Effects of Nitrogen Rate and Planting Density Interaction on Rapeseed Flowering Process Based on Visible-Light Image Features

Ming Ri; Zhong Li; Zeng Zhongwen; Li Yunjuan; Mao Lingli; Mao Fengling; Cheng Fangli; Zhang Zongji; Qin Jing; Yang Guangzhao

doi:10.15933/j.cnki.1004-3268.2026.08.005

Abstract：Using visible-light images,this study investigated the effects of nitrogen rate and planting density interaction on rapeseed flowering,and optimized their combination to enhance the economic and ecological value of the flowering period.A two-factor randomized block field experiment was conducted with five nitrogen gradients(0,90,180,270,360 kg/ha)and four planting density gradients(300 000,525 000,750 000,975 000 plants/ha).UAV digital images were used to establish the flower pixel area index(FPAI)dynamic equation for flowering process,and nitrogen-density combinations were optimized with cumulative FPAI during full bloom(MS2)and input cost as core indicators.Results showed that FPAI was significantly positively correlated with single-plant flower quantity(R2=0.941 2-0.984 0).Compared with no-nitrogen control(N0),nitrogen treatments(N 90-360 kg/ha)advanced the start of initial bloom(Bx),full bloom(Fx)and peak bloom(Tx)by 2.5,1.9 and 1.0 d,and extended initial bloom(S1),full bloom(S2)and theoretical flowering duration(S1+2)by 0.6,1.5 and 2.1 d,respectively.Planting density had no significant effect on key flowering time points[Fx,Tx,Ex(the start of final bloom)];increased density shortened S1 and S1+2 by 1.5 d and 1.0 d on average,but extended S2 by 0.5 d.At fixed density,key flower quantity indicators all increased first and then decreased with rising nitrogen rate.The average values of theoretical maximum flowering accumulation(A),MS2 and mean full-bloom FPAI(MAve)under N 90-360 kg/ha treatments increased by 50.1%,64.7%and 48.1%compared with no nitrogen application treatment.When nitrogen application rate reached 360 kg/ha,increasing planting density would instead reduce flower yield.Cost optimization showed that combinations in optimized zoneⅠ achieved the lowest input cost for target flower quantity.The optimal combinations were 580 000-780 000 plants/ha with N 157.7-195.9 kg/ha for high flower quantity,MS2=197.2-203.1,cost 1 201.8-1 515.8 yuan/ha);479 000-580 000 plants/ha with N 92.0-157.7 kg/ha for medium flower quantity,MS2=176.5-197.2,cost 801.2-1 201.8 yuan/ha;438 000-479 000 plants/ha with N 55.4-92.0 kg/ha for low flower quantity,MS2=155.7-176.5,cost 589.1-801.2 yuan/ha.Sub-optimal combinations outside zone Ⅰ could reduce cost by 2.1%—16.0%via density-increasing&nitrogen-reducing or nitrogen-increasing&density-reducing strategies.In conclusion,FPAI is an effective indicator for monitoring rapeseed flowering dynamics.Rational nitrogen-density synergy can increase flower quantity and regulate flowering process,with nitrogen rate playing a dominant role.The optimization zoning provides targeted nitrogen-density configuration schemes,and offers decision support for cost-saving and efficiency-enhancing production of flower-use rapeseed.