Biology Reference
In-Depth Information
two different categories. Initially, they appear as chloroplasts with thylakoid
membranes. With time, however, the plastids morph into forms that accumu-
late first starch (see below), then high levels of carotenoids. These unusual
plastids can best be described as amylochromoplasts (Horner et al., 2007).
The mechanisms responsible for β-carotene accumulation or for the conversion
of chloroplasts into amylochromoplasts have not yet been elucidated. Further
studies along these lines are in progress.
3.3
Filling of the nectary
As can be seen in Fig. 1, one of the most significant changes that the orna-
mental tobacco nectary undergoes during development is its dramatic
increase in size. This swelling is a prelude to the active secretion of floral
nectar that occurs just a few days later. Our early light and electron micro-
scopic studies of developing ornamental tobacco nectaries revealed that
neither phloem nor xylem vessels innervate the tobacco nectary. However,
the composition of ornamental tobacco floral nectar is 35% (w/v) sugar. The
source of this sugar was not clear.
Developmental studies of the nectaries of other species suggest that the
accumulation of starch is a prominent feature of many nectaries (Zauralov &
Pavlinova, 1975; Nepi et al., 1996a; Peng et al., 2004; Stpiczyńska et al., 2005).
To evaluate whether ornamental tobacco nectaries accumulated starch during
their development, we stained nectary tissue from two stages of development
with I
2
/KI. Nectaries from stage 9 (late ripening stage/early maturation stage)
and stage 12 (anthesis, secretory stage) both stained a deep purple, a positive
indication for starch. Subsequently, total glycans were analysed from nectar-
ies at these same developmental stages. The data in Table 2 show that stage
9 nectaries contain four to five times more starch than mature nectaries.
In recent studies, we have evaluated the production of starch throughout
nectary development. This was demonstrated using three different methods
(PAS staining and transmission electron microscopy (TEM) of nectaries, as
well as direct starch isolation from nectaries). All three methods gave good
agreement and provide experimental evidence that permits us to conclude
that the nectary stores starch during its development and that the starch is
degraded immediately prior to anthesis. We also monitored the production of
nectar sugars. These studies demonstrated that the time frame of nectar pro-
duction correlated with that of starch degradation. Thus, the degradation of
starch that occurs immediately prior to anthesis is likely the source of sugars
flowing into nectar.
