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Flowers pollinated by nocturnally active animals expose nectar at night
(Cruden et al., 1983). As far as the quantity of nectar produced per flower is
concerned, numerous works demonstrate that flowers pollinated by high-
energy requiring animals, such as bats, hawkmoths, and birds, produce sig-
nificantly more nectar containing more sugar than flowers pollinated by
low-energy requiring animals, such as butterflies, bees, and flies (Cruden et al.,
1983 and references therein). Considering the nectar secretion rate, i.e., the
quantity of nectar secreted in a unit of time (generally an hour), Cruden et al.
(1983) recognized three classes of nectar producers:
Slow producers secrete 5-10% of their maximum accumulation per hour.
Fast producers secrete 22-68% of their maximum per hour.
Super producers secrete two or three times as much as fast producers.
It is reasonable to suppose that the different classes of nectar producers
have different nectary parenchyma features. According to Pacini et al.
(2003), nectar sugars can be derived directly from photosynthesis or from
storage material. In species whose pollinators require rapid unloading of
large quantities of nectar, storage of starch in nectary parenchyma cells un-
doubtedly provides the most efficient means of accumulating nectar
constituents (Belmonte et al., 1994). Amyloplasts in nectaries may not only
serve as a source of nectar sugars, but also of energy through starch hydroly-
sis (Belmonte et al., 1994). These plants may start to produce nectar at any
hour of the day or night. Most plants that produce nectar at night have stor-
age tissues in the nectary itself, other floral organs or the stem, leaves, or
roots. Since many of these species are crassulacean acid metabolism (CAM)
plants, the timing of nectar secretion may be linked to stoma opening
(Búrquez & Corbet, 1991). On the other hand, the nectar of many plants is
derived directly from phloem exudates, i.e., directly from photosynthesized
products, especially when the nectary parenchyma has a reduced mass. In
this case a very high nectar secretion rate cannot be expected. Defoliation
experiments conducted with Impatiens glandulifera (Balsaminaceae) dem-
onstrated that only a fraction of the day's nectar secretion depends on that
day's photosynthesis, while another fraction must be mobilized from stored
photosynthates in storage organs (Búrquez and Corbet, 1998). Some
Rosaceae that flower before leaf emission certainly use vegetatively stored
substances for nectar production. In fact Radice and Galati (2003) observed
amyloplasts in the nectary parenchyma of Prunus persica before nectar pro-
duction.
In most cases flowers begin to secrete nectar before pollinators start their
foraging activity and in some cases before the flowers open (e.g., Pleasants,
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