Biology Reference
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Figure 2. Correlation of floral maturation with nectary development. The floral development
in tobacco extends from stage +1 through stage 12. The minus stages refer to the development
of the bud, prior to floral development. After stage 12, the maturation of the flower continues
with little change for several days; these stages are referred to as 12A to 12D. After fertiliza-
tion (PF), fruit development begins.
2
THE ORNAMENTAL TOBACCO NECTARY
In ornamental tobacco, the nectary consists of a single large torus-shaped
structure at the base of the ovary. As it matures, the nectary changes from
lime green to yellow to bright orange due to accumulation of high levels of
β-carotene (Fig. 1). Nectar is secreted only from a limited region on each lat-
eral face of the nectary. These regions have an abundance of open stomata
that together make up the nectary pore (Thornburg et al., 2003). Outside of
this region, stomata do not occur on the nectary surface.
Flower development in tobacco has previously been divided into 12
stages based upon morphological floral features (Koltunow et al., 1990). We
have correlated the development of the floral nectary in ornamental tobacco
with the overall floral developmental patterns, and a profile demonstrating
this correlation is shown in Fig. 2.
The development of the floral nectary consists of a number of discrete
stages. Organ initiation of the floral nectary occurs at a primordial floral
stage while the flower bud is still quite small. By floral stage 1, the nectary is
a well-differentiated ring of cells surrounding the developing gynoecium.
The filling stage of nectary development is characterized by an engorgement
of the nectary. During this stage, the nectary enlarges two- to three-fold. This
begins at early stages and continues through floral stage 7 (approximately
72-96 h). Ripening begins about floral stage 6 and lasts until about floral
stage 10. It is characterized by the synthesis and accumulation of β-carotene.
During this time frame, the nectary turns from lime green to bright orange
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