Biology Reference
In-Depth Information
The third hypothesis is the mechanism commonly invoked for leaf
chloroplasts, but the first and the second are possible modifications for rapid
production and use of nectar carbohydrates derived immediately and directly
from photosynthesis.
Chloroplasts and amyloplasts differ in morphology and physiology dur-
ing development and nectar secretion. Amyloplasts change during their
development because the starch content increases before secretion and de-
creases during secretion. Secretion may only last a few hours. Chloroplasts
change during the day and night because they store starch during the day and
starch is hydrolized during the night. This implies that ultrastructural obser-
vations must be done at different times of day in order to determine the
pathway of plastid differentiation. Available ultrastructural observations
rarely consider plastid morphology changes in time. Night observations are
totally absent.
The environment may affect nectar production irrespective of plastid type
in the nectary parenchyma (chloroplasts without starch, chloroplasts with
starch grains, amyloplasts), but the effects of environmental conditions act at
different times. If nectary parenchyma cells have chloroplasts, nectar pro-
duction is affected by immediate environmental conditions, whereas it is
affected by the environmental conditions of previous days when nectary pa-
renchyma cells contain amyloplasts.
An advantage to having nectary parenchyma with chloroplasts is that it
continues to function when nectar secretion is over (persistent nectary), con-
veying photosynthate to other parts of the flower or fruit. Other advantages
and disadvantages of the two types of plastids in nectary parenchyma are
listed in Table 3.
On the other hand, an advantage of nectaries with amyloplasts in the nec-
tary parenchyma cells is that nectar may be available for consumers, and in
large quantities, at any time of the day or night. The nectar may also have a
high sugar concentration and can be produced in a short time. In the case of
C. pepo
, nectar becomes available from 6 am and has a high sugar concen-
tration (30-40%). This high rate of nectar production is impossible with
nectary parenchyma containing chloroplasts. In
C. pepo
, nectar is secreted
for only 3-4 h but the nectary does not immediately degenerate. Empty amy-
loplasts may be involved in temporary storage of reabsorbed carbohydrates
if the nectar is not totally consumed by flower visitors (Nepi et al., 1996a, b).
Unconsumed nectar carbohydrates are temporarily polymerized to spherical
electron dense bodies which react positively to the PAS test for total insoluble
