Aspects related to the germination of Themeda triandra seed.

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[3] 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.