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基于机器学习算法钩藤叶片叶绿素含量预测模型

Prediction Model of Chlorophyll Content in Uncaria rhynchophylla Leaves Based on Machine Learning Algorithms

  • 摘要: 针对药用植物钩藤叶片生理状态快速监测需求,基于高光谱技术构建叶片叶绿素含量(LCC)预测模型,比较不同样本划分、特征提取与机器学习算法组合对模型性能的影响,并筛选适用于钩藤LCC估测的最优建模方案。以四川栗子坪国家自然保护区270份钩藤叶片样本为研究对象,采用Savitzky-Golay平滑对350~2500 nm高光谱数据进行预处理,分别利用Kennard-Stone(KS)和基于X-Y联合距离的样本划分算法(SPXY)按8:2划分建模集和预测集。采用连续投影法(SPA)和主成分分析(PCA)提取光谱特征,分别构建偏最小二乘回归(PLSR)、BP神经网络(BPNN)、随机森林回归(RFR)、极限学习机(ELM)和支持向量机(SVM)预测模型。通过5折交叉验证结合网格搜索优化模型参数,并以决定系数(R2)、均方根误差(RMSE)、相对均方根误差(rRMSE)和调整决定系数(Adjusted R2)评价模型性能,同时分析特征变量与LCC的相关关系。结果表明:KS和SPXY划分条件下,SPA分别筛选出7个和12个特征波段;PCA在两种划分方式下均保留4个主成分,累积方差贡献率分别达96.82%和96.54%。KS划分下各模型预测集R2为0.639~0.766,整体高于SPXY条件下的0.435~0.575。SVM在KS-SPA、KS-PCA和SPXY-PCA三种组合中均取得最高预测性能,其中KS-PCA-SVM表现最优,预测集R2为0.766,RMSE为0.0092,rRMSE为0.567%。相关分析表明,KS-SPA和SPXY-SPA条件下与LCC相关性最高的波段分别为751 nm(r=0.658,P<0.01)和759 nm(r=0.729,P<0.01),均位于红边—近红外过渡区域;两种PCA条件下PC3均与LCC呈最强正相关。钩藤LCC高光谱预测性能受样本划分方式、特征表达方式及机器学习算法共同影响,其中KS-PCA-SVM在本研究数据条件下表现出最佳预测能力。751~759 nm红边附近光谱区域对钩藤LCC变化具有较稳定响应,可作为钩藤叶片叶绿素监测的重要敏感区域。

     

    Abstract: To address the need for rapid monitoring of the physiological status of the medicinal plant Uncaria rhynchophylla, hyperspectral technology was used to develop prediction models for leaf chlorophyll content (LCC). The effects of different combinations of sample partitioning methods, feature extraction techniques, and machine learning algorithms on model performance were compared to identify the optimal modeling strategy for estimating LCC in U. rhynchophylla. A total of 270 leaf samples were collected from the Liziping National Nature Reserve in Sichuan Province, China. Hyperspectral data covering 350–2500 nm were preprocessed using Savitzky–Golay smoothing. The samples were divided into calibration and prediction sets at a ratio of 8:2 using the Kennard–Stone (KS) algorithm and the sample set partitioning based on joint X–Y distances (SPXY) algorithm. Spectral features were extracted using the successive projections algorithm (SPA) and principal component analysis (PCA). Prediction models were subsequently developed using partial least squares regression (PLSR), a backpropagation neural network (BPNN), random forest regression (RFR), an extreme learning machine (ELM), and a support vector machine (SVM). Model parameters were optimized through grid search combined with five-fold cross-validation. Model performance was evaluated using the coefficient of determination (R2), root mean square error (RMSE), relative root mean square error (rRMSE), and adjusted coefficient of determination (adjusted R2). Correlations between the extracted features and LCC were also analyzed. Under KS and SPXY partitioning, SPA selected 7 and 12 characteristic wavelengths, respectively. PCA retained four principal components under both partitioning methods, with cumulative variance contributions of 96.82% and 96.54%, respectively. The prediction-set R2 values of the models under KS partitioning ranged from 0.639 to 0.766, generally exceeding those obtained under SPXY partitioning, which ranged from 0.435 to 0.575. SVM achieved the highest predictive performance in the KS–SPA, KS–PCA, and SPXY–PCA combinations. Among all models, KS–PCA–SVM performed best, with a prediction-set R2 of 0.766, an RMSE of 0.0092, and an rRMSE of 0.567%. Correlation analysis showed that the wavelengths most strongly correlated with LCC under KS–SPA and SPXY–SPA were 751 nm (r = 0.658, P<0.01) and 759 nm (r = 0.729, P<0.01), respectively, both located in the red-edge–near-infrared transition region. Under both PCA conditions, PC3 exhibited the strongest positive correlation with LCC. The performance of hyperspectral models for predicting LCC in U. rhynchophylla was jointly affected by the sample partitioning method, feature representation approach, and machine learning algorithm. Under the conditions of the present dataset, KS–PCA–SVM demonstrated the best predictive ability. The spectral region near the red edge at 751–759 nm showed a relatively stable response to variations in LCC and may therefore serve as an important sensitive region for monitoring chlorophyll in U. rhynchophylla leaves. These findings provide a methodological basis for rapid and nondestructive monitoring of the physiological status of U. rhynchophylla leaves. Nevertheless, the model requires further validation across broader LCC gradients and under different seasonal and regional conditions.

     

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