Study On Influential Mechanism Of Nanoplastics On Physiolog Of Barley Under Doubling CO₂ Concentration

The impact of plastic particles is being widely studied,especially in agricultural ecosystems.Nanoplastics（NPs）are plastic fragments or particles with size less than100 nm that can accumulate in plants.NPs not only directly affect the growth and development and a variety of physiological processes of plants,but also may affect the response of plant to environmental changes（such as elevated atmospheric CO₂concentration）.The sixth report of the IPCC states that the atmospheric CO₂concentration from 2017 to 2022 is approximately 400～420 ppm.With the continuous increase of atmospheric CO₂ concentration,it is expected to reach about 800 ppm by the end of this century,which will directly affect the growth and development of plants.CO₂ is an essential raw material for plant photosynthesis.As an essential raw material for plant photosynthesis,elevated CO₂ concentration not only directly affects plant photosynthesis,but also affects the physiological response of plants to stress and pollutants.Both NPs and elevated atmospheric CO₂ concentration are inevitable in the future plant growth cycle,the impact of their interaction on plant growth and development is worth further study.In addition,research has shown that NPs can be accumulate in barley（Hordeum vulgare L.）plants,inducing oxidative stress,directly or indirectly affecting the growth and development of barley,and ultimately potentially affecting food security.Therefore,in this study,spring barley was taken as the research object through the hydroponics and soil culture pot experiments,and the doubling CO₂ concentration（800 ppm,e[CO₂]）and normal CO₂ concentration（400ppm,a[CO₂]）were set.The physiological and biochemical methods and key metabolic enzyme analysis system were used to explore the effects of interaction between NPs and e[CO₂]on the absorption of NPs in barley roots,plant growth,osmotic regulation,active oxygen metabolism,antioxidant enzyme system,photosynthesis and respiratory metabolism.This study clarifys the influential mechanism of NPs on physiology of barley under e[CO₂],which may provide theoretical basis for agricultural sustainability under new pollution and environmental change in the future.The main findings are as follows:（1）The content of NPs in NPs-treated barley roots increased from 0.0082 mg g^-1under a[CO₂]condition to 0.0275 mg g^-1 under e[CO₂]condition.e[CO₂]increased the content of NPs in the roots by three times.NPs treatment significantly decreased the total root length,root surface area,average root diameter,total root volume,root tip number and root activity of barley roots.However,e[CO₂]significantly increased the total root length,root surface area,average root diameter,total root volume and root activity under NPs treatment.The results showed that NPs could be absorbed by barley roots and inhibit root growth,e[CO₂]could promote root to absorb more NPs,could alleviated the damage of NPs to root growth.（2）Under a[CO₂]condition,NPs treatment significantly increased concentration of hydrogen peroxide（H₂O₂）in barley roots and leaves by 36.21%and 8.80%.e[CO₂]significantly decreased concentration of H₂O₂ in NPs-treated roots and leaves by43.29%and 28.41%.Under a[CO₂]condition,NPs treatment improved the activities of most antioxidant enzyme in barley roots and leaves in response to the accumulation of H₂O₂.However,e[CO₂]reduced the ability of antioxidant enzyme system in NPs-treated roots and leaves,which may be caused by e[CO₂]reducing H₂O₂ level in NPs-treated roots and leaves.The results showed that NPs could promote the accumulation of more H₂O₂ in barley roots and leaves,and disrupt the reactive oxygen species（ROS）metabolism.e[CO₂]could reduce the accumulation of H₂O₂ in NPs-treated barley plants by regulating the antioxidant enzyme system,and reduced the increase of ROS induced by NPs,effectively alleviated the oxidative damage of NPs on barley plants.（3）Under a[CO₂]condition,NPs treatment significantly increased the net photosynthetic rate（P_n）,stomatal conductance（g_s）,transpiration rate（T_r）,V_j,M_o,S_m,N,φD_o,DI_o/RC,ATP and NADPH of barley,significantly decreased chlorophyll a,chlorophyll b,carotenoid,F_v/F_o,W_k,Ψ_o,φP_o,φE_o,RC/CS_o,ET_o/RC and PI_ABS;e[CO₂]significantly reduced g_s,T_r,V_j,M_o,S_m,N,φD_o,DI_o/RC,V_i,ABS/RC,ET_o/RC,TR_o/RC,ATP and NADPH under NPs treatment,significantly increased P_n,C_i,carotenoids,F_v/F_o,W_k,Ψ_o,φP_o,φE_o,RC/CS_o and PI_ABS.It showed that e[CO₂]reduced the electron production under NPs treatment,weakened the promotion effect of NPs on photosynthesis of barley,and reduced the assimilation ability of NPs treatment.Moreover,NPs significantly decreased the activities of glyceraldehyde-3-phosphate dehydrogenase（GAPDH）and ribulose-1,5-bisphosphate carboxylase/oxygenase（Rubisco）,and significantly increased the activities of3-phosphoglycerate kinase（PGK）,triose phosphate isomerase（TPI）and sucrose phosphate synthas（SPS）and content of sucrose;e[CO₂]significantly improved the activities of Rubisco,PGK and GAPDH,as well as contents of sucrose and starch under NPs treatment,but only significantly decreased the activities of TPI and SPS.The results showed that NPs inhibited the photosynthetic carbon assimilation of plants,but promoted the production of carbon assimilation products（sucrose and starch）.e[CO₂]promoted the photosynthetic carbon assimilation of plants under NPs treatment,and further produced more carbon assimilation products.（4）Under a[CO₂]condition,NPs treatment only significantly reduced the activities of sucrose synthase（Su Sy）,phosphoglucoisomerase（PGI）,glucose-6-phosphate dehydrogenase（G6PDH）and aldolase（Ald）related to glycolysis.However,e[CO₂]significantly reduced the activities of most key enzyme under NPs treatment,included cytoplasmic invertase（cyt Inv）,vacuolar invertase（vac Inv）,cell wall invertase（cw Inv）,ADP-Glucose pyrophosphorylase（AGPase）,UDP-glucose pyrophorylase（UGPase）,Phosphoglucomutase（PGM）,phosphofructokinase（PFK）,PGI,G6PDH and Ald,indicated that e[CO₂]reduced the glycolysis efficiency of plants under NPs treatment.In addition,NPs treatment significantly increased the activities of key enzyme and the contents of metabolite in the tricarboxylic acid cycle,that is,citrate synthase（CS）,isocitrate dehydrogenase（ICDH）,α-ketoglutarate dehydrogenase（α-KGDH）,pyruvic acid（PA）,citric acid（CA）,ATP and NADH,while e[CO₂]showed the opposite trend of these parameters under NPs treatment.It showed that NPs promoted the tricarboxylic acid cycle of barley plants,while e[CO₂]inhibited the tricarboxylic acid cycle in NPs-treated plants.At the same time,e[CO₂]inhibited the efficiency of respiratory electron transfer by reducing the activities of NADH dehydrogenase（Complex I）,succinate dehydrogenase（Complex II）,cytochrome c oxidase（Complex IV）and ATP synthase（Complex V）in NPs-treated plants.In summary,e[CO₂]promoted barley roots to absorb more NPs,but reduced the negative effects of NPs on roots.e[CO₂]reduced the oxidative damage of NPs on barley plants by regulating the antioxidant enzyme system.e[CO₂]weakened the promotion effect of NPs on the photosynthesis of barley plants,but promoted the carbon assimilation of NPs-treated plants,produced more carbon assimilation products（sucrose and starch）.e[CO₂]inhibited the efficiency of glycolysis,tricarboxylic acid cycle and respiratory electron transfer by affecting the activities of core enzyme under NPs treatment.