The present-day cycads represent the diverse, modified remnants of a
much larger group of gymnosperms which flourished in the Mesozoic Era.
The approximately 148 surviving species of the Cycadales are sparsely
distributed through tropical and sub-tropical floras in a variety of
habitats. About one-half of the extant taxa are considered endangered,
vulnerable or rare and, because of their scarcity and decorative appeal,
have attracted much public interest. Their slow growth rate, the
paucity of viable seeds and limited potential for vegetative reproduction
severely limit both the natural regeneration and the controlled propagation
of cycads.
Over the past 40 years, various attempts have been made to establish
in v~o systems for cycad culture but none has been successful in
establishing a functional protocol for the artificial propagation of these
plants. The author has made renewed attempts to establish in vitro
cultures from a range of haploid and diploid tissues from South African
Encephalartos and Stangeria species. Callus proliferation was readily
obtained from most explant sources of most species using a variety of
media. Addition of the growth factors 2,4-dichlorophenoxyacetic acid
and kinetin in the 10[-7] to 10[-6]M range was beneficial but not essential.
Culture vessels which allowed relatively free gaseous interchange were
advantageous and dark conditions were marginally better than constant
light. Explants from cycad taxa which are mesic in habit gave a more
rapid response than similar explants from xeric plants.
Attempts to induce any form of differentiation other than, or after,
callus formation were unsuccessful in all Encephalartos cultures, but
two forms of morphogenesis were obtained from Stangeria-derived material.
Megagametophytic tissue occasionally developed spherical outgrowths
analogous to coralloid root primordia. More significantly, primary root
cultures after callus formation, subculture and transfer to a light
environment, regularly gave rise to meristematic zones and subsequent
leaf emergence. This is the first recorded case of in vitro morphogenesis
of a South African cycad.
The order Cycadales shows several distinctive phytochemical features,
principally the presence of the unique methylazoxymethanol glycoside
toxins and a-amino-s-methylaminopropionic acid together with some
unusual phenolic compounds, flavanoids, carotenoids and cyclitols.
Stangeria differs from other cycads in at least two phytochemical
aspects; the absence of biflavonoids in the leaves and the absence of
rhamnose and methyl rhamnose in the hydrolysed mucilages. These
diffe.rences may indicate broader physiological differences which would
in turn explain the observed differences in morphogenetic competence
of tissues from Stangeria and Encephalartos. Analyses of various
tissues from these and other cycad taxa were performed with respect to
moisture, protein, enzyme, toxin and hydrocarbon content. Significant
differences, both between organs and between taxa, were noted. The
results of peroxidase analyses were particularly important in that high
levels of this enzyme correlate with the rapidity of callus formation
in vitro. Furthermore, a sharp increase in peroxidase activity signals
the onset of callogenesis in Stangeria megagametophyte cultures.
An important incidental aspect of the phytochemical analyses is that of
potential value of these data to the taxonomist. In particular, the
leaf wax hydrocarbon profiles appear to be species-specific and are
ideally suited to processing by numerical taxonomy computer programmes.
It is anticipated that extension of this work will make a significant
contribution to the resolution of existing problems in cycad taxonomy
and, additionally, provide a means to construct phylogenetic sequences
in the order. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1988.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10519 |
| Date | January 1988 |
| Creators | Osborne, Roy. |
| Contributors | Van Staden, Johannes. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.4465 seconds