Physiological Mechanism Of Increasing Nitrogen Absorption And Utilization Efficiency And Yield Of Peanut By Melatonin

As a leguminous crop,peanut has its own nitrogen fixing ability,but the nitrogen fixed by peanut root nodules cannot meet the needs of peanut growth and development,so nitrogen supplement is needed.The overapplication of nitrogen fertilizer in peanut production leads to the decrease of nitrogen utilization rate and environmental pollution.Melatonin is a novel growth regulator that promotes growth and development and enhances plant stress tolerance.However,the effects of melatonin on peanut growth,nitrogen uptake and utilization efficiency and yield under different nitrogen levels were not clear.Therefore,in this experiment,peanut variety "Yuhua 37" was used as the test material to explore different nitrogen levels(6 Kg/ mu(low nitrogen),8.4 Kg/ mu(medium nitrogen)and 12 Kg/ mu(high nitrogen)under field conditions.Effects of melatonin seed dressing treatment on peanut biomass,root growth,nodule number,photosynthesis,carbon metabolity-related enzyme activities,nitrogen assimilation-related enzyme activities,nitrogen efficiency,active oxygen metabolism,yield and other indexes under bucket and column watering with nutrient solution containing 5 m M NO3-(high nitrogen)and 1 m M NO3-(low nitrogen),respectively.The test results show that:1.Under different nitrogen levels,melatonin could promote the growth of roots and nodules,improve the nitrogen fixing ability and nitrogen absorption of peanut roots,reduce the demand for nitrogen fertilizer,and increase biomass.In the field condition,the promotion effect was most significant under the medium nitrogen level.In the pod maturity stage in 2021 and 2022,compared with 8.4 Kg/ mu nitrogen alone,the upper dry weight of groundnut under the melatonin seed dressing treatment increased by 20.70%and 18.06%,and the number of nodulas increased by 47.69% and 16.66%,respectively.Total root length increased by 11.13% and 14.82%,respectively.In the barrel and column condition,the promotion effect is better under high nitrogen level.Compared with high nitrogen treatment alone,the upper ground dry weight under melatonin treatment increased by 24.05% and 17.06% in 2021 and 2022,respectively.The number of root nodules increased by 31.78% and 13.87%,respectively.Total root length increased by33.23% and 36.11%,respectively.2.Under different nitrogen levels,melatonin can improve the activities of enzymes related to nitrogen metabolism,promote nitrogen assimilation in peanuts,increase the content of available nitrogen in plants,and further improve the nitrogen content and nitrogen efficiency of all organs.At pod maturity stage in 2021 and 2022,compared with 8.4 Kg/ mu nitrogen alone,the nitrogen use efficiency and nitrogen harvest index under melatonin treatment were significantly increased by 25.08%,30.91%,16.46% and29.21%,respectively.Kernel nitrogen content increased by 50.03% and 52.52%,respectively.Compared with simple application of low nitrogen level(6 Kg/ mu),the nitrogen absorption efficiency under melatonin treatment was increased by 44.35% and 42.92%,respectively.The nitrogen content of fruit shell increased by 33.33% and 50.03%,respectively.Total nitrogen content of plants increased by 44.91% and38.27%,respectively.3.Under different nitrogen levels,melatonin can increase chlorophyll content,net photosynthetic rate and chlorophyll fluorescence,reduce active oxygen content,prolong leaf functional period and promote photosynthesis.Further increase the enzyme activity related to carbon metabolism,promote the increase of starch and sucrose content.At pod maturity stage in 2021 and 2022,chlorophyll content under melatonin treatment increased by 62.08% and 21.72% compared with 8.4 Kg/ mu nitrogen alone.Pn increased by 39.35% and 27.31%,respectively;Fv/Fm increased by 9.38% and 11.43%,O2.-content decreased by 25.66% and 25.83%,and H2O2 content decreased by 18.23% and 15.21%,respectively.Sucrose content increased by 68.05% and 28.42% under melatonin treatment.Starch content increased by24.20% and 30.44%,respectively.4.Seed mixing with melatonin under different nitrogen levels could increase fruit weight and kernel weight,thus increasing peanut yield,and the promotion effect was most significant under medium nitrogen level.Compared with 8.4 Kg/ mu nitrogen alone,melatonin treatment increased yield by 24.18%and 19.75% in 2021 and 2022.Compared with the simple application of 12 Kg/ mu nitrogen,the yield of 6Kg/ mu nitrogen and 8.4 Kg/ mu nitrogen combined with melatonin treatment increased by 48.00%,27.92% and 46.53% and 21.25%,respectively,in 2021 and 2022.In conclusion,the application of exogenous melatonin at different nitrogen levels promoted root growth and root nodule formation,and improved nitrogen assimilation enzyme activity,so as to improve nitrogen absorption,fixation and utilization efficiency.At the same time,by maintaining the balance of active oxygen metabolism and enhancing the enzyme activity related to carbon metabolism,the photosynthetic area and photosynthetic efficiency could be increased,so as to increase dry matter accumulation,increase pod number,pod weight and 100 fruit weight per plant,and finally increase peanut yield.This study provides theoretical basis for nitrogen reduction and increase in practical production.