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University of KwaZulu-Natal (1996)

Aspects related to the germination of Themeda triandra seed.

Baxter, Brent J. M.

Titre : Aspects related to the germination of Themeda triandra seed.

Auteur : Baxter, Brent J. M. 

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy (PhD) 1996

Résumé partiel
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


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