Laticifers are cells or series of connected cells containing latex of different composition (Esau, 1977). The first descriptions of laticifers in the capsules of P. somniferum were reported by Tschirch and Oestrele (1900) and Fedde (1936). In the genus Papaver the laticifers are articulated, i.e. they develop from single vertical files of parenchymaric cells as a result of absorption of their end walls. Lateral anastomoses occur between the neighbouring laticifers. These anastomoses may be complete, resulting in laticifers with increased diameter, or they may only be partial, in which case protrusions (bulges) emerge on the lateral walls of the adjacent laticifers and at the common surface of protrusions (oval orifices) which serve the anastomoses (Fairbairn and Kapoor, 1960).
Laticifers always occur in association with the phloem of all organs of Papaver with the exception of the seeds. The existence of laticifers in the filaments of the stamens only became clear in 1976, when Nessler and Mahlberg observed them using optical and electron microscopy.
3.2 Fine Structure
The laticifers of P. somniferum can be distinguished from the other cells by the large, irregularly shaped vesicles that are located in the cytosol. The cytoplasm also contains all the organelles of an ordinary plant cell. The ellipsoid and structurally intact nucleus is located peripherally. Mitochondria have their normal size and form. Plastids in laticifers do not contain starch, and their membrane structure resembles that of etiolated plastids in parenchyma cells. The endoplasmic reticulum is mainly rough in the early stages of development, but smooth areas are also visible. The cytosol also contains some dark unidentified particles (Thureson-Klein, 1969). Dickenson and Fairbairn (1975) described a rare organelle covered by a double membrane, with lipid inclusions and tubular structures. The function of this particle is unknown. The vesicles in the cytoplasm are covered by a unit membrane and exist in two principal Figure 18 Germination of Papaver seed: 1, radicle; 2, cotyledon; 3, hypocotyl; 4, leaf; 5, leaf bud
forms. The two forms never exists simultaneously in the same laticifer and Dickenson and Fairbairn (1975) regarded them as different developmental stages. The juvenile form can possibly be characterized by osmiophilic granules which adhere to the exterior of the vesicle membrane. In the older form the boundary membrane has one or more osmiophilic ‘caps’.
Griffin and Nessler (1989) separated vesicles in terms of their content and mechanical stability. The cell-specific major latex protein (MLP) is deposited in fragile vesicles and isoquinoline alkaloids were detected in vesicles which were more resistant to centrifugal forces and osmotic stress.
3.3 Development of the Laticifer Vesicles
There are two models which explain vesicle formation in laticifers. The formation of vesicles from endoplasmic reticulum was suggested by Sárkány et al. (1964), Thureson- Klein (1969) and Nessler and Mahlberg (1976). In the early stages of laticifer differentiation, rough endoplasmic reticulum accumulates along the periphery of the plasma membrane and alternates with smooth endoplasmic reticulum. Vesicles detach from localized dilation of both smooth and rough endoplasmic reticulum and these small particles develop into larger ‘capped’ vesicles concentrated along the surface of the laticifer. Finally enlargement of the vesicles ceases and they become arranged irregularly in the cytoplasm.
According to the second model the vesicles originate from the central vacuole of the laticifer initials (Griffin and Nessler, 1989). The first step of this process is the filling of the central vacuole with different storage products. At this stage the future laticifer is indistinguishable from the other parenchyma cells. The only peculiar characteristic is the existence of main laticifer protein (MLP) in the vacuole, as the first sign of the further differentiation. As the vacuole grows, it begins to branch and separate into smaller vacuoles containing different storage products. This model suggests that vesicles do not arise as individual particles but are rather built from the previously existing central vacuole.
ACKNOWLEDGEMENTS
We thank Prof. Dr. Zsolt Bor and Dr. Béla Hopp (József Attila University, Department of Optics and Quantum Electronics, Szeged, Hungary) for the laser ablation facility in the preparation of transversal sections for SEM.
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JENÕ BERNÁTH and ÉVA NÉMETH
Department of Medicinal Plant Production, UHFI, 1114-Budapest, Villányi str. 29, Hungary
1 PHASES OF GROWTH AND DEVELOPMENT