Effect Of F Supply On Photosynthesis,Reactive Oxygen Metabolism And Ultra-Structure Of Tea Plant (Camellia Sinensis) Leaves

Tea plant is a kind of plant that has high tolerance of fluoride(F)and no F-induced macroscopic toxicity symptoms were observed in tea plant under much higher dose of F than other plants in certain range.However,the F-tolerance mechanism is not clear.And,there are very few researches concentrated on effects of F on physiology and biochemistry of tea plant.Shu cha zao,the main cultivar in Anhui,in hydroponics for 30 d,was used to study mineral nutrition elements,photosynthesis,level of reactive oxygen species,ultra-structure of tea plant under F stress.In addition,the important role of antioxidant defense systemand response of cell ultrastructure of tea plant under different F concentrations was analyzed.The main results are as follows:1.High doses of F inhibited photosynthetic system of tea plant.With the increase of F concentration,content of chlorophyll gradually declined.Chl.a/b in tea plant leaves supplied with F was lower than control,which proved chlb was more sensitive to F than chla.Additionally,chlorophyll fluorescence parameters did not change much at 1~5mg/L F treatments but it decreased slowly when 10mg/L F and above were supplied into tea plant.F stress caused obvious effect on photosynthetic parameters,photosynthetic rate(Pn),transpiration rate(Tr),intercellular CO2 concentration(Ci),stomatal conductance(Gs)presented a decreased trend due to F toxicity.2.With increasing of F concentration,F concentration had no significant effect on content of basic elements C and H in tea plant leaves.However,content of N and S presented increasing first and then decreasing trend.Content of nutrient elements like K,Ca,Mg declined with increase of F concentration.Additionally,elevated F concentration inhibited accumulation of trace elements Mn,Fe,Zn in tea plant leaves and moderate F facilitated uptaking of Cu and Al.3.Reactive oxygen species(ROS)including superoxide anion(O2-)and hydrogen peroxide(H2O2)accumulated in tea plant leaves,obvious increase of content of malondialdehyde(MDA)and rose of leaves conductivity demonstrated that F stress caused oxidative damage in tea plant.It was conducted that the activity of ROS scavengers like peroxidase(POD)and catalase(CAT)increased at 0~5mg/L,but sharply decreased under10~50mg/L F.Trend of superoxide dismutase(SOD)was similar to them but reached the peak at 1mg/L.The results suggest that at 5mg/L F concentrations,tea plant leaf antioxidant enzymes can eliminate reactive oxygen species(ROS)to enhance sustainstability,which however,ability of scavenging reactive oxygen of antioxidant enzymes decreased exposed to 20~50mg/L F.4.Ascorbate-glutathione(AsA-GSH)cycle played an important role in tea plant against F stress.The activity of main enzymes in the cycle like dehydroaacorbate reductase(DHAR)and glutathione reductase(GR)in tea plants rose first and then reduced supplied with different concentrations of fluoride.And,GR remained high activity at 20mg/L F,which played a key role in scavenging ROS and protect tea plant from oxidative damage.However,activity of ascorbic acid peroxidase(GPX)falled down with the increase of F concentration.In addition,content of two antioxidant in AsA-GSH cycle including ascorbic acid(As A)and glutathione(GSH)significantly reduced at 50mg/L F supply.No obvious change was observed in GSH/GSSG or AsA/DHA at 1mg/L F supply,however,which presented a decreased trend with increase of F concentration.Thus,excessive ROS caused oxidative damage in tea plant.5.Microstructure was studied by using transmission electron microscope(SEM).F(1mg/L),enhanced stomatal aperture,but at higher concentrations,it decreased obviously.Stomata density were significantly reduced at 1-10mg/L when compared with control but it obviously increased in 50mg/L F treatment.There is a remakable increase of the number of epidermal hair on lower epidermis supplied with 50mg/L F compared with control.It can reduce transpiration and water loss,which was a kind of manifestation of tea plant to adapt to F stress.Result of transmission electron microscope(TEM)showed that,no obvious damaged was discovered about cell ultra-structure under 1~50mg/L F but plenty of starch grains were in accumulating trend in chloroplast with increasing concentration of F.