Alternating temperature requirements in Leucospermum R.Br. seed germination and ecological correlates in fynbos

Abstract

The effect of germination incubation regimes, which were chosen on the basis of temperature patterns found within the fynbos (Mediterranean fire-prone shrubland-type) habitat in the Cape Floristic Region during the autumn germinative period, was tested on seeds of 6 Leucospermum species over 5 taxonomic sections (L. cordifolium, L. cuneiforme, L. erubescens, L. glabrum, L. reflexum, L. vestitum). Seeds of each species were scarified, soaked in water and incubated at 15 combinations of diurnal low (16h) and high (8h) temperatures. The results indicated that Leucospermum seeds generally require alternating temperatures for germination. Regression analysis of germination percentage responses were used to estimate provisional optimum low [TO(LOW)] and optimum high [TO(HIGH)] temperature requirements for individual species. These requirements differed markedly among species, and across species they averaged 9.9 and 21.2°C, respectively. Individual requirements correlated positively with mean late autumn air temperatures obtained from weather stations near the natural habitats of species. We conclude that a diurnal alternating temperature requirement is a character syndrome in Leucospermum in which the TO(LOW) and TO(HIGH) components are narrowly adaptive, independent and genetically stable characters relating to the known ecophysiological roles of low and high temperature requirements (Brits, Cutting, Brown and Van Staden, 1995). Ecological and adaptive roles of these requirements are compared with those in some persistent small-seeded soil-stored seed banks.