Biology Reference
In-Depth Information
Table 2.
Common substances contained in nectar and their possible origins and functions.
Some of these substances may have different origins according to the type of nectar secretion
(i.e., holocrine, granulocrine, or eccrine) because they involve the activity or degeneration of
different cell types. The presence of antioxidants becomes a necessity in the case of exposed
nectar presented for long periods.
Substances
Possible origin
Functions
Water
Xylem and/or phloem sap
A medium in which soluble sub-
stances can dissolve
A reward, especially in dry eco-
systems
Ions
Xylem and/or phloem sap
Carbohydrates
Phloem sap, photosynthesizing
and/or starch storing paren-
chyma
A reward for pollinators
Amino acids and solu-
ble low molecular
weight proteins
Phloem sap and/or a product of
the activity of certain nectary
parts
(parenchyma or epidermis)
A reward for pollinators
Enzymes
Certain nectary parts
(parenchyma or epidermis)
Responsible for secretory and
post-secretory modifications
Lipids
Certain nectary parts
(parenchyma or epidermis)
A reward for pollinators and a
way to reduce water evaporation
when lipids form a surface layer
Volatile substances
Certain nectary parts
(parenchyma or epidermis)
Involved in attraction or repulsion
of visitors
Toxic compounds
Certain nectary parts
(parenchyma or epidermis)
To discourage certain consumers
and to reduce the potential visitors
(e.g., alkaloids,
phenols)
Antibiotics
Certain nectary parts
(parenchyma or epidermis)
To reduce growth of moulds and
bacteria
Antioxidants
Certain nectary parts
(parenchyma or epidermis)
To avoid oxidation of substances
such as lipids
pigments than the nectary. Nevertheless, this research did not demonstrate
whether this type of nectary is completely autonomous for nectar carbohydrates.
When a nectary lies below the ovary (Ericaceae, Scrophulariaceae) or
forms a protuberance at the base of the ovary (Brassicaceae, Fabaceae), where
it is often covered by the calyx, corolla, or stamen filaments, it is more diffi-
cult to believe that it is totally self-sustaining, even if its parenchyma cells
contain chloroplasts. This could be possible only if the photosynthetic pig-
ments “concentrate” the little available light. In this case we expect to find
