Biology Reference
In-Depth Information
3.5
Nutrient stress
Lack of nutrients combined with water shortage may constitute another
stress limitation to nectar secretion in Mediterranean plants. A first attempt
to investigate the effect of nutrient application on nectar secretion was made
by Shuel (1955) on non-Mediterranean plants. He concluded that nectar
secretion is higher under low nitrogen supply. This conclusion was experi-
mentally confirmed by Petanidou et al. (1999) in an extensive study carried
out on Mediterranean plants. The authors investigated the effect of artificial
nutrient supply on nectar secretion in three Lamiaceae species, both potted
and naturally growing. Interestingly, they found similar results to those for
irrigation (increased nectar secretion in the case of
Thymus capitatus
, no
change in
Satureja thymbra
, and mixed trends in
Stachys cretica
). They con-
cluded that irrigation is more important than nutrient supply in increasing
per-flower nectar secretion, implying that the most influential external factor
in shaping the physiology of nectar secretion in the Mediterranean is pri-
marily drought, not nutrient scarcity. Owing to the production of surplus
flowers on artificially fertilized plants (as a result of extended vegetative
growth), a much higher number of nectarless flowers were found than on
untreated plants (Petanidou et al., 1999). The presence of empty flowers has
been considered to be of evolutionary significance, as it may enhance insect
movements between flowers and plants, increasing visitation rates, reducing
geitonogamy and thus increasing plant fitness (Brink & de Wet, 1980; Bell,
1986; Gilbert et al., 1991; Sakai, 1993). The results of Petanidou et al. (1999)
indicate that this may apply particularly to regions experiencing long periods
of drought, such as the Mediterranean.
Another effect of nutrient supply is the alteration of the chemical compo-
sition of nectar. Petanidou et al. (1999) found that nutrient application results
in nectars having higher sucrose/hexose ratios than controls (although in
Thymus capitatus
the results were not significant). In the same series of ex-
periments, amino acid concentration of nectars remained statistically
unchanged after treatment with fertilizer. Treated flowers and controls, how-
ever, differed markedly in the relative abundance of certain amino acids,
which were different among the three study species. In a similar study car-
ried out in the UK, Gardener and Gillman (2001a) found that the concentration
of total amino acids together with those of glutamine and proline increased
significantly with increasing fertilizer treatment in
Agrostemma githago
,
whereas the concentration of GABA decreased. Fertilizing also resulted in a
significant decrease of the relative abundance of about half of the amino ac-
ids in the nectar of
Agrostemma
, with the exception of glutamine, which
increased. The results of both studies show that the nectar complement can
