INTRODUCTION - Nectaries and Nectar - page 8

producing honeydew are also present (Rashbrook et al., 1992). In these ex-
periments the ants strongly preferred honeydew to foliar nectar.

4 NECTAR COMPONENTS

Nectar composition varies widely, quantitatively more than qualitatively,

presumably because it is produced to reward different kinds of animals

(Faegri & van der Pijl, 1979; Cruden et al., 1983). Not surprisingly, nectar

consumers and pollen vectors are primarily taxa that have evolved the ability
to fly—insects, birds, and bats (Pellmyr, 2002). However, nectar rewards
also attract many non-pollinators. Dissolved substances in nectar have mul-
tiple functions: in addition to rewarding animals with water, ions,
carbohydrates, amino acids and low molecular weight proteins, nectar con-
tains scented compounds to attract consumers (Raguso, 2004), and enzymes
and antioxidants to maintain homeostasis of nectar composition (Carter &
Thornburg, 2004). It may also contain toxic compounds to discourage un-
wanted consumers (Adler, 2000). For solutes other than sugars and amino
acids, there is generally much more information available for floral nectars
than for extrafloral nectars. Although many constituents of nectar originate
in phloem sap, the latter fluid is more difficult to sample than nectar and
studies comparing the composition of both fluids in the same plant are rare
(for studies comparing phloem sap and extrafloral nectar see Baker et al.,
1978 for Ricinus communis ; Pate et al., 1985 for Vigna unguiculata ).

The major constituents of nectar (see Nicolson & Thornburg, 2007,
Chapter 5 in this volume) are given below, with a brief indication of their
origins and their importance for animal consumers. Only for the carbohy-
drate component of nectar are the origins well understood. The early
emphasis was on the energetics of the relationship between flower and polli-
nator, based on considering nectar as predominantly a sugar solution and
also on the high energy demands of many pollinators. More attention is now
being paid to the non-sugar components of nectar (it was Herbert and Irene
Baker who first drew attention to these; Kevan, 2003) and to their role in
pollinator attraction and nutrition. This is by no means an exhaustive list: see
Jakubska et al. (2005) for an example of the chemical complexity that be-
comes evident when nectar is subjected to suitable analytical techniques.
Water. Depending on nectary structure, water may be derived from both
xylem and phloem or phloem alone, with a lower water content being ex-
pected as the proportion of phloem in the vasculature increases. Nectar water
content depends on floral microclimate, and may be greatly affected by