Breaking the dormancy of selected asteraceae annuals in the winter-rainfall region of South Africa

Abstract

The aim with study was to investigate whether the germination of Arctotis hirsuta, Cotula duckittiae and Oncosiphon suffruticosum are three Asteraceae annuals can be improved by determining its individual temperature- and light-condition requirements and subjecting the achenes to pre-germination treatment such as scarification, GA₃ and after-ripening at high temperatures. Experiments were conducted between 2019 and 2020. The first experiments evaluated the temperature and light required per species. Treatments consisted of inducing germination at 5 different temperatures namely 7 °C, 12 °C, 17 °C, 22 °C, 27 °C and alternate temperatures at 12 °C/22 °C and 17 °C/27 °C in conditions of light, darkness and alternate lightning (12 hours each of light and darkness). In the second experiment, pre-germination treatments consisted of scarification (pericarp-pricking) and the soaking of seed over 4 periods in 4 different gibberellic-acid (GAɜ) concentrations. Seeds of all species were then subjected to pre-determined light and temperature conditions as follows: A. hirsuta (collection year 2016) [22 °C in light], C. duckittiae (collection year 2017) [22 °C in light], O. suffruticosum (collection year 2016) [12 °C in light] and O. suffruticosum (collection year 2017) [22 °C in dark] to ensure optimum germination. The third experiment, after-ripening, consisted of two different treatments over two periods (30 °C & 45 °C for 4 and 8 weeks). Seeds of all species were then subjected to pre-determined light and temperature conditions as follows: A. hirsuta (collection year 2016) [22 °C in light], C. duckittiae (collection year 2017) [22 °C in light], O. suffruticosum (collection year 2016) [12 °C in light] and O. suffruticosum (collection year 2017) [22 °C in dark] to ensure optimum germination. Chapter 2 reviewed the importance to alleviate the germination-challenges for the three species when grown from seed of wild plants. It was found that when incubated at specific temperature- and light-requirements, some winter-growing annuals can improve their germination. The utilization of these pre-determined light- and temperature-requirements, in conjunction with other pre-germination treatments such as scarification, soakings in GA₃ and after-ripening at elevated temperatures have been deemed very effective to counter challenges with germination for some species of Asteraceae-annuals. Chapter 3 indicated that germination of A. hirsuta achenes is the result of a temperature and light interaction for temperatures between the 22 °C and 27 °C in the light. The impact of the interaction between temperature and light/dark phases on the germination of C. duckittiae (collection year 2016) were insignificant through a range of temperatures and light- or dark-conditions. This could also be observed from the interaction of alternating temperature under cyclic light/dark conditions on the germination of C. duckittiae (collection year 2016) at 12 °C/22 °C compared to 17 °C/27 °C. Achenes of C. duckittiae (collection year 2017) achieved optimum germination in the dark at 22 °C, but very low to no germination in all the other treatment combinations. In O. suffruticosum (collection year 2016) the best germination was achieved at 12 °C in the light, however this did not differ very much from 17° C, (in light), 22° C ( in light) and 7° C (in dark). O. suffruticosum (collection year 2017) recorded very low germination over all the treatments. In chapter 4 the pre-germination treatments, scarification and the GA₃-treatment, had a significant impact on the germination of A. hirsuta (collection year 2016), but could not effect improved germination for C. duckittiae (collection year 2017) and O. suffruticosum (collection years 2016 & 2017). These results suggested that the use of GA₃ or scarification as a single pre-treatment application are not effective in improving germination for C. duckittiae (collection year 2017) and O. suffruticosum (collection years 2016 & 2017). Chapter 5 investigated whether the impact of accelerated high temperature after-ripening treatments of 30 °C and 45 °C over 4 and 8 weeks may alleviate dormancy and result in improved germination. Findings from this experiments indicated that none of the treatments could improve germination. Overall this study has found that some species stored (after-ripened) at controlled conditions in a seed room, will have no specific requirements in order to alleviate dormancy as was the case of C. duckittiae (collection year 2016) when stored for between 28 - 32 months. Good germination was obtained over several temperature-ranges in light or dark conditions. A. hirsuta achieved optimum germination and only exhibited a mild form of dormancy between 22 °C - 27 °C. Pre-treatments with scarification and GA₃ yielded very average results for this species. After-ripening, at the temperatures of 30 °C and 45 °C, as well as scarification and a GA₃-treatment, did not improve germination for C duckittiae (collection year 2017), O. suffruticosum (collection year 2016) and O. suffruticosum (collection year 2017).