Stress responses of Azolla sp. and its symbiotic Anabaena azollae against oxyfluorfen.

Abstract

Non-target aquatic plants in agricultural lands are highly vulnerable to various stresses including agrochemicals. The present study assessed the stress responses of aquatic macrophyte Azolla sp. and its symbiotic Anabaena azollae against a common herbicide; oxyfluorfen. The capability of atmospheric nitrogen fixation by Anabaena azollae is an added advantage for Azolla and its living environment. Azolla sp. (2 g fresh weight) was exposed to six different concentrations of oxyfluorfen [0 (control), 60, 80, 100, 120, and 140 μgL-1]. These concentrations were selected based on several preliminary trials. Each treatment with three replicates was randomly allocated into 18 experimental units in a complete randomized design and the experiment lasted for 96 hours under laboratory conditions. The plant growth, pigment content (chlorophyll and anthocyanin), H2O2 concentration, peroxidase activity (POD), and Indole acetic acid (IAA) content of Azolla sp were measured at the end of the experiment while the microscopic observations were recorded daily. The growth rate of Azolla sp. was significantly different among treatments (P<0.05). The chlorophyll-a and anthocyanin content were significantly reduced by the oxyfluorfen (P<0.05), where the highest concentrations of chlorophyll-a (460 μg/g Fresh Wt) and anthocyanin (0.6 μg/g Fresh Wt) were found in the control and 100 μgL-1 treatments respectively. Increased H2O2 concentration and the POD activity together with low IAA content indicated oxidative stress in plants exposed to oxyfluorfen. Except in the control, roots of Azolla exposed to oxyfluorfen detached after 24 hours of exposure, while plants became pink in colour in 80, 100, 120, and 140 μgL-1 treatments. Shrinking of the filaments of Anabaena azollae was observed in each concentration of oxyfluorfen at 96 hours of exposure and the absence of filaments was observed in plants exposed to 140 μgL-1 concentration. The stress caused by oxyfluorfen on Azolla sp. disrupted its symbiotic relationship with Anabaena azollae at the concentration of 140 μgL-1 after 96 hours of exposure. Further studies are recommended to observe the possible recovery process of Azolla sp. and Anabaena azollae.