NECTAR PRODUCTION AND PRESENTATION - Nectaries and Nectar - page 184

used by the nectary to produce nectar. Notwithstanding the problem of par-
tial autonomy of the flower, the various researchers working on nectar
biology commonly have not considered the sources of the different nectar
components. Pacini et al. (2003) showed that nectar components may have
different sources, depending on the type of nectary and its cytological struc-
ture.
Certain previous researchers (Frey-Wyssling, 1955; Fahn, 1979b) work-
ing on nectary cytology and physiology attributed much importance to
phloem sap as precursor of pre-nectar. According to these authors, pre-nectar
is transformed into nectar by nectary parenchyma cells.
The two main soluble nectar components are carbohydrates and amino
acids/proteins. These have different origins according to secretion type. If
secretion is of the merocrine type, at least part of the two main components
are derived from phloem or xylem sap transformed by nectary parenchyma.
In the case of holocrine secretion, nectar components may be derived from
xylem and phloem sap and nectary cell cytoplasmic content.
Nectar carbohydrates are directly or indirectly derived from the photo-
synthetic activity of the nectary itself or of other floral or vegetative parts,
generally close to the flower. If the nectary is responsible for the formation
of part of the nectar carbohydrates it must have a photosynthesizing part.
This photosynthesis may occur in the parenchyma or in cells close to it (sub-
nectary parenchyma, Nepi, 2007). The parts most often involved in the
production of nectar carbohydrates by photosynthesis are the flower pedicel,
calyx, ovary, and even adjacent leaves. This is, however, only a hypothesis,
as no conclusive demonstration is yet available.
These inner carbohydrate sources of nectar (coming from the nectary pa-
renchyma itself) and outer sources (coming from tissues situated outside the
nectary, mentioned previously) cannot generally be considered mutually ex-
clusive.
The presence of phloem or xylem in the floral nectary complex may indi-
cate whether a nectary is partially or totally autonomous, but the little
research on this topic is inconclusive. Authors sometimes omit to describe
the type of vascular bundle when they describe the structure of a nectary.
The presence of phloem only, or both xylem and phoem, their relative abun-
dance and disposition have a physiological meaning, i.e., transport of complex
molecules such as sucrose, amino acids, and soluble proteins. Only phloem
is present in B. napus (Brassicaceae; Davis et al., 1986), V. faba (Fabaceae;