Germination characteristics of dimorphic honeybush (Cyclopia spp.) seed

Abstract

Cyclopia is a genus of leguminous shrubs endemic to the fynbos biome of South Africa. Cyclopia spp. are used to make honeybush tea for which a high market demand has led to the overharvesting of Cyclopia species in the wild. Consequently, it has become important to study the cultivation requirements of honeybush in order to support farmers with the cultivation of this crop. Honeybush seeds are known to have either physical or combinational dormancy upon maturity, depending on the species. The plants also produce colour-dimorphic (green and brown), mature seeds in the same pod. A study of honeybush (Cyclopia maculata, Cyclopia genistoides and Cyclopia subternata) seed anatomy was carried out using light and dissection microscopy. It is hypothesized that seed physical dormancy is attributable to a relatively thick (3.5μm) cuticular layer of the integument, a layer of macrosclereid cells, and the presence of a hygroscopically activated hilar valve. There is an absence of other structural openings by which moisture may enter the seed. No consistent structural differences were found amongst the three species or their colour-dimorphic seed. Seeds of the same three honeybush species were divided by colour and their germination response to selected variables (scarification, stratification, seed age and germination temperature) was evaluated. Three dormancy-breaking treatments (wet heat, dry heat and microwave energy) were identified as potential alternatives to conventional sulphuric acid scarification. Highest germination percentage of scarified seed for the three species tested was obtained after 3weeks of cold stratification at 2°C and incubated at 15°C. Brown C. subternata seeds stored for three years gave a significantly higher germination percentage than seed stored for one year only. In non-treated seed, brown seeds had a higher germination percentage than green seeds. In treated seed (i.e. scarified and/or stratified), green seeds had a better germination percentage than brown seeds.