基于叶片光谱特性的玉米品种抗倒伏性预测

    Predicting lodging resistance of maize varieties using leaf hyperspectral imaging

    • 摘要: 针对玉米叶片各区域光谱特性与玉米品种抗倒伏性能之间关系未知的问题,该研究探讨了叶脉区、正常反射区和整片叶的平均光谱对玉米品种抗倒伏性预测效果的影响。试验采集了2018年和2019年8个玉米品种的叶片高光谱图像,使用阈值分割和K-means聚类方法提取各叶片区域的平均光谱数据。用最大相关最小冗余(Max-Relevance and Min-Redundancy,MRMR)特征选择算法,提取各叶片区域平均光谱的抗倒伏和不抗倒伏品种分类特征。使用交叉验证的方式,对MRMR方法选择的特征数量进行优化,并使用支持向量机(Support Vector Machines,SVM)方法建立各叶片区域的抗倒伏性预测光谱模型,用网格搜索法对各模型参数进行优化。两年试验结果显示,各叶片区域约有35~50个可以反映品种抗倒伏性的光谱特征,其中非叶脉区光谱相比叶脉区光谱的抗倒伏特征更多,分类效果更好。参数优化训练后,整叶片、叶脉区和正常反射区的光谱模型对训练集数据的预测正确率达到98.46%、98.52%和100%,正常反射区的光谱模型对测试集数据的分类效果最好,2018年和2019年测试集数据的预测正确率分别达到了91.00%和94.34%。与基于整片叶平均光谱的预测模型相比,基于叶片各区域的光谱特征模型可以排除不平整叶面反射的干扰,有助于提高模型预测结果的稳定性。研究表明,基于正常反射区光谱的预测模型更适用于品种抗倒伏预测,研究结果可为基于玉米叶片光谱预测品种的抗倒伏能力提供借鉴。

       

      Abstract: This study aims to investigate the effect of the average spectra of leaf vein, normal reflectance, and whole leaf on the lodging resistance classification of maize varieties. A field trial was conducted in 2018 and 2019, where the hyperspectral image data was collected at the 9-leaf stage for the top leaves of eight maize varieties. The threshold segmentation was used to identify the leaf area, and then the K-means clustering was used to distinguish the leaf into three areas: the normal reflection, dark reflection, and vein area, and finally the average spectral curves were extracted to determine the spectral data characteristics of the lodging resistant samples and the control. The result showed that the average spectral curves of the normal reflectance region and the whole leaf were the typical plant spectral curves, with the distinct absorption bands for blue and red light, whereas, the reflection peaks for green light. However, the whole leaf spectrum contained the spectrum of the dark reflectance region, indicating the lower overall spectral reflectance. There was a low chlorophyll content in the leaf veins. There was almost no absorption band in the spectral curve, where the reflectance in all bands was higher than that in the normal reflectance region, indicating a broader band distribution of the spectral curve. A Kennard Stone sampling was selected to sort the spectral data of each leaf region in each variety. Two parts were divided in the ratio of 3:1, including the training set and test set. The division of each variety was combined into the final training and test set data to ensure the uniform distribution on each variety. The Max-Relevance and Min-Redundancy (MRMR) feature selection was used to extract the categorical features for each type of leaf area spectra for both lodging resistance and varieties, thereby tranking them in the order of importance. Support Vector Machine (SVM) classification models were built separately for each leaf region using the selected features, where the penalty and kernel parameters of the SVM models were optimized using the grid search to obtain better model predictions. The number of selected features was optimized using the cross-validation method on the training set data. As such, the number of features was selected for the final model considering the prediction accuracy and computational complexity of the model. The experimental results showed that there were significant differences between the spectra of the leaf vein region and the non-vein region of maize leaves. The MRMR was greatly contributed to quickly finding the spectral features that the most associated with the lodging resistance, further improving the prediction of the model. Specifically, the spectral model presented the highest accuracy for the normal reflectance region, with 91.00% and 94.34% predictions for the test sets of the 2018 and 2019 trials, respectively. There were about 35-50 spectral features in each leaf region with the lodging resistance of maize. Among them, there were more features spectra of non-vein areas, compared with the vein areas, indicating more outstanding classification. Therefore, the accurate extraction and the model were greatly contributed to removing the influence of uneven leaf surface on spectral reflection. As such, the higher stability of model prediction was achieved, compared with the average spectrum of the whole leaf. Furthermore, the spectral features of the non-vein region were more suitable for the prediction of the lodging resistance of varieties, compared with the vein region. The spectral model of the normal reflectance region presented a better comprehensive prediction ability for the lodging resistance of varieties. The finding can also provide a strong reference to predict the lodging resistance of varieties using the spectrum of maize leaves.

       

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