Studies on weed populations in sugarcane in Sri Lanka
Laboratory experiments were conducted to characterise the germination of some tropical weed seeds. Field experiments in Sri Lanka characterised the seed bank and monitored seedling emergence after sugarcane planting. In laboratory experiments, final germination, median germination time and rate of germination were determined on a Temperature Gradient Plate and used to define values for optimum temperature (To), base temperature (Tb), and upper temperature limit for germination (Tm). Final germination, median germination time and rate of germination were also evaluated to study germination response to water availability. Field studies were conducted in Sri Lanka to estimate the seed rain and the bank and also to monitor seedling emergence after sugarcane planting. Taylor's Power Law (TPL) parameters were used to describe the spatial distribution of seeds in the seed bank. The Principal Components Analyses (PCA) was used to summarise the variation between samples and species. The Shannon-Weaver diversity index was used to estimate species diversity in the seed bank. Chi-square analyses (x²) were performed to assess the species similarity of the seed bank and the emerged vegetation. Seedling emergence in the field was examined for relationships with rainfall, soil moisture and soil temperatures. The seed rain fluctuated with time. The higher seed rain in fallow land increased the density of the seed bank particularly near the surface. Ploughing substantially reduced the seed bank (by about 80%) and mixed seeds more evenly through the ploughed layer. The majority of seedlings (98%) emerged from the top 4.0cm of soil. Hot and humid conditions favoured the germination and emergence of weed seedlings. This was seen not just in comparison of plantings in October 1995, January 1996 and April 1996 but also more seedlings emerged in the furrows than on ridges. There was no similarity of the species composition of the seed bank and the seedling population arising. Also, the abundance of common species in the seed bank and emerged vegetation was poorly correlated. There was a little relationship between seedling emergence and soil temperature in this investigation.