Biology Reference
In-Depth Information
In
Rosmarinus officinalis,
the vacuole volume increases after secretion,
the cytoplasm darkens and its volume decreases. There is a distinct increase
in ER cisternae, which appears to be related to the lytic process of the disin-
tegrating protoplast (Zer & Fahn, 1992). Similar processes of nectary
degeneration after secretion were observed in septal nectaries of banana
flowers (Fahn & Benouaiche, 1979) and in extrafloral nectaries of
Sambucus
nigra
(Fahn, 1987). Multilamellar bodies characterized by membranous con-
glomeration have been associated with degradative processes in a number of
species (Davis et al., 1986 and references therein; Nepi et al., 1996).
According to Durkee et al. (1981), a complete breakdown of the secre-
tory tissue occurs in the floral nectary of
Passiflora
in the post-secretory
phase. Intercellular spaces enlarge considerably and cell walls become com-
pressed and collapsed. The cytoplasm becomes electron-translucent and the
internal membranes of plastids and mitochondria show signs of considerable
disorganization. Collapsed and compressed cells were also observed in the
epidermis of
Hexisea imbricata
(Orchidaceae) (Stpiczyńska et al., 2005a).
2.2.1
Patterns of plastid development in nectary parenchyma cells
Plant cell differentiation is a process in which almost all cell compartments
are involved, among which plastids always play a crucial role. Proplastids of
meristematic cells differentiate into other types of plastids. In vegetative
organs, such as leaves, plastid differentiation is commonly unidirectional,
while in flower cells plastids may interconvert and dedifferentiation is more
frequent than in other plant parts (Pacini et al., 1992; Clement & Pacini,
2001).
Proplastids are the “meristematic” plastid type always encountered in all
the young stages of nectaries studied ultrastructurally (Nepi et al., 1996).
Generally proplastids undergo some divisions before beginning to differenti-
ate (Pacini et al., 1992; Nepi et al., 1996).
Plastid differentiation may follow different pathways according to the
species and the stage of nectar development. The features of plastids in adult
parenchyma nectary cells vary widely at the moment of nectar secretion, be-
cause of the different development of thylakoids and grana and the different
degrees of starch storage (Fig. 9). Undifferentiated plastids (proplastids) are
present in the very early stages of nectary development. Close to flower an-
thesis, chloro-amyloplasts may differentiate and are generally present in
nectary parenchyma when secretion begins (Figs. 9 and 10). They contain
very few small starch grains per plastid. In some cases, chloro-amyloplasts
