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Nectar volume and concentration data is sparsely recorded for other members of the
Lamiaceae. One record is given in Goldblatt & Manning (2000) for the long-tubed
O. tubiformis which had nectar volumes of 2.7 – 4.1 μl and concentration of 24.5% (SD
2.2). Nectar volumes in three bird-pollinated species of Leonotis (Pers.) R.Br. were
measured by Vos et al. (1994), and ranged from 5.3 – 11.3 μl per flower with
concentrations of 19 – 28%. These volumes reflect the nectar-feeding capacity of
sunbirds, but the nectar concentration falls within the range of insect-pollinated
species. Nine species of the Lamiaceae were studied by Dafni et al. (1988) and nectar
volume per flower was found to be small, ranging from 0.14 – 6.3 μl, with most of the
species having less than 1 μl per flower. Nectar sugar concentration was generally
high, from 25 – 52%, and no correlation was found between honeybee preference for
different plants and nectar volumes or sugar concentrations (Dafni et al. 1998). A
subsequent study, measuring floral parameters in thirteen species of Labiatae in Israel
(Dafni 1991), presents nectar concentration values of 17 – 60% sucrose equivalents,
with nectar volumes of 0.4 – 18.2μl per flower over 24 hours. Larger flowers were found
to offer more calorific reward per flower, and large flowers lasted longer. Nectar
rewards in large flowers, with longer floral tubes, were also better protected from shorttongued
insects and general climatic conditions (Dafni 1991). The study by Ford &
Johnson (2008) showed four species of Syncolostemon (Lamiaceae) with tube lengths
varying from 9.6 – 28.0 mm to have corresponding nectar volumes of 0.7 – 3.2 μl and
variable nectar concentrations of 21.6 – 29.3 %.
In the current study there does not appear to be a relationship between corolla tube
length and maximum volume of nectar (Table 4), since short-tubed species such as P.
hadiensis (Forssk.) Schweinf. ex Spreng. and P. ciliatus had nectar volumes similar to
those of the long-tubed P. ambiguus and P. hilliardiae. This is contrary to results in
Syncolostemon, where Ford & Johnson (2008) found a positive correlation between
nectar volume and corolla tube length. The occurrence of saccate corolla bases in
Plectranthus shows no obvious relationship to nectar volume either (Table 4).
There is, however, a general trend for lower concentrations of nectar to be found in
longer-tubed flowers of Plectranthus that are pollinated by S. wiedemanni, but no other
trend is evident with respect to pollinator class (Tables 5 & 6). The nectar concentration
values for Plectranthus species pollinated by S. wiedemanni vary from 4 – 55%, which
extends the 20 – 32 % range suggested by Goldblatt & Manning (2000) for longproboscid
flies. The limited nectar concentration data presented in Potgieter et al.
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