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Highlights

An abnormal accumulation of sucrose and glucose was found in HLB-infected leaves, and it presented a similar pattern in different orchards from cool to hot seasons.

A decreasing value of the actual quantum yield of PSII (ΦPSII) in HLB-infected leaves was mainly related to an increase of non-regulated energy quenching (ΦNO) due to the irreversible damage of PSII.

Chlorophyll fluorescence imaging combined with a random forest was able to identify HLB at the asymptomatic stage.

Abstract Citrus Huanglongbing (HLB) poses a serious threat to citrus production. This research aimed to explore chlorophyll fluorescence imaging for characterizing the photosynthetic response to HLB-infected citrus leaves in different orchards and seasons. Chlorophyll fluorescence images of citrus leaves were acquired with an in-house chlorophyll fluorescence imaging system. It was found that sucrose and glucose accumulated earlier than starch in HLB-infected leaves, and a similar carbohydrate metabolic pattern was observed in HLB-infected leaves grown in different orchards from cool to hot seasons. The pathogen damaged the thylakoid structure of chloroplasts with a higher value of Fo. It decreased photosynthetic activity of the host by reducing the number of active photosynthetic centers and the maximum quantum yield of PSII (Fv/Fm) with lower values of Fv/Fo and Fv/Fm. Additionally, the pathogen modified the allocation of excitation energy in citrus leaves by reducing the actual quantum yield of PSII (ΦPSII) due to an increase of non-regulated energy quenching (ΦNO), which indicated irreversible PSII damage before symptom development. Moreover, photosynthetic signatures combined with the random forest method were able to identify HLB in the asymptomatic stage with an overall accuracy of 91.8%. These results demonstrated the potential of chlorophyll fluorescence imaging for evaluating the photosynthetic response to HLB as well as disease diagnosis.