Abstract:
Application of organic or chemical nitrogen fertilizers in Sri Lanka plays a dominant role
in increasing rice yield and the efficiency of agronomic practices. However the increasing
cost of nitrogen fertilizers is a heavy burden on farmers. Biological nitrogen fixation by
Cyanobacteria is an alternative source of nitrogen in rice fields. However it is reported that
the population of Cyanobacteria is gradually diminishing. This phenomenon has been
observed by farmers and some researchers. Application of herbicides may be one of the
major reasons for the decrease in cyanobacterial population and therefore the critical
effects of two common herbicides on Cyanobacterial flora were investigated.
Some cyanobacterial species were isolated from water samples, collected randomly from
ten paddy fields in Matara District. Thb Cyanobacterial species, isolated from those
samples, were Oscillatoria sp, Synecococcus sp, Microcystis sp, and Merismopedia sp.
They were grown in cyanobacterial culture meaia. Two selected herbicides (Propanil and
Ronstar PL) which are commonly used by the farmers in Southern province were tested on
those species isolated. Two different concentrations of each herbicide as one equal to the
concentration recommended for field application and the other equal to the 50% of the
former, were applied separately to each species. The number of cells in 1ml of sample
before adding the herbicide and 3, 6 and 9 days after application of the herbicide were
counted by using a counting chamber and a haemocytometer. Cell numbers in each
treatment and in the control (no herbicide) was analyzed by SPSS computer package.
Both herbicides had clear inhibitory effect on the cells of all the species studied. Inhibition
was greater at the high concentration of the herbicides. Inhibitory action of both herbicides
was nigh for Oscillatoria sp and Merismopedia sp ,compared to other two species.
Microcystis was the least affected. Ronstar PL showed higher degree of inhibition than
Propanil for all the species studied.
