Physiological And Biochemical Characteristics And Glyphosate Tolerance Of Alligator Weed (Alternanthera Philoxeroides) Under Manganese Stress

Alligator weed,Alternanthera philoxeroides (Mart.)Griseb.,is one of the mostinvasive alien weeds in China.Hydroponic culture experiments were conducted in thepresent study to investigate the growth and propagation of alligator weed,and theassociation between Mn accumulation and absorption of other ions under Mn stress,and the physiological responses of photosynthesis and antioxidant enzymatic systemsof alligator weeds to Mn stress.The susceptibility of alligator weed to glyphosate andthe behavior of ¹⁴C-glyphosate in this weed were also analyzed.Furthermore,thedeposition characteristic of glyphosate in alligator weed leaves was determined.Theresults were summarized below:The growth of alligator weed was significantly enhaced in high Mn concentrationranging from 0.31～2.45 mM treatment as compared to that of the control (0.0091 mMMn),but inhibited when Mn concentration increased to more than 4.90 mM.Thenumber of new rhizomes developed from the parent rhizomes on 40,120,and 365 daysafter treatment (DAT)of 0.31 mM Mn were 2.3～2.6 times of those of the control,andthe biomass of underground parts were significantly increased by 24.1%～32.2%.Theshoot fresh weight per year was significantly increased by 18.4 % as compared to thatof the control.The propagation of alligator weed was also stimulated in 2.45 mM Mntreatment.The number of new rhizomes developed from the parent rhizomes on 40,120,and 365 DAT were 2.4～3.0 times more than that of the control,and the biomassof underground parts increased from 82.2% of the control at 40 DAT to 125.1% of thecontrol at 365 DAT.However,the biomass of shoots per year was only 71.8 % of thecontrol.The results indicate that the potential of alligator weed fitness to Mn stressprobably contribute to its invasive ability.The light saturation point of alligator weed was not altered under Mn stress,PPFDapproximately around 1000μmolm^-2s^-1.Compared to the control,the content of bothChlorophyll b and total Chlorophyll increased in 0.31 mM Mn treatment,and theexcitation energy capture efficiency of PSâ…¡reaction centers (F_v′/F_m′)and electrontransport rate (ETR)was improved,with more excited energy used for photochemistry reaction,and the photosynthetic rate (P_n)was also increased by 22.4%.In 2.45 mM Mntreatment,the content of Chlorophyll b and the ratio of Chlorophyll a/b were thesame as the control,but the effective PSII quantum yield (Φ_PSâ…¡)and ETR as well asphotochemical quenching (q_p)declined compared to the control,and consequently CO₂assimilation probably be inhibited.Therefore,the photosynthesis of alligator weed wasimproved under 0.31 mM Mn treatment,but was inhibited under 2.45 mM Mntreatment.The antioxidant enzymatic systems under high Mn stress were characterized.Thecontent of malondialdehyde (MDA),and the activity of superoxide dismutase (SOD)and ascorbate peroxidase (APX)in alligator weed leaves were similar to those of thecontrol in 0.31 mM Mn treatment.The content of MDA was similar to that of thecontrol,the activity of SOD was significantly increased by 69.0%,and the activity ofAPX was increased and then followed by decrease in 2.45 mM Mn treatment ascompared to those of the control.After glyphosate (68 g ae/ha)treatment,the solubleprotein content was not affected under Mn stress;however,the activity of SODdecreased earlier than that of the control in 0.31 mM Mn treatment;and the MDAcontent increased earlier and the activity of SOD and APX decreased earlier comparedto those of the control in 2.45 mM Mn treatment.The above results indicate that theantioxidant enzymatic systems might have been initiated in alligator weeds in responseto Mn stress.Mn is absorbed by the roots of alligator weed and then translocated in the plant.Excess Mn is mainly accumulated in leaves and underground parts of the weed.On120 days after 0.0091 mM Mn treatment (the control),the concentration of Mn inleaves,stems and underground parts were 0.13,0.042 and 0.15 mg/g,respectively.In0.31 and 2.45 mM Mn treatments,the concentrations of Mn in leaves were 5.2 and44.6 times those of the control,and the concentrations of Mn in underground partswere 5.0 and 45.3 times those of the control.The concentrations of Iron (Fe),copper(Cu),zinc (Zn),boron (B),and molybdenum (Mo)in leaves of the control were 163.0, 15.7,74.0,0.7,and 34.4μg/g,respectively,however,the concentrations of Fe,Cu,Zn,B,and Mo in leaves were significantly increased by 21.2%,17.1%,5.0%,47.9% and33.9%,respectively,in 0.31 mM Mn treatment and by 12.5%,25.3%,7.9%,73.1%and 43.4%,respectively,in 2.45 mM Mn treatment.The concentration of calcium (Ca),magnesium (Mg)and potassium (K)in underground parts of the control were 30.1,3.4and 30.1 mg/g,respectively.However,the concentration of K in underground partswas decreased by 25.0% in 0.31 mM Mn treatment,and those of Ca,Mg and K inunderground parts were decreased by 44.7%,31.1% and 19.0%,respectively,in 2.45mM Mn treatment.Alligator weeds showed remarkably variable susceptibility to glyphosate under Mnstress.On 120 days after treatment of 0.31 and 2.45 mM Mn,shoots fresh weightreduction declined significantly by 17.4 % and 44.6 %,respectively,compared to thatof the control on 20 days after glyphosate application (DAA).The number of survivingrhizomes in both high Mn treatments was approximately 3 times more than that of thecontrol at 50 DAA.The ¹⁴C-glyphosat absorption by alligator weed was not reducedunder Mn stress 3 days after ¹⁴C-glyphosate application,but more ¹⁴C-glyphosate wastranslocated.Consequently the ¹⁴C-glyphosate concentration in both shoots andrhizomes were improved in 0.31 mM Mn treatment.Though more ¹⁴C-glyphosate wasremained in the treated leaves in 2.45 mM Mn treatment,the ¹⁴C-glyphosateconcentration in rhizomes and roots were remained at the same level as the control.Less ¹⁴C-glyphosate was accumulated in the treated basal leaves under Mn stress 7days after ¹⁴C-glyphosate application.About 10 ng per plant of ¹⁴C-glyphosate residuewas detected in rhizomes and roots 14 days after ¹⁴C-glyphosate application,whichwas approximately 0.8% of total ¹⁴C-glyphosate applied.The main metabolic productof ¹⁴C-glyphosate——AMPA was not detected using thin-layer chromatography (TLC).These results suggest that the tolerance of alligator weed to glyphosate under Mn stressis probably not due to the decrease of absorption and translocation of glyphosate andmight not caused by rapid metabolism of glyphosate in the weed. A noteworthy strategy for tolerant or resistant weed management is to improveherbicide application technique.Amogy the droplet sizes ranging from VMD 149.5μmto 233.7μm,the deposition of glyphosate on alligator weed leaves treated in VMD157.3μm was more than that in larger droplets.The deposition of glyphosate treated inless spray volume (339 L/hm²)and smaller droplets (VMD 157.3μm)was 1.54 timesmore than that in more spray volume (694.5 L/hm²)and coarse droplets (VMD 233.7μm).The deposition of glyphosate was increased slightly when spray volume wasmore than 382.5 L/hm²,and the maximum deposition of glyphosate on alligator weedleaves was approximately 4.92μg ae/cm² when glyphosate was applied inconcentration of 800 mg ae/L and VMD 149.5μm.The above results indicate that inrelatively smaller droplets of VMD (149.5～157.3μm)and lower spray volume(approximately 339 L/hm²),the efficiency of glyphosate application would beimproved significantly in the process of alligator weed management.