Biology Reference
In-Depth Information
Recently, the last of the five ornamental tobacco nectarins (
Nec4
) was charac-
terized as a xyloglucan-specific endoglucanase-inhibiting protein (XEGIP),
which serves to protect the gynoecium from invading fungi (Naqvi et al.,
2005). Xyloglucan-specific endoglucanases (XEGs) are fungal enzymes that
hydrolyse the plant cell wall hemicellulases, thereby weakening plant cell
walls and offering fungi opportunities for invasion. The XEGIPs function to
inhibit these fungal pathogenesis factors much in the same way that polyga-
lacturonase inhibitor proteins inhibit fungal polygalacturonases (Pressey,
1996). The expression pattern of
Nec4
mRNA implies that the nectary con-
tinues this protective function even after pollination has occurred. The
identification of
Nec4
as a potent inhibitor of fungal endoglucanases pro-
vides strong support for the hypothesis that nectar also functions to protect
the floral base from invading microorganisms.
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OTHER NECTAR CONSTITUENTS
A wide variety of other biochemicals also accumulates in plant nectars.
Some of these may enrich the nectar, providing a better diet for visiting
pollinators, while others are thought to decrease the palatability of nectar to
unwanted floral visitors. Alkaloids, coumarins, saponins, and non-protein amino
acids in nectar may render it toxic or repellent to some animals (Guerrant &
Fiedler, 1981; Detzel & Wink, 1993; Adler, 2000). However, Rhoades and
Bergdahl (1981) argued that the ability of specific pollinators to tolerate
toxic compounds would serve as a co-evolutionary mechanism to manipulate
animal behaviour to the plant's advantage and to exclude nectar thieves. Fur-
thermore, if insects co-evolved to prefer toxic compounds, this would focus
pollinators on a single plant species or group of closely related species,
thereby maximizing the effectiveness of pollinator visitation. Pyrrolizidine
alkaloids in nectar are inhibitory to generalist-feeding butterflies but attrac-
tive to specialist feeders (Masters, 1991). Iridoid glycosides in
Catalpa
speciosa
(Bignoniaceae) are feeding attractants for caterpillars, and also oc-
cur in the nectar of this species. They were shown to deter nectar thieves but
not the legitimate pollinators (Stephenson, 1982).
A nectar property rarely considered is pH, which ranges from 3 in
Silene
alba
(Caryophyllaceae) to 10 in
Viburnum costaricanum
(Caprifoliaceae),
but is generally slightly acidic (Baker & Baker, 1983a). A more extreme
example is the parasitic
Lathraea clandestina
(Scrophulariaceae), which
flowers at ground level and produces pungent nectar-containing ammonia,
which is tolerated by pollinating bumblebees but deters ants. The freshly
secreted nectar is slightly acidic (about pH 6.5), becoming alkaline (pH 11.5)
