Do
they make sugar?
If a plant lacking a conventional nectary
nevertheless produces nectar it is reasonable to ask where the nectar comes
from. It is this question that has motivated the research of
Machado et al. (Botucatu, Brazil, pp. 357–369). The
plant in question is
Zeyheria montana, a member of the
Bignoniaceae that grows in the tropical savanna of
Brazil. The pollinators are humming
birds, attracted to the flowers by copious quantities of nectar. However,
Z. montana has only a
rudimentary, non-functional nectariferous disk round the base of the ovary. It
had been suggested previously by this research group that nectar is produced
from glandular trichomes also located around the base of the ovary. However,
according to the old proverb, the proof of the pudding is in the eating (or,
for the humming birds, the drinking). What actually do these trichomes secrete?
The authors have shown that the trichomes are present from an early stage in floral
development, certainly long before nectar is secreted. Further, new trichomes
are initiated through flowering and into fruit set so that trichomes at
different stages of development are present at the same time. The timing of
secretion from the trichomes is also significant, again occurring from
flower-bud stage into fruiting. This is not a pattern expected of nectariferous
organs and, indeed, chemical analysis indicates strongly that these glandular
trichomes are not substitute nectaries. The majority of the secreted substances
are terpenoids, accompanied by alkaloids, while sugars are undetectable.
Consistent with this, the authors observe the accumulation of lipophilic
substances initially in plastids (the site for terpenoid synthesis) and then
movement from the plastids via the endomembrane system to the secretions. The
authors thus reject their earlier views of the function of these trichomes and
suggest that rather than functioning as nectaries, they are involved in defence
of the flower and young fruit.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
