Biology Reference
In-Depth Information
analysis” on page 276 will help in a more global understanding of the role of
general metabolism in nectary function.
6.3
Hormones and nectar production
The involvement of plant hormones in flowering is a complex problem
marred by myriad studies with contradictory results. Similarly, the role of
plant hormones in nectar production is not entirely clear-cut. Two plant
hormones have been studied repeatedly in relation to nectar production. The
first of these is auxin, which appears to inhibit nectar secretion. In 1956,
Philippe Matile examined the effect of IAA on nectaries from two plant spe-
cies. These studies showed that IAA strongly inhibited nectar secretion in
Euphorbia pulcherrima
nectaries; this inhibition was much weaker in
Abuti-
lon striatum
(Matile, 1956). In later studies, both pretreatment and constant
exposure of
Antirrhinum majus
nectaries to 500 µM IAA inhibited nectar
secretion by 25-50%. Interestingly, Ca
++
ions nullified the inhibitory effects
of IAA on sugar secretion (Shuel, 1964, 1967). Later, using radio-isotopes,
Shuel (1978) demonstrated that incubation with IAA resulted in an inhibition
of nectar secretion within hours, and the decrease in radioactivity incorporated
into nectar was accompanied by increased incorporation of radioactivity into
nectary protein. Furthermore, these investigators demonstrated that such lev-
els of IAA initiated a reorganization of the nectary, which led to a cessation
of secretion, an increased incorporation of labelled uracil into RNA, and a
stimulation of growth that resulted in a considerable enlargement of the nec-
tary (Shuel & Tsao, 1978). Finally, in very recent work, Aloni et al. (2005)
demonstrated that one of the functions of IAA in
Arabidopsis thaliana
flow-
ers was to retard nectar secretion up to the time of anthesis.
The second plant hormone whose role in nectar production has been ex-
amined is gibberellic acid. Flowers from plants treated with GA
3
had the
maximum volume of nectar per flower and the highest levels of sugar con-
centration in nectar. As might be expected, the GA
3
-treated plants that
showed increased sugar and nectar levels also showed increased pollinator
visitation (Mishra & Sharma, 1988). By contrast, GA
3
treatments inhibited
the initiation of nectaries in normal buds of
Nigella damascena
(Raman &
Greyson, 1978). The contradictory nature of these studies suggests that addi-
tional work is needed to examine the role of these and other plant hormones
in the process of nectar secretion. Although many of these studies are dated
and were performed prior to the “omics” scientific era, they suggest direc-
tions of nectary research that should be renewed with the modern tools that
are available today.
