A method has been developed to facilitate shoot formation from leaf explants of almond. Leaves were dissected from micropropagated shoot cultures of the commercial cultivars Nonpareil and Ne Plus Ultra, and sections incubated on Almehdi and Parfitt's (1986) basal medium (AP) with varied plant growth-regulator conditions. Three auxins, 2,4-dichlorophenoxyacetic acid (2,4-D), a-naphthaleneacetic acid (NAA), and indole-3-butyric acid (IBA), in combination with two cytokinins, benzylaminopurine (BA) and thidiazuron (TDZ), were tested at various concentrations along with casein hydrolysate (CH) to determine the conditions most conducive to adventitious shoot regeneration. Response to the tested plant growth-regulator conditions varied with genotype. Of the three auxins tested, NAA and IBA induced adventitious shoots from Ne Plus Ultra explants, but only IBA was effective for Nonpareil. For the cytokinins, shoot development from Ne Plus Ultra occurred in the presence of either BA or TDZ, whereas for Nonpareil only TDZ was effective unless CH was incorporated in the basal medium. The inclusion of CH (0.1% w/v) improved callus morphology, and increased regeneration frequencies for both cultivars. Maximum regeneration frequencies for Ne Plus Ultra (44.4%) and Nonpareil (5.5%) were achieved on AP basal salts supplemented with CH, IBA (9.8 mM), and TDZ at 22.7 and 6.8 mM, respectively.
Physical dormancy is common in seeds of arid-land legumes. Improved understanding of germination requirements of hard-seeded species will further our understanding of arid lands and aid restoration projects. We studied the germination responses of Acacia papyrocarpa (Benth.), A. oswaldii (F.Muell) and Senna artemisioides (Gaudich. ex DC.) Randell ssp. × coriacea (Benth.) Randell from a chenopod shrubland in South Australia. Imbibition testing indicated that all three species had physical dormancy, but the proportion of dormant seeds was lower in A. oswaldii. This corresponded to a thinner testa in this species. Mechanisms tested to scarify seeds included mechanical scarification and different durations of wet or dry heat. Mechanically scarified seeds germinated readily, reaching maximum numbers in 10–15 days, independently of incubation temperatures, with the exception of S. artemisioides seeds, which germinated at a slower rate in cooler temperatures. Overall, wet heat was more effective than dry heat to alleviate physical dormancy, whereas dry heat in some cases resulted in seed mortality. On the basis of these results, it is recommended that seeds of A. papyrocarpa and S. artemisoides be pretreated with wet heat in future restoration programs. No pre-treatment is required for dormancy loss in A. oswaldii seeds. The different responses of seeds of these species suggest that their populations have varying strategies for persistence in this unpredictable environment.
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