Objective  Aiming at the problem of high temperature stress encountered in the growth of tomato at seedling stage in actual production scenarios, a method for detecting high temperature stress of tomato at seedling stage based on thermal infrared and RGB images was proposed.

Method  Firstly, the tomato canopy temperature parameters were obtained by inversion through the thermal infrared image of tomato plant at seedling stage, and the canopy temperature characteristic indicators were extracted by the partial least squares (PLS) model. Then, a Mask-RCNN model using three different backbone feature extraction networks was established, and the RGB images of tomato seedlings were input into the Mask-RCNN model by means of transfer learning for instance segmentation of high temperature stress symptoms. The characteristic indicators of tomato stress symptoms at seedling stage were obtained. Finally, the extracted temperature and stress symptom characteristic indicators were used to construct a hierarchical data set and fed into the high temperature stress classification model to obtain the high temperature stress level.

Result  The cumulative contribution rate of the canopy temperature characteristic indicators extracted based on the PLS model reached 95.45%. The high temperature stress symptom segmentation network based on ResNet101+Mask-RCNN had the highest segmentation accuracy for mild and severe stress of tomato at seedling stage, with mean average precision (mAP) of 77.3% and 73.8% respectively. Among the four high temperature stress grading models constructed based on temperature and stress symptom characteristic indicators, the back propagation neural network (BPNN) showed the best high temperature stress grading performance with the grading accuracy rate of 95.6%.

Conclusion  The proposed method in this study achieves better detection performance for high temperature stress of tomato at seedling stage, and provides a technical support for early detection and rapid automatic warning of high temperature stress of tomato at seedling stage.