Seed dormancy and germination in Protea compacta R.Br. and Leucadendron daphnoides Meisn.

Abstract

The germination responses of seed to a wide variety of treatments showed that both the pericarp and the embryo contribute to the dormant condition in Protea compacta. Germination can be improved to a greater or lesser extent by scarification, high oxygen tensions, stratification and applied hormones. In Leucadendron daphnoides dormancy is mainly imposed by the pericarp which apparently acts as a barrier to oxygen diffusion to the embryo. A considerable increase in germination is brought about by scarification and high oxygen tensions applied to intact seeds. Stratification and applied hormones improve germination to a lesser extent. Leaching and light treatments fail to improve germination in both species. Attempts to characterize the major inhibitor present in seed leachates indicated that it was coumarinlike in its properties. No evidence could be found that inhibitors leached from seed were actually involved in the regulation of germination. Poor germination in both species is apparently due to the lack of germination promoters rather than to the presence of inhibitors. Germination of seed of Leucadendron daphnoides was increased by 50% chilling at 5°C and 500% by incubation in oxygen. Both these treatments brought about a four-fold increase in the level of butanol-soluble cytokinins. The latter apparently play the primary role in promoting germination. The greater effectiveness of high oxygen tensions improving germination appears to be due to the additional effect oxygen has in stimulating the production of acidic gibberellin-like substances, to a level 30 times higher than with chilling. The acidic gibberellin-like substances do not appear to have a primary role in promoting germination but their effect is additive in the presence of a threshold level of butanol-soluble cytokinins. Maximum germination does not appear to depend on phasic changes in promoter levels, but on whether an increase in the level of gibberellin-like substances coincides with an increase in the levels of butanol-soluble cytokinins. In Protea compacta the effectiveness of chilling and oxygen incubation in improving germination by approximately 50%, is apparently mediated through their effect in increasing the levels of butanol-soluble cytokinins. The latter appear to play the primary role in promoting maximum germination, which apparently does not depend on a phasic change in promoter levels.