Response mechanism of rapeseed to nitrogen application rate during flowering based on digital image features

【Objective】This study aimed to investigate the response of rapeseed to different nitrogen application rates during flowering period based on visible-light digital image features of corolla， thereby providing technical support for accurate management of nitrogen nutrient to improve economic and ecological value of rapeseed flower utilization mode.【Method】Visible-light digital images of rapeseed corolla under different nitrogen gradient treatments N₀（0 kg/ha），N₉₀（90 kg/ha），N₁₈₀（180 kg/ha），N₂₇₀（270 kg/ha）， and N₃₆₀（360 kg/ha） acquired by unmanned aerial vehicle were analyzed through pixel segmentation statistics and CIELab color space interpretation， and parameters including flower area index （FAI）， theoretical flowering period （S_f）， average daily FAI accumulation （M_Ave）， FAI increase rate （V_up）， FAI decrease rate （V_down）， and CIELab color space b^* were determined to investigate response of rapeseed to different nitrogen application rates during flowering period.【Result】As nitrogen application rate increased， rapeseed flowering period was gra-dually delayed and shortened. Specifically， theoretical initial flowering stage（E）， full bloom stage（F）， and final flowe-ring stage （B） were delayed by 42.7%-129.4%， 10.3%-26.7%， and 0.0%-4.7% respectively； S_f decreased from 25.1 d under N₀ treatment to 12.3 d under N₃₆₀ treatment. M_Ave increased first and then decreased with the increase of nitrogen treatments， peaking under the N₁₈₀ treatment， which was 56.3% higher than that under the N₀ treatment. The distribution of CIELab color space b^* of rapeseed corolla image during full bloom stage was skewed under nitrogen gradient treatments. The median and mode of b^* distribution increased at first and then decreased with the increase of nitrogen application rate. The yellow saturation of corolla was the best under the N₉₀ treatment （median of b^* was 0.685 and mode was 0.804）， while the flower color under the N₂₇₀ and N₃₆₀ treatments faded. In terms of comprehensive evaluation of rapeseed response to nitrogen fertilizer during flowering period， the highest weighted comprehensive score （0.60 points） was observed under the N₉₀ and N₁₈₀ treatments. By constructing a fitting equation of nitrogen application rate and S_f， M_Ave， and b^*， the optimal nitrogen application rate that could balance S_f and M_Ave and yield highly saturated yellow color in northern Guangxi was 118.9 kg/ha， and the appropriate range of nitrogen application rate was 90-180 kg/ha.【Conclusion】Visible-light digital images of corolla effectively elucidate the response mechanism of rapeseed to nitrogen application rates during flowe-ring period， further simplifying studies on rapeseed flowering dynamics. Given that under the background of shifting climate， nitrogen deficiency or excess can amplify population instability during flowering period under temperature stress， and appropriate nitrogen fertilizer application （90-180 kg/ha） can balance nutrition， reproduction， and growth， thereby mitigating climate risks.