Heat desiccation as a seed scarifying agent in Leucospermum spp. (Proteaceae) and its effects on the testa, viability and germination

We quantify the newly preposed seed biological phenomenon of desiccation-scarification of different testal layers, found in Leucospermum spp. in Cape fynbos. Intact seeds oft. cordifolium were exposed for varying periods to different temperatures, and breakage rests were then performed on both the exo- and endotesta, the latter following immersion in water. Heat intensity and duration correlated positively with the degree of breakage of both testal layers. The viability of embryos, tested with tetrazolium, was not affected. The effects were furthermore determined on germination itself, of desiccation-scarification by means of heat, and other oxygenation treatments. These treatments distinguished between the effects of oxygenation, desiccation and heat in the germination of deeply dormant seeds of L. cordifolium, L, glabrum and L. reflexum. The promotive effects of heat desiccation on germination were similar for the three species and could not be linked to possible physiological changes caused by high temperature per se. Combined heat desiccation pulsing (scarification) of the exo- and endotestas resulted in strongly increased germination compared with controls, however the exotesta appears to act as the primary barrier to oxygen diffusion to the embryo. We conclude that, in natural fynbos, desiccation mediated scarification of the testal layers is the main means of regulating oxygenation, and thus synchronous germination, of dormant Leucospermum seeds after fire.