INTRODUCTION - Nectaries and Nectar - page 10

concentrations in the nectar of onion flowers have a deterrent effect on hon-
eybees (Waller et al., 1972).
Antioxidants such as ascorbate are involved in nectar homeostasis (Carter &
Thornburg, 2004).
Lipids are a high energy source but usually occur only in trace amounts in
nectar. In some flowers, oils secreted by elaiophores or glandular trichomes
are offered as rewards instead of nectar.
Terpenoids. Volatile terpenoids are important components of floral scents
(Raguso, 2004) and may accumulate in nectar.
Secondary compounds associated with resistance to herbivory have often
been documented in floral nectar (Adler, 2000). Toxic compounds such as
phenols and alkaloids may have a selective effect on pollinators, deterring
some and attracting others.
Cytoplasmic remnants result mainly from holocrine secretion where the
secretory cells break down in the process, e.g., Strelitzia reginae (Kronest-
edt-Robards et al., 1989), Glycine max (Horner et al., 2003).
Spores of fungi and bacteria dispersed in the air may fall into nectar, es-
pecially if it is exposed, and grow. Thus nectar may be a portal for plant
pathogen infections. However, antimicrobial substances with a homeostatic
function may prevent the spread of harmful organisms (reviewed by Carter
& Thornburg, 2004), because examples of infection are rare. In only a few
cases have these invasions been demonstrated to occur via the nectar. Spores
of the mould Aureobasidium pullulans and Cladosporium herbarum enter
the nectary via the nectar and destroy extrafloral nectaries in the leaves of
Ailanthus altissima (Clair-Maczulajtys & Bory, 1982). The pathogen bacte-
rium Erwinia amylovora , the agent of fire blight also enters flowers via
nectar (Bubán et al., 2003).
Nectar should be seen as a complex and dynamic fluid. Pollinators reduce
the volume, sometimes stimulating further secretion in the process, and con-
taminate it with microbes. Changes in nectar sugar are caused by activity of
the nectary (secretion or reabsorption) as well as removal by foragers, which
may stimulate further secretion. Nectar water content depends on activity of
the nectary, removal by foragers, and is additionally affected by equilibration
with ambient humidity (Corbet, 2003). This is particularly noticeable in the
more exposed extrafloral nectaries. The resulting spatial and temporal varia-
tion is a frequent theme in subsequent chapters.