Effect Of Indoleacetic Acid On Photosynthesis Of Two Arsenic-stressed Pteris Plants

Although heavy metals and metalloid arsenic(As)inhibit plant growth and photosynthesis,exogenous indoleacetic acid(IAA)not only promotes the growth and photosynthesis in plants but also promotes the enrichment of heavy metals and As in plants.However,exogenous IAA how to adjust the photosynthesis of plants under the arsenic stress,there was little available information for this study.A hydroponic experiment was conducted to investigate the stress effect of As on Pteris cretica var.nervosa and Pteris ensiformis in the presence of IAA(0-40mg·L-1)under the stress of 2 mg·L-1 As(V).Growth,As accumulation,photosynthetic pigment,chlorophyll fluorescence parameters,activities of two dark reaction enzymes and the chloroplast ultrastructure of the two plants were evaluated to identify the stress effect at different IAA concentrations.The results showed that:1.Compared with the control,under 2 mg·L-1 As stress dry weight of the two plants significantly increased at 20 mg·L-1 IAA treatment.However,dry weight of the two plants remained unchanged at 10 and 40 mg·L-1 IAA treatments.The fresh weight of the two plants significantly increased in the presence of 20 mg·L-1 IAA compared to those in the respective control.What's more,P.cretica var.nervosa was higher than that P.ensiformis.Two-factor variance analysis showed that there were significantly effects between plant species,auxin concentration on plant biomass(P<0.01).Under different concentration of IAA treatment,there was no significant difference in the plant height of P.cretica var.nervosa compared to the control(0 mg·L-1 IAA).However,the plant height in P.ensiformis increased in the presence of 20 mg·L-1 IAA,but there was no significant change in P.ensiformis in the presence of 10 and 40 mg·L-1 IAA.The roots length of the two plants significantly increased in the presence of 10 and 20 mg·L-1 IAA compared to those in the respective control,which reached maximum at 20 mg·L-1 IAA treatment.At 40 mg·L-1 IAA treatment,roots length of P.cretica var.nervosa was similar to that of the control samples,but it significantly increased in P.ensiformis.Two-factor variance analysis showed that there were significantly effects between plant species,auxin concentration on plant height and roots length(P<0.01).Therefore,20 mg·L-1 IAA contributed to plant growth and increase of biomass.3.2.The total and fronds As concentration of the two plants significantly increased in the presence of 20 mg·L-1 IAA compared to those in the respective control,and the concentration of As in P.cretica var.nervosa was much higher than that in P.ensiformis.At 20 mg·L-1 IAA,the photosynthetic pigment content in P.cretica var.nervosa fronds was similar to that of the control samples,but it significantly decreased in P.ensiformis.Increasing IAA concentrations did not affect the chlorophyll alb value of P.cretica var.nervosa compared to the control samples.However,the chlorophyll alb value of P.ensiformis significantly reduced in the presence of 40 mg·L-1 IAA.The maximal photochemical efficiency of photosystem II(Fv/Fm),actual quantum yield(?PSII),and non-photochemical quenching coefficient(qN)remained unchanged in P.cretica var.nervosa fronds with increasing IAA concentrations,but decreased profoundly in P.ensiformis in the presence of 20 mg·L-1 IAA.The photochemical quenching coefficient(qP)showed an increase in P.cretica var.nervosa but diminished significantly in P.ensiformis at 10-40 mg·L-1 IAA.The activity of ribulose-1,5-bisphosphate carboxylase(RuBPC)and glyceraldehyde-3-phosphate dehydrogenase(GAPDH)reduced significantly in P.cretica var.nervosa fronds at 20 mg·L-1 IAA,then it deceased slightly,and there was no significant difference in it compared to the control;Compared with the control,the activity of RuBPC in 10-40 mg·L-1 IAA and GAPDH in 40 mg·L-1 IAA decreased significantly in P.ensiformis while the GAPDH increased significantly at 10 and 20 mg·L-1 IAA.Therefore,at 20 mg·L-1 IAA.the photosynthetic pigment content,chlorophyll a/b value and chlorophyll fluorescence parameters was similar to that of the control samples,and the activity of RuBPC significantly increased in P.cretica var.nervosa fronds;however,in P.ensifornmis,the photosynthetic pigment content,chlorophyll fluorescence parameters and significantly increased significantly decreased.3.Electron microscope observation of the chloroplast ultrastructure,no visible damage was detected in P.cretica var.nervosa at 20 mg·L-1 IAA treatment,even though enrichment of a large amount of arsenic in frond.With smooth and intact external double membranes,chloroplast from control fronds were oval shape,and its grana thylakoids well arranged,which was a chloroplast with typical structure of healthy but only few starch grains and osmiophilic particles appeared.The smaller disturbance of chloroplast structure was observed at 10 and 40 mg·L-1 IAA,the number of grana decreased or disorganised,and the grana thylakoid were swollen,distorted or thin and loosely arranged,meanwhile,chlorophyll content significantly decreased.However,the larger disturbance of chloroplast structure in P.ensiformis was observed at only As stress.Chloroplasts lost its elongated shape and appeared round with irregular external membrane.Grana thylakoids lost its arrangement and small.More and big starch grains and a few osmiophilic particles were appeared.At 10 mg·L-1 IAA treatment,chloroplast were swollen.grana thylakoid structure changed and convergence phenomenon appeared.At 20 mg·L-1 IAA treatment,chloroplast double membrane structure almost disappeared.Besides,few grana thylakoids and convergence phenomenon appeared.A few osmium particles appeared.At 40 mg·L-1 IAA treatment,chloroplast double membrane structure disappeared completely.The chloroplast grana thylakoids almost disappeared.The emergence of grana was also very fuzzy.Starch grains appeared,and the number of osmium particles increased.And,chlorophyll content significantly decreased at 20 and 40 mg·L-1 IAA treatments.According to the changes of chlorophyll content and chloroplasts ultrastructure,we found that the chloroplast ultrastructure of the two plants was damaged,at the same time,the chlorophyll content also declined significantly.Therefore,the damage of chloroplast ultrastructure resulted in the decrease of chlorophyll content.According to the changes of the chlorophyll fluorescence parameters and chloroplast ultrastructure,we found when the chloroplast ultrastructure of the two plants was damaged,qN significantly reduced in P.ensiformis;Although there was not statistic difference of qN value in P.cretica var.nervosa at 10 and 40 mg·L-1 IAA treatments,it showed a downward trend.Therefore,20 mg·L-1 IAA could protect the phytosynthesis of P.cretica var.nervosa under As stress,resulting in normal growth and hyperaccumulation of As.