BIOLOGICAL STUDIES OF THE SPECIES OF ACANTHUS IN SINGAPORE

Abstract

1. The main objective of this project was to study the various aspects of the morphology and biology of Acanthus so as to determine the number of species in Singapore. 2. Studies conducted in this project included: a) the general morphology of the taxa based on specimens collected from the field in Singapore and Peninsular Malaysia and from herbarium specimens in the Singapore Botanic Gardens Herbarium (SING), National Univ. of Singapore Herbarium (SINU) and specimens loaned from the Forest Research Institute, Malaysia, Herbarium (KEP); b) pollen analysis; c) wood anatomy; d) chromosome counts; e) general ecology; f) intra- and inter-specific pollination experiments; and g) soil and light microhabitats. 3. New morphological discoveries included a) A. ebracteatus with whorled leaves and inflorescences (3 leaves/flowers per node) and pink abaxial corolla lip; and b) A. volubilis with pink abaxial corolla lip and corolla median band. Additionally, A. ebracteatus and A. volubilis inflorescence were found to have 35 and 53 pairs of flowers respectively, increasing the Malesia range of 30 and 20 pairs respectively, previously recorded. 4. All three species have similar pollen morphology. But pollen grains of A. volubilis tend to be more globular while those of A. ebracteatus are more elliptic. But grain size is not diagnostic due to the size range (35-50x20-40 µm) overlap between species. 5. The wood ray width and densities of the three species varied. A. volubilis has the widest rays and A. ebracteatus the highest ray density. 6. The chromosome studies showed that Acanthus ilicifolius is most likely a tetraploid (c. 70) while A. ebracteatus (c. 38) and A. volubilis (c. 34) are most likely diploids. 7. Pollinators showing foraging inconstancy are Xylocopa spp. and Amegilla sp. As such, natural interspecific pollination can occur. 8. Extra-nectary secretions from sepals attract ants, one of which (Oecophylla smaragdina) is an ant guard. 9. Pollination experiments have shown the A. ilicifolius is highly self-fertile, while A. ebracteatus has lower rates of selfing and A. volubilis (restricted to the one clump studied) could be male-sterile or self-incompatible. All interspecific pollinations, except between A. ilicifolius (pollen donor) x A. volubilis (pollen recipient), yielded no fruits. 10. Soil salinity was correlated to lamina spininess in all three species, but only A. ebracteatus and A. volubilis showed salinity correlations to lamina shape. Soil pH was correlated to lamina form in A. ebracteatus only. Red:far-red fluence ratio was correlated to lamina form for all three species. Light intensity quotient was correlated to lamina form only in A. ilicifolius and A. volubilis. As such, lamina form in Acanthus spp. is highly plastic. 11. The results support the delimitation of three species of Acanthus in Singapore, as proposed by previous authors (e.g. van Steenis, 1937; Bremekamp, 1955b), and refute the delimitation of A. ilicifolius and A. ebracteatus as conspecific to A. ilicifolius (Percival and Womersley, 1975).