| The ability of Pennisetum to degrade triazine herbicides at the rhizosphere level has been reported. However, sparse information exists on the physiological responses of the plant to increasing concentrations of atrazine. Moreover, information on the use of calcium（Ca） to alleviate atrazine induced physiological damages on Pennisetum seedlings is not yet available. Thus, in this study, it was hypothesised that increase in atrazine concentration induced increase in oxidative damages in Pennisetum plants, a ssociated with other stress related signals. The second hypothesis was that Ca applied to the stressed plants induced physiological recovery and alleviated atrazine induced oxidative damages.In order to investigate the above mentioned hypotheses, experim ents were conducted testing varying concentrations of the herbicide( 0, 5, 10, 20, 50, 100 and 200 mg?kg-1) on the plant for a period of 68 days. Various biomarkers measured included the antioxidant enzymes（glutathione-S-transferase GST and glutathione reductase GR） and non-antioxidant enzymes（ascorbate As A, dihydroascorbate DHA, reduced glutathione GSH, and oxidized glutathione GSSG）. Moreover, the production of reactive oxygen species（ROS; hydrogen peroxide H2O2 and superoxide anion radical O2.-） as signals of oxidative damage was also measuredAscorbate contents（As A and DHA） increased significantly with increase in atrazine concentration and exposure time; but the increase was more evident under higher(50 and 100 mg?kg-1) atrazine concentrations. Increase in atrazine concentration to 200 mg?kg-1 significantly decreased As A, but increased DHA, throughout the experiment. Atrazine at 200 mg?kg-1 significantly lowest GSH content; while GSSG was not affected, after 68 d. Atrazine at 100 mg?kg-1 increased GST activity after 48 d and 68d; while 200 mg?kg-1 atrazine significantly increased GR after 58 d.In order to determine the effect of Ca to alleviate atrazine induced physiological damages in Pennisetum plants, Ca was applied as Ca（NO3）2 to atrazine(100 mg?kg-1) stressed plants, and the plants were grown for 25 days. Plants without atrazine exposures were also amended with Ca; the plants without Ca or atrazine were used as the control plants. Biomarkers measured during the study included the enzymes activiti es（guaiacol peroxidase POD, ascorbate peroxidase APX and catalase CAT）, chlorophyll contents（a, b, a+b and a/b）, and expression levels of genes of the antioxidant enzyme（APX, POD, and Cu/Zn SOD） as well as the psb A. Moreover, the production of ROS（H2O2 and O2.-） as signals of oxidative damage was also measured in the plants.Results showed that atrazine increased ROS production and enzyme activities; but decreased antioxidants and psb A gene transcripts. Atrazine also decreased the chlorophyll contents, but increased chlorophyll（a/b） ratio. Contrarily, Ca application to atrazine pre-treated seedlings lowered the harmful effects of atrazine by reducing ROS levels, but enhancing the accumulation of total chlorophyll contents. Ca-protected seedlings in the presence of atrazine manifested reduced enzyme activity. Antioxidant gene transcripts that were down-regulated by atrazine toxicity were up-regulated with the application of Ca. Calcium application also resulted in up-regulation of the psb A gene.In conclusion, this study suggests that Pennisetum may tolerate lower atrazine concentrations; However, higher concentrations(≥50 mg?kg-1) which could have longer residency period in the soil, could induce more physiological damage to the plant. Moreover, Ca application can be used to reverse oxidative damage induced by abiotic stress such as atrazine induced stress. The significant highlights of this study include（1） Pennisetum has the tendency to tolerate low(< 50 mg?kg-1) atrazine toxicity（2） Pennisetum tole rance to atrazine could involve the glutathione conjugation process（3） calcium application lowered atrazine uptake into Pennisetum seedling（4） calcium decreased atrazine-induced increase in chlorophyll（a/b） ratio（5） there might be a cross-talk between Ca, ROS and psb A gene under atrazine stress. |
PDF Full Text Request