Biology Reference
In-Depth Information
be identified by the HPLC methods used in their study.) Phenylalanine was
highest in the Lamiaceae, where it comprised on average 47% of the total
amino acids, and high proportions of phenylalanine have also been recorded
among the nectar amino acids of
Salvia fruticosa
and
Satureja thymbra
in
Israel (Dafni et al., 1988). Phenylalanine is known to be a phagostimulant for
bees (Inouye & Waller, 1984). However, the fact that it stimulates the sugar
cell in flies (Shiraishi & Kuwabara, 1970) is unlikely to be important in the
responses of bees to the concentrated nectars in Mediterranean systems (see
also Petanidou, 2007, Chapter 8 in this volume).
6
PROTEINS
The existence of proteins in nectar was reported long ago (Pryce-Jones,
1944; Lüttge, 1961). The first enzymatic activity to be identified in nectar
was invertase, found in the floral nectar of
Tilia
sp. (Beutler, 1935). How-
ever, for the most part, these earliest studies did not characterize nectar
proteins. Baker and Baker (1975) studied 129 species of plants using a bro-
mophenol blue assay and found that 17% showed a positive test for the
presence of protein.
Table 4 lists proteins that are reported to be secreted into various plant
nectars. The term
nectarin
refers to any protein that is secreted into the nectar
of plants. Lüttge (1961) reported the presence of a tyrosinase in the nectar of
Lathraea clandestina
, but its function in the nectar of plants is still not clear
45 years later. Tyrosinase catalyses the hydroxylation of phenolic compounds
(Metzler, 2003) and consequently could function to modify phenolic com-
pounds secreted into nectar (see “Phenolics” on page 247). Transglucosidase
and transfructosidase identified in nectar (Zimmerman, 1953, 1954) are
involved in the production of glucosyl or fructose oligosaccharides. The
function of oligosaccharides in nectar is not clear, but they could provide
additional nutritional benefits.
Phosphatases, enzymes that catalyse the removal of phosphate from or-
ganic molecules, have been reported in the nectar of a number of species
(Cotti, 1962; Zalewski, 1966). Phosphatases are known to have multiple
functions in eukaryotes, e.g., recovery of phosphate during times of phos-
phate starvation (Bozzo et al., 2002), signal transduction (Li et al., 2001),
metabolism of nucleic acids (del Pozo et al., 1999), and defensive functions
(Chandra & Low, 1995). Each of these functions is important in plants;
none, however, has been examined for nectar phosphatases. Zauralov (1969)
reported the presence of oxidizing enzymes in the nectar of milkweed.
The enzymes identified were polyphenol oxidase, cytochrome oxidase, and
