Biology Reference
In-Depth Information
are presumably possible in a single nectary (Schnepf & Pross, 1980; Pate
et al., 1985).
The source of nectar carbohydrates may be immediate photosynthesis by
the nectary itself or by any other part of the plant, or may require temporary
starch storage in the parenchyma cells (Pacini et al., 2003). The two modes
are strictly related to the rate of secretion: a very high nectar secretion rate
requires starch storage in the parenchyma with big amyloplasts differentiat-
ing before secretion (Durkee et al., 1981; Belmonte et al., 1994; Nepi et al.,
1996; Maldonado & Otegui, 1997), whereas a low rate of nectar secretion is
often associated with chloro-amyloplasts with poor thylakoid structure, ir-
regular shape and plastoglobuli (Stpiczyńska, 1997, 2003; Razem & Davis,
1999). Floral nectaries may manifest both modes of carbohydrate supply,
whereas in extrafloral ones nectar is always derived from direct photosyn-
thesis.
In plants with a high nectar secretion rate and starch-storing nectary pa-
renchyma, there is a dramatic increase in the number of mitochondria just
prior to anthesis, indicating that the comparatively rapid breakdown of
stored
starch requires more immediate energy than the gradual storage of starch
during flower bud development (Durkee et al., 1981).
The source of nectar carbohydrates and the manner of nectar secretion
seem correlated:
Passiflora
,
Cucurbita
,
and
Rosmarinus
have eccrine secre-
tion and contain a lot of starch; other species with little or no starch at all in
the nectary may have granulocrine or eccrine nectar secretion (O'Brien et al.,
1996; Nepi et al., 1996) (Table 1).
Although some nectaries are green, presumably due to chlorophyll in
their plastids, it seems unlikely that the nectary parenchyma plastids them-
selves produce the starch grains observed inside them. Nectaries are often
concealed and only receive very diffuse light. This may be why the thylako-
ids and grana are underdeveloped. When nectaries are exposed directly to
the light, photosynthesis in the nectary parenchyma cannot be excluded
a priori
. The main photosynthetic activity probably takes place in the subnec-
tary parenchyma where a greater quantity of chlorophyll is located (Fig. 7).
The vacuoles of the nectary parenchyma or subnectary parenchyma cells
may contain different types of inclusions. Calcium oxalate crystals in the
form of druses or raphides have often been found in floral and extrafloral
nectaries (Davis et al., 1988; Horner et al., 2003; Stpiczyńska et al., 2003). It
has been demonstrated that Ca
2+
inhibits plasma membrane ATPase (Leonard
