Research On The Mechanism And Effect Of Different Nitrogen Application Rates On The Quality Of Rice

As a main staple food for human beings,rice is the crop second only to wheat in the world for the yield and planting area.Because of its seasonal growth,it is usually stored after harvest to meet the needs of the whole year.With the increase of trade between countries all over the world and the improvement of people’s living quality,the requirements of agriculture for rice have gradually shifted to the improvement of rice quality.However,the research on improving rice quality seems to be disjointed before and after harvest due to different disciplines.Agronomists mainly focus on the impact of pre-harvest factors on rice quality,such as varieties,cultivation techniques,field management,etc.Experts in the field of food mainly focused on the change of rice quality and its control during postharvest storage,such as the research on the aging mechanism of rice,the effect of delaying quality deterioration by temperature and humidity control,controlled atmosphere storage,vacuum packaging and so on.However,the post-harvest storage quality of rice is fundamentally closely related to the quality of raw materials,that is,it is closely related to the pre-harvest factors affecting the formation of rice quality,such as variety selection and fertilization.Therefore,the research in combination of the factors of pre-harvest and post-harvest on rice end-used quality is expected to provide a certain theoretical basis and practical guidance for field production of rice from the perspective of raw material quality,so as to fill the gap in this field.The main research results are as follows:(1)Firstly,the quality of 16 rice varieties which mainly planted in Panjin,Liaoning Province was analyzed,we found that according to the national standard,all 16 varieties failed to reach the tertiary level in rice milling quality,indicating that the rice milling quality is generally poor;However,1,4 and 10 varieties reached the national first,second and tertiary level in rice appearance quality respectively,indicating that the appearance quality of rice is better;meanwhile,the eating quality are prior good but with great differences among varieties,which may be related to the great difference of protein content among varieties.Then the potential high-yield and high-quality variety"Yanfeng 47"was selected to carry out the experiment on the effect of relevant fertilizer application rates on rice quality.The results showed that the nitrogen application rates had a great impact on rice quality,which significantly affected rice milling quality,appearance quality,eating quality and protein content;The differences of phosphorus application rates on rice quality appeared more between no-phosphorus group and phosphorus-application group.The quality of rice applied phosphorus fertilizer is better,and there is little difference in rice quality between different phosphorus application rates.Considering the cost,the recommended application rate of phosphorus fertilizer is 90 kg/hm~2.In a word,the key fertilization method affecting rice quality was different nitrogen application rate,which provide a reference for the selection of follow-up test conditions.(2)According to the results of the first part above,nitrogen application significantly improved the milling quality and appearance quality of Yanfeng 47 rice,but the eating quality of rice may be significantly deteriorated due to the increased protein content.In order to solve the deterioration of eating quality caused by nitrogen application,this paper further discusses the correlation between the changes of protein components content and structure in rice and eating quality under different nitrogen application rates.It is found that nitrogen application changed the content and structure of protein component,resulting in the change of eating quality.Applying nitrogen fertilizer remarkably increased glutelin and globulin contents(P<0.05),especially the methionine content of globulin,leading to increased hardness of cooked rice.Moreover,excessive nitrogen significantly reduced the ratio of the prolaminα‐helix structure(P<0.05),which had a significant correlation with hardness and pasting temperature(P<0.05),at correlation coefficients of-0.859 and 0.828,respectively.Besides protein content,the changes in amino acid content and protein secondary structure were also closely related to the eating quality of rice.(3)The quality of rice treated with 0,160,210,260,315 and 420 kg/hm~2 was analyzed during 12 months of storage.Results showed that Nitrogen application rate not only has a significant effect on the initial quality of rice at harvest,but also affects the change trend of rice quality during storage.Field nitrogen application is expected to be a measure to improve the milling quality and appearance quality of rice during postharvest storage.The milling quality and appearance quality of rice treated with medium nitrogen(260 kg/hm~2)were better than those of other treatment groups in the whole storage process.The effect of nitrogen application on rice eating quality is complex,low nitrogen application(0-160 kg/hm~2)has advantage to obtain high eating quality;although excessive nitrogen application(315-420 kg/hm~2)did significantly increase the hardness of rice paste(P<0.05),reduce the viscosity(P<0.05),but it delayed the decline of rice paste viscosity for 2 months,which was conducive to the stability of rice eating quality during storage.In conclusion,low nitrogen application rate is conducive to obtain high eating quality rice,medium nitrogen application rate is conducive to improve the milling quality and appearance quality of rice during storage,and excessive nitrogen application is conducive to the stability of eating and cooking quality during storage.(4)Comparative proteomics technology was used to explore the mechanism of this effect.It was found that high nitrogen application increased the proteins involved in the following pathways:cell wall synthesis(UDP-glucose 6-dehydrogenase),protein degradation(peptidase,endopeptidase,and Lon protease homolog),flavonoid synthesis(chalcone--flavonone isomerase),energy metabolism,and response to stress(thioredoxin proteins),while reduced the proteins involved in starch degradation(α-amylase,1,4-α-glucan starch branching enzyme,and isoamylase)and histone modification(histone-glutamine methyltransferase),resulted a higher ability to resist storage stress in rice.The discovery of key differentially expressed proteins in the related pathways provides a list of potential targets for further control of the deterioration of rice quality during storage.