冬小麦叶片SPAD遥感探测的光谱尺度效应

    Spectral scale effects on the optical estimation of winter wheat leaf SPAD value

    • 摘要: 叶片SPAD(soil and plant analyzer development)值表征了叶片叶绿素相对含量,是监测农作物长势和营养状况的重要参数。光学遥感是大面积无损探测叶片SPAD值的重要手段。然而,由于不同光谱尺度数据探测光谱变化存在差异,影响了光学探测作物生化参数的精度,但目前很少有研究系统评估不同光谱尺度对探测冬小麦叶片SPAD值的影响。为优化光谱尺度提升叶片SPAD探测精度,该研究通过连续4年田间试验,获取冬小麦4个关键生育期(拔节期、抽穗期、开花期和灌浆期)和3种施氮水平(N1、N2和N3)条件下的冠层光谱反射率和叶片SPAD值,评估了5种光谱尺度(1、5、10、25 和50 nm)下单一波段反射率和植被指数对叶片SPAD值敏感性差异及对机器学习模型估算SPAD值的影响。结果表明,红光波段反射率对SPAD值敏感性最大,光谱尺度敏感性变异系数Var为0.497。红边波段波长710 nm反射率受到光谱尺度影响最大,在全生育期敏感性变异系数Var为1.000。全生育期敏感性最佳植被指数为mND705,在50 nm光谱尺度对SPAD的敏感性最高(R2 =0.685)且光谱尺度敏感性变异系数低(Var = 0.014)。在4个单一生育期中,mND705在灌浆期对SPAD的敏感性最佳(R2=0.895)且受到光谱尺度的影响小(Var = 0.014)。施氮水平的增加提升了植被指数对SPAD的敏感性。优化光谱尺度提升了机器学习模型估算SPAD的能力,全生育期中以25 nm光谱尺度构建的偏最小二乘回归模型对SPAD的估算精度最佳(R2 = 0.816 和均方根误差RMSE = 4.04)。该研究为从优化光谱尺度角度优化光学传感器选择和设计、光谱植被指数波段选择和机器学习模型光谱特征构建提供了理论基础。

       

      Abstract: Chlorophyll is one of the most important photosynthesis pigments in crops. The photosynthetic capacity of the plant can also indicate the plant's growth and nutritional status. Leaf chlorophyll content can be accurately acquired to monitor crop growth and yield. The portable optical instrument SPAD-502 can be used to rapidly acquire the SPAD value in a non-destructive way, indicating the leaf's relative chlorophyll content. However, the tremendous amount of hand labor cannot fully meet the needs to estimate the leaf SPAD value in a large area and then monitor the dynamic of crop growth and agricultural management. Alternatively, optical remote sensing can be expected to non-destructively measure the leaf SPAD value at a large scale. This study aims to evaluate the effects of spectral resolution on the optical SPAD estimation of winter wheat leaf using remote sensing. A four-year field experiment was carried out under four growth stages (jointing, heading, anthesis, and filling) and three levels of nitrogen application. A systematic investigation was implemented to determine the canopy spectral reflectance and leaf SPAD values of winter wheat. The leaf SPAD estimation model was constructed using 25 commonly used chlorophyll content-sensitive spectral indices combined with machine learning. An evaluation was also made on the effects of five spectral resolutions on the reflectance of a single band, spectral indices, and the estimation of SPAD value using machine learning. The results show that the sensitivity of single-band reflectance to SPAD was dominated by the spectral resolution, growth stages, and nitrogen application levels. In the whole growth period, the reflectance of the red band was more sensitive to the SPAD than the rest bands, where the determination coefficient R2 was between 0.411 and 0.579 at the five spectral resolutions. The reason was that there was a strong absorption of chlorophyll in the red band. Specifically, the R2 was between 0.242 and 0.700, and the Var was between 0.313 and 0.952 at the most sensitive spectral resolution for the red band in each growth stage. The red edge band reflectance at 710 nm shared the largest variation coefficient of spectral resolution sensitivity, and Var was 1.000 in the whole growth period. The main reason was that the reflectance of green vegetation changed sharply in the red edge band, indicating outstanding differences in the reflectance at different spectral resolutions. The spectral resolution also dominated the sensitivity of spectral indices to SPAD at different growth stages and nitrogen application levels. Except for the filling and anthesis stage, the spectral resolution range of the optimal sensitivity spectral index was between 25 and 50 nm. The potential reason was that the broad spectral resolution caused the spectral index to contain more spectral information, and then reduce the influence of noises, particularly for the optimal sensitivity spectral index. However, there were limited effects of spectral resolution on the sensitivity of the optimal spectral index to SPAD, compared with the single band reflectance. In the whole growth period, the spectral index mND705 presented the strongest sensitivity to SPAD at 50 nm (R2 = 0.685) and a low variation coefficient of spectral resolution sensitivity (Var = 0.014). In each growth stage, the R2 of the most sensitive spectral index to SPAD was between 0.387 and 0.895, and the coefficient of sensitivity variance Var was between 0.01 and 0.05. The nitrogen application level enhanced the leaf chlorophyll content to improve the sensitivity of the spectral index to detect SPAD. Spectral resolution of spectral indices was optimized as the feature inputs for machine learning models, in order to improve the estimation accuracy of SPAD. In the whole growth period, the best accuracy of estimation was achieved in the model with 25 nm spectral resolution spectral index and PLSR, indicating the R2 of 0.816 and RMSE of 4.04. In each growth period, the model with the optimized spectral resolution spectral index as the input also shared the best estimation accuracy with R2 between 0.531 and 0.916 and the RMSE between 2.76 and 4.47. At the three levels of nitrogen application, the R2 was between 0.780 and 0.884 and the RMSE was between 2.67 and 4.55, according to the model with the optimized spectral resolution. The nitrogen application level improved the chlorophyll content of leaves. Thus the detection of SPAD was enhanced in the machine learning model using spectral characteristics. The spectral index and the spectral resolution were optimized to improve the estimation accuracy of winter wheat leaf SPAD using optical remote sensing. The finding can provide theoretical support to design the optical remote sensing sensor.

       

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