Time bound physiological stress responses of fresh water aquatic plant species to water flow, irradiance and heavy metal toxic environments

概要

Abiotic stressors exerted on macrophytes are more complex process in nature and understanding each stressor time interrelation with biochemical activities are not yet fully discovered. Although one particular stress variable provides limited knowledge to ecological perspective compared to the conceptual studies focus on several stress factors. Considering the anthropogenic activity related issues, on present aquatic environment appropriate stressors such as flow movement, irradiance variation and heavy metal exposure were selected for study. Thus the present thesis work specifically assesses the temporal scale biochemical activity fluctuations to define how particular stressor effect mediates by aquatic plants. The main objective of study was to evaluate the flow and irradiance effect on macrophyte species during different diurnal time points for understands suitable habitat niches. For achieve this objective flow and irradiance conditions were simulated with outdoor flume channel integrating two flow conditions and two irradiance levels. Then in order to understand the different flow velocities effect on macrophyte antioxidative activities and scavenging capabilities by two velocity gradient and stagnant flow compared with fully shaded condition on Egeria densa macrophyte. Third and fourth objectives were concern on heavy metal effect on biochemical activities of Ceratophyllum demersum and Chara braunii, respectively considering time interactions. The flow and irradiance experiment results showed that compared to the other treatments, the open-flow treatments had the highest hydrogen peroxide (H₂O₂) concentration which occurred with the acceleration of antioxidant activity at midday, mainly due to the independent effect of irradiance and flow movement. The plant diurnal H₂O₂ and antioxidant activity rhythms had varying fluctuations, and peroxidase (POD) activity showed a rapid response to oxidative stress. The increase in POD activity followed a more similar pattern aligned with H₂O₂ synthesis, while catalase (CAT) and ascorbate peroxidase (APX) activities were dominant in the late hours of the day. Concerning the velocity gradient experiment, between the two mean flow velocity generated H₂O₂ content there was no any significant difference. However, compared to the stagnant and shaded treatments velocity effect was significantly contributed to the ROS synthesis at midday time. Although antioxidative activities and chlorophyll pigments, fluctuated with the 12hr day duration and stabilized at evening phase. Oxidative stress related cellular starch granule accumulation was pronounced within high flow velocity, low flow velocity and shade treated E. densa plants while bigger starch granules observed from same treatments respectively.
In heavy metal study C. demersum macrophyte was exposed to two heavy metal (Cu and Zn) gradients in order to test the hypothesis whether biochemical activities may vary over time. The heavy metal concentrations were arranged as 0 to 10 mg L-¹. The concentration of H₂O₂ accumulation was found to be accelerated with short duration and higher concentration of Cu and Zn whereas afterward it was appeared to decline. With upregulated H₂O₂ content, indole acetic acid IAA content decreased significantly in all Cu treatments whereas only 5 and 10mg L-¹ of Zn caused the IAA changes. The oxidative stress was not observed at 1 mg L-¹ of Zn, whereas all Cu gradients caused dramatic damage to the antioxidative activities (POD and CAT). In addition, chlorophyll pigments were negatively influenced by both heavy metals suggesting that the chlorophyll destruction was inevitable with metal accumulation. Furthermore, the IAA reduction with heavy metal stress made C. demersum antioxidative potential in to lower level. These results suggest that C. demersum may grow well at 1mg L-¹ threshold Zn polluted water bodies through regulating the antioxidative system whereas the plant species seems not suitable for phytoremediation in Cu polluted water with a range of 1 to 10mg L-¹. Although, Chara braunii, H₂O₂ synthesis rapidly accelerated with the high Cu concentrations and time exposure. This oxidative stress leads to the higher Catalase activity and Guaiacol peroxidase activity in shorter durations thus afterwards it reduced to the lower concentration. Considering the overall experimental results flow and irradiance fluctuation caused more prominent effect on both macrophyte species biochemical activities. These fluctuations might be associated with morphological characteristics of two macrophytes and based on flow resistance which they can survive in each flow movements by regulating biochemical activities. However, heavy metal exposure should be limited to the required range of heavy metal and shorter time durations for maintain optimum biochemical activities of C. demersum and Chara braunii.

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