Biology Reference
In-Depth Information
Two orchid genera ( Aerangis and Rangaeris ) in Kenya are pollinated by
sphingid moths, which drink from floral spurs that can exceed 10 cm in
length (Martins & Johnson, 2007). Sampling of the nectar at intervals along
the spurs showed steep concentration gradients, from about 1% at the mouth
of the spur to 20% at the tip. It is not clear how the gradients in the nectar
columns are generated or maintained, but perhaps they encourage moths to
probe more deeply.
As an example of the complex factors influencing nectar rewards,
Búrquez and Corbet (1998) carried out a detailed study of nectar dynamics
in the annual herb Impatiens glandulifera (Balsaminaceae), a Himalayan
species that is now invasive in Europe. They examined the interacting effects
of nectary activity, microclimatic modification, and animal visitors. Most
importantly, the variables affecting nectar secretion are different from those
affecting its solute concentration. Nectar secretion depends on air tempera-
ture and plant age. It declines in older plants where fruits constitute an
additional carbohydrate sink. Nectar concentration depends on short-term
microclimatic factors, especially relative humidity, which has an immediate
effect (slowed by high secretion rates, large volumes, and long corollas). In
that study, a wide spectrum of visitors ensured that the nectar-standing crop
was too small to measure for most of the day. Incidentally, the successful
invasion of I. glandulifera may, in part, be due to the fact that it offers bum-
blebees higher rewards than native plants (Chittka & Schürkens, 2001). In
considering associations of nectar concentration with pollinator type,
Búrquez and Corbet (1998) stressed that selection does not act on the nectar
concentration itself, but on the factors that determine it—corolla structure,
the rate of secretion, and the chemical composition, which alters concen-
tration/humidity relationships.
Table 1. Effect of rain on nectar concentration (% w/w) in Protea species with and without
furred inflorescences. Values are means ± SE (n). (Unpublished data of S.W. Nicolson and
C.A. Beuchat, collected in Kirstenbosch Botanic Gardens, Cape Town.)
Protea species
Inflorescence
type
Concentration in
dry weather
Concentration after rain
P. coronata
Furred
25.0 ± 0.5 (6)
23.7 ± 1.1 (8)
P. longifolia
Furred
27.6 ± 1.5 (14)
21.8 ± 2.4 (12)
P. neriifolia
Furred
27.5 ± 0.9 (12)
22.1 ± 3.2 (8)
P. aurea
Smooth
16.6 ± 0.6 (4)
3.0 ± 1.7 (4)
P. compacta
Smooth
23.6 ± 1.2 (12)
0.8 ± 0.2 (6)
P. eximia
Smooth
29.9 ± 2.9 (7)
3.3 ± 2.2 (7)
P. obtusifolia
Smooth
22.1 ± 0.6 (6)
6.9 ± 2.0 (10)
P. repens
Smooth
21.6 ± 1.0 (13)
1.9 ± 0.7 (11)
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