Desiccation as the active principle in heat-stimulated seed germination of Leucospermum R. Br. (Proteaceae) in fynbos.

Abstract

A new seed biological phenomenon in the myrmecochorous, fire-recruiting fynbos species Leucospermum cordifolium (Salisb. ex Knight) Fourcade is reported. This is the extensive breaking, following desiccation treatment, of the seed testa which in intact, soil-stored seeds imposes dormancy on the embryo by means of oxygen exclusion. In this study the pericarps of L. cordifolium seeds were removed and seeds with intact exotestas were selected stereomicroscopically. Samples of seeds were placed in nylon bags and suspended over (cone.) H2 SO4 or dried silica gel. A third series of seeds were exposed for varying periods to 40 °C, cooled and immersed in water before performing breakage tests on both the exo- and endotesta. All treatments resulted in extensive breaking of the exo- as well as the endotesta of seeds. The two 'cool' desiccating treatments both caused more effective breaking than heat treatments. This indicates strongly that the breakage phenomena are desiccation effects and not caused directly by heat intensity. Rehydration in air before wetting seeds completely prevented breakage effects in all treatments in the endotesta, but not in the exotesta. This further supports the conclusion that endotesta breakage is a function of desiccation plus wetting and not of heat per se. The results suggest that in nature desiccation by fire could break the exotesta and the endotesta as well, if fire is followed soon by rain. These effects amount to effective scarification of the seed and thus suggest a positive correlation, in nature, between heat intensity of fire and emergent seedling numbers.