Regulation Effects Of Exogenous Silicon On Tomato Growth And Development Under Low Phosphorus Condition

Phosphorus(P)deficiency in soils is one of the abiotic stress factors for sustainable crop production.Improving crops’ absorption and utilization of P and alleviating the low P stresses on plants through biological approaches have become a current research hotspot.Silicon(Si)is a beneficial element that can promote plant growth,development and resistance to various stresses.However,the effect of Si on tomato(Solanum lycopersicum L.)growth and development under P deficit conditions and underlying mechanisms remains unclear.In this study,we used ‘Zhongza 9’ tomato cultivar,with eight treatments of two Si levels(0 and 1.5 mmol·L-1 K2 Si O3·n H2O)combined with four P levels(0.66,0.44,0.22,and 0 mmol·L-1 Na H2PO4),under hydroponic cultivation.We analyzed the effects of Si(1.5 mmol·L-1)on plant growth,photosynthesis,reactive oxygen species,osmotic adjustment and element absorption properties.We also studied the impacts of Si at the transcriptional levels on tomato roots under low P stresses.The main results are as follows:1.Under low P supply,The root function of tomato and leaves’ microstructure was damaged.The relative water content,photosynthetic pigment content and net photosynthetic rate of the leaves were reduced,and the biomass was significantly reduced.Exogenous Si alleviated the inhibitory effects of low P on tomato seedling growth and photosynthesis,and had the best alleviation effect on mild low P stress.2.Under low P supply,the content of malondialdehyde in tomato leaves and roots was increased significantly.The integrity of the plasma membrane was impaired,and the main antioxidant enzyme activities and antioxidant content were decreased to different levels.Exogenous Si enhanced its antioxidant capacity of tomato seedlings by inducing the antioxidant enzyme activity(SOD,CAT,POD)and accumulation of antioxidants(GSH),thereby maintaining the cell membrane completeness under low P stresses.3.Under low P supply,The osmotic potential of tomato leaves showed a downward trend,but did not reach a significant level(P<0.05),while the osmotic potential of the root system decreased significantly by 37.93% and 41.38%.Exogenous Si effectively promoted the accumulation of osmotic adjustment substances in tomato seedlings,and the proline and betaine in the leaves had higher contributions,and the increase was more than 20%.The contributions of soluble sugars and free amino acids in roots were also significantly higher than the mildly low P supply with increases of 40.59% and 54.55%,which maintained a good osmotic regulation ability to protect cell function.4.Under low P stresses,the absorption of Ca,Mg,Na,K,Fe,Mn,Zn,and Cu in tomato roots were significantly decreased.Exogenous Si promoted the absorption of mineral elements by the root system,The distribution ratios of P,Ca,Mg,Fe,Mn,Zn,Cu in stems and leaves under mildly low P were also all significantly increased.Under severe low P stresses,the content of amino acids in the root system decreased,with the largest decrease in histidine by 51.48%,Exogenous Si under low P content significantly induced the accumulation of threonine,aspartic acid,histidine,and alanine.5.Transcriptome study showed that the main differentially expressed genes(DEGs)of tomato roots induced by exogenous Si treatment under low P stresses were involved in antioxidant defense response,organic acid metabolism,glutathione metabolism,photosynthesis and post-transcriptional regulation.Gene Ontology analysis of DEGs showed that unregulated genes induced by exogenous Si upregulation under low P were significantly enriched in organic acid metabolism pathways,which demonstrated that exogenous Si under low P conditions could mediate the expression of key enzyme genes involved in the citrate cycle in the root system.The secreted organic acids were used to activate fixed P in the cultivation medium,thereby alleviating the damage caused by low P stress.