Comprehensive Evaluation Of Low Nitrogen Tolerance Of Cucumber Germplasm At Seedling Stage And Utilization Of Related Growth Regulators

China’s cucumber(Cucumis sativus L.)production ranks first in the world,accounting for 81.2% of the global planting area.Cucumber has a high demand for nitrogen fertilizer and is often over-applied in order to pursue yield production.However,excessive application of nitrogen fertilizer can lead to soil acidification and leaching of nitrogen fertilizer into water bodies,causing serious environmental pollution problems such as eutrophication of water bodies.The selection and breeding of low nitrogen tolerant cucumber varieties to improve the low nitrogen tolerance of cucumber is an effective way to achieve green production of cucumber.In this study,we identified some cucumber germplasm with high tolerance to low nitrogen by identifying low nitrogen tolerance indexes in seedling stage,and selected some cucumber germplasm with high tolerance to low nitrogen by transcriptome analysis,mining differential expression pathways and differential gene analysis,further discovering endogenous substances that can improve low nitrogen tolerance in cucumber,which can provide reference for cucumber breeding for low nitrogen tolerance on one hand,and promote the role of exogenous substances in improving low nitrogen tolerance in cucumber on the other hand.On the other hand,it can promote the application of exogenous substances in improving the growth and development of cucumber under low nitrogen.The results are as follows:1.Thirty-two sequenced cucumber germplasm were used as test material,and two nitrogen levels,normal(10 m M)and low(0.1 m M),were set in an artificial climate chamber under controlled light and temperature,and seedling hydroponics was performed.The changes of 19 indicators,including plant height,root dry weight,aboveground dry weight,root to crown ratio,net photosynthetic rate,transpiration rate,aboveground nitrogen content and aboveground nitrogen accumulation of cucumber were analyzed after 10 d of treatment.And the low nitrogen tolerance of cucumber was comprehensively evaluated by principal component analysis,affiliation function method and cluster analysis,and the low nitrogen sensitive germplasm 9930 and low nitrogen tolerant germplasms 23 and 47 were screened out of 32 germplasm.2.Transcriptome analysis of leaves and root systems of germplasm 9930,23 and 47 at 2 and 8 d of low nitrogen treatment revealed that differentially expressed genes of low nitrogen tolerant germplasms 23 and 47 were jointly enriched in molecular functions such as Antioxidant activity,Oxidoreductase activity and Further analysis revealed that the genes enriched in the Alanine,aspartate and glutamate metabolism pathway were mainly associated with GABA synthesis.The KEGG analysis of the genes that were up-regulated in the root systems of germplasm 9930,23 and 47 at 2 d of low N stress showed that the genes that responded to growth hormone were upregulated in the root system of germplasm 23.These results suggest that both germplasms 23 and 47 can resist low nitrogen stress by regulating antioxidant,nitrogen metabolism and GABA-related metabolism,and germplasm 23 can also resist low nitrogen stress by regulating genes related to IAA metabolism in the root system.3.Different concentrations of GABA and its inhibitor 3-MP and IAA and its inhibitor yucasin were exogenously sprayed on low N-sensitive germplasm 9930,and it was found that 200 μM 3-MP and 100 μM yucasin inhibited germplasm 9930 under low N stress to the highest extent compared with the control(clear water),and 50 m M GABA and 50 μM IAA improved germplasm 9930’s tolerance to low The best effect was achieved by 50 m M GABA and 50 μM IAA to improve the low N tolerance of germplasm 9930.Under low nitrogen,exogenous sprays of 200 μM 3-MP and 100 μM yucasin significantly inhibited the growth of germplasm 9930 under low nitrogen stress and reduced the photosynthetic capacity,antioxidant capacity and nitrogen metabolizing enzyme activities of germplasm 9930.50 m M GABA and 50 μM IAA both improved the low nitrogen tolerance of cucumber by increasing antioxidant enzyme activities and nitrogen metabolizing related enzyme activities,among which 50 m M GABA reduced the growth of germplasm 9930 under low nitrogen stress.m M GABA could alleviate the inhibition of photosynthetic capacity of cucumber by low nitrogen stress,while 50 μM IAA could alleviate low nitrogen stress by promoting stem and root elongation.