Study On The Mechanism Of Panax Notoginseng Photosystem Adapting To Shade Environment Based On The Synergistic Strategy Of Nitrogen And Potassium

Panax notoginseng（P.notoginseng）is a typical shade C₃ plant,which has strict requirements for environmental factors.In order to facilitate the intensive planting of P.notoginseng,a suitable lighting environment is created by erecting shade nets in production.However,the scientific problem of the photosynthetic regulation mechanism of P.notoginseng in the shade environment has not been effectively explained.The photosynthetic efficiency of crops is most affected by nitrogen（N）and potassium（K）.However,can N and K regulate the photosynthetic mechanism of P.notoginseng under shade conditions to maintain it relatively high activity?Can it alleviate the limitation of photosynthesis?Can the distribution of photosynthetic nitrogen pool and storage nitrogen pool in P.notoginseng leaves be adjusted?This series of questions are the problems that must be solved to reveal the mechanism of P.notoginseng light system adapting to the shade environment.In response to the above problems,in this study,different light intensity environments were set up to analyze the synergistic effect of N and K synergy on（1）the activity of reaction center of P.notoginseng photosystem;（2）the physical structure of photosynthesis and photosynthesis limitation in P.notoginseng leaves;（3）the photosynthetic N substrate supply system;（4）the photosynthetic product metabolism and key enzyme activities.The purpose of this study is to clarify how P.notoginseng photosystem adapts to the shade environment by regulating the absorption and distribution of N and K,thereby improving the mechanism of P.notoginseng adapting to the shady environment.And from the perspective of photosynthetic efficiency improvement,it will provide a new theoretical basis for N and K to synergistically increase the yield and fertilizer utilization rate of P.notoginseng.The main results of this study were following:（1）The variation trend of photosynthetic effective radiation intensity（PAR）and temperature both increased with the increase of light transmittance,and the humidity showed LL>ML>HL under different light transmittance（LL:5.3%,ML:11.5%,HL:20.4%）.Principal component analysis of temperature,humidity and PAR showed that light transmittance had the greatest impact on PAR.The biomass of one-year old P.notoginseng decreased with the increase of PAR,and the biomass of two-years and three-year-old P.notoginseng reached the maximum value at PAR of 200μmol/m²/s and400μmol/m²/s,respectively.The expression of key genes（NAC1、GL2、Exp1）in the root system of P.notoginseng can be down-regulated,and root growth can be inhibited,thereby reducing the accumulation of N,P,K and other large elements in P.notoginseng under LL or HL treatment.At the same time,the photosynthetic rate and triose phosphate utilization rate of P.notoginseng leaves were reduced under strong or weak light,resulting in a significant decrease in the accumulation of dry matter in P.notoginseng leaves,stems,roots,flowers and the whole plant.In addition,The specific leaf area of P.notoginseng increased（LL>ML>HL）with the increase of light transmittance.（2）The results of field experiment showed that the biomass of P.notoginseng root increased first and then decreased with the increase of N and K fertilizer application.The combined application of N1K2(225 kg N·hm^-2 and 450 kg K₂O·hm^-2)significantly increased the surface area and projected area of the main root,optimized the development of the root system of P.notoginseng,and then promoted the absorption of nutrient elements in the leaves of P.notoginseng and the distribution of nitrogen in the photosynthetic system.Eventually the photosynthesis of P.notoginseng is improved.The pot experiment showed that N1K2 significantly promoted the synthesis of photosynthetic pigments of P.notoginseng under different light level,alleviated the inhibitory effect of strong light or low light on photosynthetic efficiency,and enhanced the adaptability of P.notoginseng to shade environment.（3）The chlorophyll content of P.notoginseng decreased under strong light and weak light environment.Fv/Fm,ΦPSII and q P all showed ML>HL>LL,while NPQ showed HL>Ml>LL.Nitrogen and potassium synergistically improve the leaf characteristics and chlorophyll content of P.notoginseng,thereby alleviating the decrease of Fv/Fm,ΦPSII and q P values of P.notoginseng leaves induced by strong light or weak light,and promoting the heat dissipation of leaves.It shows that NK synergy can reduce the disorder of photosynthetic system structure and function induced by unsuitable light environment and the damage to electron transfer,which ensures that the PSII of P.notoginseng maintains a high photochemical activity,thereby adapting to the shade environment.（4）The relative variable fluorescence showed that the L-peak（50-300μs）was negative under low light,while the H-peak and G-peak did not change significantly.However,the L-peak,K-peak,H-peak and G-peak were all positive under strong light conditions.The results of W_OI standardization showed that the amplitude of I-P phase had no significant difference in weak light,but decreased significantly in strong light compared with the ML.In addition,the performance indicators（PIabs）of PSII of P.notoginseng were significantly reduced under strong or weak light,resulting in a decrease in the number of PSII active reaction centers,which in turn inhibited the electron transfer rate of the photosystem of P.notoginseng.The P.notoginseng upregulated the P point under strong or weak light through NK synergy,increased the I-P phase amplitude,alleviated the decrease in PIabs and ETo/RC of P.notoginseng,and promoted the increase of the number of active reaction centers,thereby improving the quantum yield and electron transport rate of PSI and PSII under strong or weak light.In addition,the synergistic effect of N and K significantly increased the enzyme activities of Ca²⁺-ATPase and Mg²⁺-ATPase of P.notoginseng under different light,resulting in the accumulation of more ATP,which in turn promoted the repair cycle of PSII under strong or low light.（5）The stomata of P.notoginseng were large and sparse under the LL condition,while the stomata of the ML and HL treatments were small and dense,and the stomatal opening degree was expressed as ML>LL>HL,resulting in a significant decrease in stomatal conductance（Gs）under strong or weak light conditions.The thickness of P.notoginseng leaves increased,the ratio of palisade tissue to sponge tissue and the proportion of palisade tissue increased significantly,while the proportion of sponge tissue decreased significantly with the increase of light intensity.In addition,both LL and HL treatments significantly reduced the ratio of cell voids to leaf volume(f_ias)and the area of chloroplasts facing the cell voids（S_c/S）.In contrast,the cell wall thickness(T_cw)increased with the increase of light intensity,resulting in the weakening of mesophyll conductance（g_m）.NK synergism significantly increased the stomatal density,opening number and stomatal opening degree of P.notoginseng under strong or weak light,and decreased the stomatal limit value and T_cw.At the same time,NK synergism significantly increased the ratio of palisade tissue to sponge tissue,S_c/S and f_ias,then reduced the total photosynthetic limit value under strong or weak light,thereby promoting the increase of Rubisco and RCA enzyme activities,and finally improved the carboxylation efficiency of P.notoginseng.（6）LL and HL treatments significantly reduced the nitrogen content per unit area（Na）and the proportion of photosynthetic nitrogen（Npsn）in P.notoginseng leaves,while the proportion of structural nitrogen（Nstr）and stored nitrogen（Nstore）increased significantly,and then reduced the photosynthetic nitrogen use efficiency and photosynthetic efficiency of P.notoginseng.P.notoginseng significantly increased the distribution of nitrogen in the photosynthetic apparatus of P.notoginseng leaves through the coordination of N and K,and improved the distribution proportion of carboxylation system nitrogen（Ncb）.At the same time,P.notoginseng promoted the transformation of leaf non-protein nitrogen（Nnp）and fat soluble nitrogen（Nos）to photosynthetic nitrogen under strong or weak light,so as to expanded the photosynthetic nitrogen pool,thereby alleviating the reduction of photosynthetic nitrogen use efficiency and photosynthetic efficiency of P.notoginseng under strong or weak light.（7）Compared with ML,the number of differential metabolites in HL and LL treatment were 153 and 27,respectively.P.notoginseng regulated the metabolism of starch and sucrose,fructose and mannose,and then changed the synthesis and decomposition of sucrose and fructose under HL and LL treatment through NK.In addition,P.notoginseng promoted the further increase of SPS enzyme activity in leaves under strong light and reduced SPS enzyme activity in leaves under weak light,so as to alleviated the sucrose accumulation induced by unsuitable light environment.At the same time,P.notoginseng increased the activities of ADPG caramel phosphorylase（AGPase）,granule-bound starch synthase（GBSSase）and soluble starch synthase（SSSase）,and inhibited the activities of starch phosphorylase（SPase）,α-amylase,β-amylase and total amylase through the coordination of N and K,then the saponin content of P.notoginseng under different light intensities was increased.In summary,P.notoginseng alleviates the inactivation of light system caused by strong light or weak light and changes the leaf structure to reduce the limitation of photosynthesis under strong light or weak light through N and K cooperation,so as to ensure electron transfer and CO₂ conduction under unsuitable light environment.At the same time,P.notoginseng promotes the transformation of non-protein nitrogen to photosynthetic nitrogen through the NK synergy strategy,which ensured the sufficient supply of nitrogen substrate in the process of photosynthesis.In addition,P.notoginseng regulates the metabolism of photosynthetic products to adapt to the shade environment,and then maintains the yield and quality of P.notoginseng under the unsuitable light environment.