Stuck-open
stomata secrete sugary solution
The
value of continuity in research programmes is well illustrated by the work of a
regular contributor to Annals of Botany, K. L. Davies of Cardiff, UK.
The main focus of the author's work is floral biology in the orchid genus Maxillaria;
the present paper, written by Davies and colleagues from Lublin,
Poland and from Swansea, UK
(pp. 217–227) is no exception. Within the genus, a range of pollinator
rewards has been observed, although it appears that many species offer no
reward at all. Whether or not any Maxillaria species
produce nectar has, until recently, been a matter of controversy.
However, it is now clear that a small percentage of species in this genus do
so, although this has only been described in detail for one, somewhat atypical
species, the bird-pollinated M. coccinea. Thus, the authors have
focused on a more typical insect-pollinated species, M. anceps. They
describe the flower as zygomorphic with a well-developed labellum. Nectar is
secreted by a structure known as the labellar callus, consisting largely of
parenchyma cells overlain by a secretory epidermis. This structure thus
functions as a nectary and, somewhat unusually, nectar secretion occurs via modified stomata complete with
guard cells. The latter are cuticularized, which prevents full closure of these
stomata. The authors state that this is the first report of nectar-secreting
stomata in an orchid although they have been described in other angiosperm
families. Chemical analysis of the nectar revealed a very high sugar content (ca.
62 % sucrose with traces of glucose and fructose) plus free amino acids at low
concentrations. The fluorescence of the pollen, taken with a staining reaction,
indicated the presence of terpenoids, compounds also found in the resinous
secretions of some non-nectar-producing Maxillaria species. The
secretion of a ‘high-sucrose’ nectar is consistent
with bee pollination. Indeed, the authors suggest that the pollinators are
stingless bees, a suggestion that invites further careful observations in the
field.
