The population biology of straits Rhododendron (Melastoma malabathricum L.)

Abstract

Straits Rhododenron (Melastoma malabathricum L. )(Melastomataceae) is a weedy invasive and medicinal shrub in arable lands, abandoned farmlands, secondary forest openings and derelict areas in Malaysia. Experiments were conducted to determine the effects of chemical media, light and temperature regimes on seed germination and clonal growth of M. malabathricum. Branching pattern, inter-plant spacing, selfthinning, allelopathy and antioxidant activity of M. malabathricum were also investigated in this study. Seeds of M. malabathricum were collected from the campus of the University of Malaya, Kuala Lumpur (3o 8’ N; 101o 42’ E), Malaysia. Moderate germination percentages ranging from 37% to 40% were recorded at 25-30 oC for oven-dried seeds. Seed germination was positively photoblastic. No seed germination prevailed in darkness, or when exposed to chemical media, and at temperature regimes < 20 oC. Light regime strongly influenced clonal growth of M. malabathricum. Mathematical models were developed for M. malabathricum describing branching networks, directionality and dispersion of branches with respect to the mother plant as influenced by density. The aim of this study was to determine if uniform branch patterns existed or changed in relation to density or plant height. Circular statistics were applied to test whether a plant with close neighbours responded either by investing in branch growth away from the competitive pressure or simply into zones free of neighbours. Rayleigh’s uniformity tests showed that the horizontal rotation angle φ was uniformly distributed and, but the distributions of the vertical rotation angle Ө was uni-modal form. The Ө angles were distributed in plants at different densities registering respective mean direction of 52.3°, 47.8° and 41.8° from the lowest to the highest density, respectively. The effect of inter-plant spacing on the growth of individual plants in an increased space and with equal sample size (number of plants) for each spacing gradient was investigated under varying plant spacing (2, 3, 7, 13, 25, 50 and 98 plants/m-2). The experimental design consisted of plants positioned in a polar coordinate grid with seven arcs, eighteen rays and an angle of 20o. The distance between arcs increased exponentially from 6 cm to 43 cm. Rays were separated by an angle of 20o. Straits Rhododenron showed very high survival percentages even when planted in high densities for a period of 200-odd days after planting. Widely-spaced plants were significantly taller than the more closely spaced plants of the same cohort. Mean stem diameter, plant biomass, number of leaves and branches, and branch length increased with increase in plant (wider) spacing. The results support the concept of competition in the self-thinning rule. Intra-specific competition had a profound effect on the number of individuals in a population, as illustrated by the log of average leaf weight per plant plotted against the log of density of survivors for a plant population. The leaf population's trajectory was held under a line of slope -1.12 (R2 = 0.58, p<0.05). The antioxidant activities of crude extracts of M. malabathricum in the laboratory were also been assessed. The effects of methanol extracts of both root and shoot were investigated for their radical scavenging activity using DPPH assay. The methanol extracts were able to reduce the stable free-radical DPPH to the yellow – colored diphenylpicrylhydrazine with an IC50 of 141.88μg/ml and 154.48μg/ml for leaf and root methanol extracts, respectively, but these were measurably lower than ascorbic acid at 28.61 μg/ml. In the study of allelopathy, aqueous extracts were assayed from fresh materials with concentrations of 0, 50, 100, 150 and 200gL-1. The parallel concentrations of aqueous extracts of oven-dried materials used were 40, 80, 120, 160, and 200 gL-1. Inhibition of seed germination of radish prevailed at concentrations above 200gL-1 of the aqueous extract of dried materials, while with methanol extract similar inhibitions were observed at concentrations ranged of 14.28 - 30 gL-1. Total inhibition of root and shoot growths was also observed in barnyardgrass. Both species were susceptible to allelochemical extracts isolated from shoot and root of M. malabathricum. Root and shoot lengths of radish and barnyardgrass decreased with the application of the extracts. The results were discussed from the viewpoint of using the extracts as a natural herbicide to control barnyardgrass and other susceptible weed species by arresting seed germination, seedling growth and establishment. The results of studies embodied in this thesis may provide insights on the processes involved in the population regulation of M. malabathricum. Arguably, these processes precisely are involved leading to colonization and consequential establishment of open spaces by invasive plant species.