Determining optimal nitrogen concentration intervals throughout lettuce growth using fluorescence parameters

Abstract

The commonly used universal nutrient solution formula for facility-grown lettuce leads to excessive nitrogen fertilizer usage, low utilization efficiency, and severe environmental pollution. This formula keeps the nitrogen fertilizer concentration consistently high throughout the growth stages of lettuce, which is not conducive to lettuce growth because its nitrogen needs vary across different developmental stages. To address these inefficiencies, this study introduces a method for determining an appropriate interval limits for nitrogen concentration regulation for greenhouse lettuce cultivation based on chlorophyll fluorescence parameters. A single-factor experiment was designed to gather a dataset of chlorophyll fluorescence and biomass parameters at varying nitrogen concentrations and growth stages. Initial findings using the maximal information coefficient correlation analysis indicated that no single fluorescence parameter alone was sufficient for optimal regulation. Thus, the analytic hierarchy process was employed to dynamically determine the weights for comprehensive fluorescence parameters. The U-chord curvature method was then used to calculate the response curve’s upper and lower interval limits. The Technique for Order Preference by Similarity to an Ideal Solution method confirmed the rationality of the nitrogen concentration intervals for different stages, which achieved the highest comprehensive scores. Implementing these intervals led to a 49.7 % reduction in nitrogen fertilizer usage, with no significant difference in dry weight at the lower limit but a 36.2 % reduction with a 9.4 % increase in dry weight at the upper limit compared with the universal nutrient solution formula. This approach significantly reduces the use of ineffective nitrogen fertilizers while maintaining crop yield, offering a more environmentally friendly and efficient method for managing nitrogen for lettuce cultivation in greenhouses.