectar dis-
tribution.
The spatial distribution of nectar production in a population may have a
genetic basis (Leiss et al., 2004; Mitchell, 2004) and it may also be affected
by small-scale variations of environmental parameters (water availability, air
humidity, temperature, and light) (Leiss & Klinkhamer, 2005a).
4
THE SOURCE OF NECTAR COMPONENTS
Sexual reproduction represents a strong investment of resources. Vegetative
growth slackens or ceases when sexual reproduction is starting. This is par-
ticularly evident in annual plants. Resources for basal metabolism, on the
other hand, are always kept constant (Wardlaw, 1990).
The demand for resources for reproduction, in the absence of predation,
increases continuously until fruit ripens, because of increases in biomass.
The plant invests in the different parts of the flower and later fruit, but some
of these parts (pedicel, calyx, corolla, stamens, ovary, fruit integuments) are
green, contain chloroplasts and may photosynthesize and self-sustain, spar-
ing the mother plant resources.
Hoch (2005) demonstrated experimentally that developing fruits of some
European trees “exhibited complete carbon autonomy of fruiting at the level
of whole, undisturbed branchlets”. We do not know how much this self-
sustainability applies to other phases of reproduction, types of plants, and
environments; nevertheless, it represents a matter for speculation. The pres-
ence of some green and photosynthesizing reproductive structures in flowers
induces us to think that they may be autonomous, at least for part of the car-
bohydrates.
Nectary formation and nectar secretion have a high energy demand.
Southwick (1984) and Pyke (1991) quantified the energy necessary to pro-
duce nectar and demonstrated that more than 30% of daily photosynthate is
