| Tomato(Solanum lycopersicum)is one of the main crops cultivated in greenhouses during winter and spring in China.Due to the sensitivity of tomato to low temperature and the characteristics of greenhouses,low temperature is the main factor that limit tomato plant growth in greenhouses.The effects of low temperature on the physiological and biochemical processes in tomato plant have been studied.However,there are few reports on the relationship between nitrogen metabolism and photosynthesis,growth and proteomics in tomato plant under low temperature.The effects of nitrogen addition and low temperature on the nitrogen metabolism system,growth,photosynthetic system,cell structure and proteomics in tomato plant were studied,which was aimed to explore the mechanism that nitrogen absorption,metabolism and distribution affected on the photosynthesis,growth and protein expression of tomato plant at low temperature.The main results are as follows:1.When exposed to low temperature stress for a long time,the RGR and biomass accumulation of tomato plant decreased due to the inhibition of photosynthesis and the activities of nitrogen metabolism enzymes.High nitrogen treatment relieved the adverse effects of low temperature on plants,which improved the RGR and the biomass of plant.This was due to the increased in the activities of nitrogen metabolism under high nitrogen treatment,which lead to the increased of nitrogen content in stem,root and leaves.Increased in the activities of nitrogen metabolism and nitrogen content in organs also influenced stomatal limitation to photosynthesis by increasing E,gs and Ci and decreasing Ls.Meanwhile,the content of Chl a,Chl b,Car increased and the ratio of Car/Chl decreased,which was responsible for the improved of light energy absorption and the reduced of the portion of heat dissipation.Conversely,low nitrogen treatment exacerbated the adverse effects of low temperature stress on plants and reduced the RGR and biomass accumulation.These indicated that appropriate increase of nitrogen can enhance low temperature resistance and increase photosynthesis and growth at low temperature in tomato plant.2.When exposed to low temperature stress,nitrogen addition treatment improved the NAR,which lead to the increased of RGR and biomass accumulation of tomato plant.The changed RGR of leaves and root represents the acquisition of light energy and nutrition,respectively.In low temperature,nitrogen addition treatment improved the area and LAR of up leaves,these favored the increased of photosynthetic area and the improvement of the whole plant RGR.Nitrogen sources supporting the NAR improved at low temperature included the inorganic nitrogen absorbed form substrate by root and the nitrogen reallocation from old leaves.Under low temperature,the treatment of nitrogen addition enhanced the nitrogen absorbed form substrate by root and inhibited the nitrogen transferred from old leaves,which increased the nitrogen content in the old leaves,new leaves,root and stem,these favored the improvement of NAR.In addition,the increased content of nitrogen in below leaves delayed the senescence,helping to these leaves maintained photosynthetic performance.Large leaf area was favorable for photosynthetic carbon and nitrogen metabolism,which was resulted for the increased of RGR and biomass accumulation.The increased in nitrogen absorption by root under nitrogen addition treatment improved the activities of NR,GS and GOGAT and decreased the activity of GDH.The value of NAR is related to PNC and NP.Whereas NP was not changed under nitrogen addition treatment in low temperature,so the increased growth was mainly attributed to the increased of PNC.3.Nitrogen limitation is often accompanied by low-temperature stress under natural conditions and together,they may drive a greater imbalance between light absorption and the metabolic use of light energy than any one of the two factors alone.Net photosynthesis,CO2,and light-response curves improved in the NH4NO3 addition plants,demonstrating an enhanced photosynthetic performance.Upregulated photosynthetic pigments and chlorophyll fluorescence parameters,including Fv/Fm,Fv'/Fm',?PSII,ETR and q P and downregulated Car/Chl ratios and NPQ showed that NH4NO3 addition improved light absorbance and utilization,while reducing energy dissipation and ROS generation,which was also indicated by the lowered malate dehydrogenase activity.Enhanced gs and gm favored CO2 diffusion efficiency,which,together with enhanced Ru Bis CO activity,ultimately increased carboxylation efficiency.These results emphasize the importance of nitrogen addition to the favorable recovery and acclimation of the photosynthetic system to low temperature.4.When plant exposed to low temperature stress for a sustained time,excess excitation energy production may break the balance of ROS generation and scavenging,which lead to the increased production rate and the accumulation of the O2?-and H2O2 in vivo.Excessive active O2?-and H2O2 together with low temperature played an adverse impact on leaf structure and chloroplast ultrastructure.Low temperature stress significantly increased the thickness of the epidermis,spongy tissue,lower epidermis and leaves,and significantly reduced the ratio of palisade/sponge.In low temperature,NH4NO3 addition treatment mitigated the adverse effects of low temperature on leaf structureand,and reduced the thickness of spongy tissue,lower epidermis and leaves,and slightly increased the ratio of palisade/sponge.Low temperature stress swelled the structural of chloroplast,increased the content or volume of osmiophilic granule,reduced the content of SL and GL,thicken the mesophyll cell wall and higher the viscosity of cytoplasm.Compared with no NH4NO3 addition treatment,chloroplasts in different leaves under NH4NO3 addition treatment had smaller osmiophilic granule,more grana and bigger chloroplast.In addition,the degradation of chloroplast in below leaves was reduced in NH4NO3 addition plant under lower temperature.These provide the basic guarantee for the improvement of photosynthetic performance.5.By the analysis of protein expression profiles of tomato seedlings treated with different temperatures and nitrogen,we found that high number of differentially expressed proteins involved in stress response,development process,biological regulation,biological process regulation,signal and molecular function for NH4NO3 addition and NH4NO3 free treatments under low temperature.For metabolic pathways,many differentially expressed proteins were classified to photosynthesis,carbohydrate and energy metabolism,and amino acid and protein metabolism.For analysis the change of these differentially expressed proteins in these three metabolic pathways,we found that low temperature stress reduced the content of photosynthesis and photosynthesis-antenna related proteins,which was lead to inhibition of photosynthesis.Nitrogen addition at low temperature increased the differentially expressed proteins associated with photosynthesis and photosynthetic antenna proteins and reduced the content of differentially expressed proteins in carotenoid biosynthesis.These showed that NH4NO3 addition treatment can reduce carotenoid biosynthesis and the ratio of Car\Chl.The proteins linked carbon and nitrogen metabolism was up-regulated by nitrogen addition at low temperature,while the proteins involved in carbohydrate metabolism and stress response were decreased.Furthermore,the CHCB2,RPE,LHCA3,GOT1,psa H,pet E and NADP+-MDH are differentially expressed proteins in low temperature treatment,which were related to photosynthesis and were verified at transcription level.Except for GOT1,psa H,pet E and pet E,the expression of other genes at transcription level were consistent with protein level. |
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