Research On Diagnostic Method Of Nitrogen Nutrition Profit And Loss Index Of Hydroponic Lettuce

Excessive nitrate content in vegetables can cause hazards,and the key to efficient nitrogen management is the timely,rapid and accurate determination of nitrate content in vegetables.In order to explore the accurate monitoring method of crop nitrogen nutrient status,this paper takes hydroponic lettuce as the research object,analyzes the dynamic influence of nitrogen on the growth and quality of hydroponic lettuce through the indoor culture experiments of different nutrient nitrogen level treatment,constructs the nitrogen absorption model of hydroponic lettuce root;studies the nitrogen distribution mechanism of the crown of hydroponic lettuce,and explores the rapid diagnosis method of hydroponic lettuce nitrogen.Based on nitrogen nutrition diagnosis,the critical nitrogen concentration,nitrogen uptake and nitrogen nutrition index models of hydroponic lettuce were constructed,which provided a theoretical and technical basis for the accurate nitrogen application decision of hydroponic lettuce.The conclusions of the study are as follows:(1)The nitrogen concentration of nutrient solution will affect the nitrogen uptake rate of lettuce roots,the root-to-head ratio of lettuces and the nitrogen distribution of root and crowns.There was a negative exponential relationship between nitrogen uptake factor δ and nutrient nitrogen concentration:δ=4.1088CN-0.607,the active and passive nitrogen uptake of hydroponic lettuce could be judged by the δ,the nitrogen nutritional status of lettuce was further evaluated,and the nitrogen uptake model of hydroponic lettuce root system was constructed.The root-to-head ratio of lettuce decreases first and then increases with the increase of nutrient liquid nitrogen concentration,and the minimum root-to-head ratio appears under 3.50 mmol/L and 7.00 mmol/L treatment,and the plant biomass distribution can be regulated by adjusting the nutrient solution nitrogen concentration in production,so as to obtain a larger economic yield.(2)When the nitrogen concentration of plants is measured by spectrophotometer,its accuracy is high,but the operation is cumbersome,and it is not suitable for plant nitrogen diagnosis in the field.When using ion selective electrode for measurement,the overall measurement result is about 1.8 times of the spectrophotometer,and the measurement accuracy decreases with the increase of nitrogen concentration.When the reflectometer-test strip method is used,the measurement result is slightly lower overall,but this method is convenient and simple to operate,so it can be used to predict the nitrogen nutrition status of field plants.The model fit of the relationship between leaf SPAD value and nitrate nitrogen content was well fitted at 18d and 25d,which could predict and evaluate the nitrogen vegetative status of lettuce.The fitting degree of the model of the relationship between NDVI value and nitrate nitrogen content in crops is between 0.21～0.34,and the model fit is low,which cannot be used as a rapid diagnosis method for nitrogen nutrition in the field.The quantitative relationship between nitrate nitrogen content and total nitrogen content of lettuce plants under hydroponic conditions was constructed:y=-2.46x2+9.49x+5.15,R2=0.7581,and the total nitrogen content could be calculated by crop nitrate nitrogen content.(3)A critical nitrogen concentration dilution curve model describing the relationship between aerial biomass and nitrogen concentration in hydroponic lettuce was constructed,and the relationship between this two conforms to the power function relationship,and the fitting results were good.The nitrogen uptake model and the nitrogen nutrition index model were basically consistent with the nitrogen nutrition diagnosis results of hydroponic lettuce,and the two inferred that the suitable nutrient liquid nitrogen concentration of hydroponic lettuce should be controlled between 1.75mmol/L～3.50mmol/L.The critical nitrogen concentration dilution curve model,nitrogen uptake model and nitrogen nutrition index model can be used to diagnose and predict the nitrogen nutrition status of plants,which was helpful for the development of hydroponic lettuce nutrient liquid nitrogen decision-making system.