Biology Reference
In-Depth Information
6.2
Metabolism and nectar production
Since nectar contains such high levels of sugars, carbohydrate metabolism in
the nectary is extremely important in understanding nectary function. The
earliest studies involved the secretion of nectar from excised nectaries of a
number of species; the nectaries were fed with a number of sugars (Frey-
Wyssling et al., 1954; Matile, 1956; Heinrich, 1975). A few studies have ex-
amined the role of glycolysis in the secretion of nectar (Zauralov, 1969a;
Bargoni, 1972a, b; Bosia & Pescarmona, 1972; Nichol & Hall, 1988). These
studies generally indicate that glycolytic enzyme activity as well as glyco-
lytic intermediates are high in nectaries. As the nectary matures, glycolytic
enzyme activity decreased (Zauralov, 1969b), as did nectar secretion. An-
aerobic conditions significantly affected the secretion from nectaries
incubated on glucose and fructose, but had less effect when nectaries were
incubated on sucrose, implying that respiration may be required for nectar
secretion. The role of respiration in nectar secretion was confirmed through
the use of respiration inhibitors. Inclusion of respiration inhibitors (azide,
KCN, 2,4-dinitrophenol, NaF, and arsenite) in the incubation medium also
inhibited secretion of nectar in a variety of species (Matile, 1956; Zauralov,
1969b; Zauralov & Zauralova, 1970).
Of special interest is the examination of the role of sorbitol among the
woody Rosaceae. Sorbitol is a major soluble carbohydrate in the phloem of
the woody Rosaceae (Watari et al., 2004); floral nectars, however, contain
very little sorbitol (Bieleski & Redgwell, 1980). Conversion of sorbitol to
other sugars occurs within the nectary primarily during phloem unloading.
Recently a number of sorbitol transporters have been isolated from sink or-
gans of several species (Gao et al., 2003; Watari et al., 2004; Zhang et al.,
2004). Understanding the involvement of these transporters in nectary
function will be very interesting.
Only a few other studies have investigated the role of nectary metabolic
enzymes in nectary function. Nichol and Hall (1988) examined a number of
metabolic enzymes in the nectaries of
Ricinus communis
. They localized acid
phosphatase in the parenchyma of the nectar, while ATPase was localized in
the nectary epidermis. In other studies, Zauralov and Pavlinova (1975) found
that high levels of acid invertase in gourd nectaries were responsible for the
high-hexose content found in the nectaries of that species.
There is clearly a lack of recent work on the role of general nectary
metabolism in nectar secretion. We hope that the studies outlined in “EST
