|dc.description.abstract||Themeda triandra is a grass species of economic importance in Southern and
Eastern Africa, and Australia. The species is being lost from grasslands and
savannas in these areas due to poor agricultural practice, rangeland degradation,
opencast mining and increased afforestation. Based on the poor re-establishment
of the species from seed in sub-climax grasslands, dogma holds that T. triandra
can not be re-established from seed. Recent research has, however, highlighted
the potential to establish this species from seed, but the use of seed of T. triandra
in re-vegetation of disturbed areas is limited by poor understanding of the seed
biology of the species and low seed availability. In this Thesis ways to maximise
the use of available seed are reported. Areas investigated include optimisation of
seed storage conditions, overcoming primary seed dormancy, promoting
germination of available seed and pre-treatment of seed to improve germination.
The Thesis closes with an investigation of the environmental limits of tolerance of
seedlings from the T. triandra ecotypes studied, when grown under field conditions
at reciprocal sites.
Two altitudinally and geographically distinct populations of T. triandra were
studied; a high altitude grassland population at Cathedral Peak (Drakensberg:
1800 m) and a low altitude savanna population from the Umfolozi Game Reserve
(Zululand: 90 m).
At seed shed T. triandra seed is dormant. The depth and duration of primary seed
dormancy varies between populations, but appears to reflect severity of the winter
period experienced. More than 95% of T. triandra seed from the Drakensberg
population was dormant at seed shed, compared to 55% of seed from the Zululand
population. In both populations dormancy is lost during dry after-ripening.
At seed shed T. triandra seed displays a high level of seed viability (> 80%). Seed temperature range -15°C to 70°C, was achieved at 25°C (± 2°C), at which
temperature seed was held for 40 months. During this period viability decreased
from over 80% to 50% and dormancy was lost through dry after-ripening within
four (Zululand) to eight (Drakensberg) months. Loss of dormancy can be
accelerated at higher temperatures, but is accompanied by rapid loss of seed
viability. In contrast, viability can be maintained in storage at sub zero
temperatures, but loss of dormancy is retarded. Loss of dormancy coinsides with
the onset of spring.
Dormant seed is capable 'of germination at a narrow range of constant
temperatures (25 ° C to 40 ° C). With after-ripening, the range of temperatures at
which germination takes place increases (15 ° C to 40 ° C) and the optimum
temperature for germination decreases from 30 ° C in both populations to 25 ° C.
After-ripened seed is capable of germination at lower water potentials than
dormant seed. Similarly, seed from the low altitude population is capable of
germination at lower water potentials (-1.0 MPa dormant: -1.5 MPa after-ripened)
than seed from the high altitude population (-0.5 MPa dormant: -1.0 MPa afterripened).
Dormancy in seed from the high altitude population is overcome by
prolonged stratification (30d). In contrast, seed from the low altitude population
responds to short duration stratification (5d) with longer periods proving
detrimental to seed germination. Germination of dormant and non-dormant seed
of T. triandra does not differ significantly in the light or dark. Neither does
photoperiod, or red / far-red light exposure significantly affect germination.
Seed response to light and temperature, as characterised under controlled
conditions, was verified in a field seed burial experiment undertaken at the high
altitude Drakensberg site during winter. Burial in soil does not affect the response
of T. triandra seed to light or temperature. Loss of dormancy is accelerated in
buried seed. After-ripened seed germinates over a wider range of temperatures than dormant seed.
The mechanisms governing T. triandra seed dormancy and germination appear to
be universal between ecotypes. Dormancy is enforced, in part, by the seed
covering structures (glumes) which impose a mechanical restraint to radicle
emergence. Approximately 85% of dormant seed, however, contains a dormant
embryo. Embryo dormancy is enforced at seed shed by compounds inhibitory to
seed germination. The germination process in T. triandra appears to be governed
by endogenous gibberellins. Bioassay results reveal that endogenous gibberellin
synthesis commences up to six hours sooner in after-ripened seed than in dormant
seed and that the level, or concentration, of gibberellin-like compounds is
substantially lower in after-ripened seed than in dormant seed. Similarly, the
concentration of applied gibberellic acid required to achieve maximum germination
of T. triandra seed decreased from 500 mg.l ¯¹ (8 week old seed) to 50 mg.l ¯¹ (78
week old seed) as dormancy is lost during after-ripening. Contrary to previous
reports, boron does not promote T. triandra seed germination.
Plant-derived smoke significantly promotes T. triandra seed germination (5% to
43% for dormant seed from the Drakensberg population). The effectiveness of
smoke in promoting germination increased with increasing seed imbibition
suggesting smoke action at a metabolic level. This suggestion is reinforced by the
ability of smoke to bring about the germination of seed which had failed to
germinate in water. Moreover when smoke is applied in combination with
gibberellic acid the final level of seed germination following combined treatment is
significantly greater than the level of germination achieved in the presence of either
smoke or gibberellic acid alone. A similar result is achieved with joint application
of smoke and kinetin, although the results were not statistically significant.
Furthermore, smoke treatment reversed ABA-induced inhibition of germination of
non-dormant T. triandra, wheat, radish and sunflower seed to a level equal to or
greater than that achieved using GA or kinetin. The possibility that smoke
promotes seed germination by mimicking, or promoting the synthesis of
endogenous gibberellins was investigated. Bioassay results revealed that smoke
had no effect on increasing the level of endogenous gibberellin-like activity in
T. triandra caryopses. The mechanism by which smoke acts to promote seed
germination remains elusive, however results presented suggest that smoke may
act to remove an ABA-induced block to seed germination. Consequently, it is
suggested that smoke plays a permissive role in promotion of T. triandra seed
germination by removing a block to the seed germination process thereby allowing
endogenous gibberellins to act.
Treatments which significantly improved the level of T. triandra seed germination
were evaluated as seed pre-treatments. Significant improvement in germination
was obtained following smoke (aq) and gibberellic acid (100 mg.l ¯¹) pre-treatment
of seed. The effects of pre-treatment were evident on germination of seed for up
to 21 days after pre-treatment. Seed pre-treatment with smoke had no affect on
subsequent seedling growth, but gibberellic acid pre-treated seedlings developed
abnormally. In contrast, short duration exposure of dormant seed to high
temperature (70 0 C for 7 days) increased germination, seedling height and tiller
number. Priming of seed in polyethylene glycol (PEG 8000) for 7 days significantly
improves the level of T. triandra seed germination. The use of seed pre-treatment
to maximise germination of T. triandra seed is discussed.
Reciprocal transplanting of seedlings from both the Drakensberg and Zululand
populations confirmed that the T. triandra populations under investigation are
distinct ecotypes. Field transplant gardens were established in the Drakensberg,
Zululand and at an intermediate altitude in Pietermaritzburg (800m). Less than
10% of planted seedlings died at any site. With increasing altitude of the field site,
tiller number increased, but tiller allocation to reproduction decreased. Similarly,
for both Zulu land and Drakensberg seedling transplants the time taken to reach
anthesis increased with increasing altitude and the proportion of transplants which flowered decreased. These data are consistent with the climate of the field sites
where the high altitude site experiences a short growing season and harsh winter
while the Zululand site experiences a prolonged growing season and mild winter
period. These data indicate that T. triandra ecotypes are tolerant of a wide range
of environmental variables.
The application of the data presented in this Thesis, in maximising the use of
available seed of T. triandra for use in re-vegetation, is discussed.||en