Effects of leaf scorch on chlorophyll fluorescence characteristics of walnut leaves

To explore the effect of leaf scorch on chlorophyll fluorescence characteristics of walnut leaves, we used the FMS-2 pulse-modulated chlorophyll fluorometer to measure parameters of chlorophyll fluorescence. Two walnut varieties were selected (Juglans regia 'Wen185' and 'Xinxin2') as test materials. During diurnal variation, the initial fluorescence (F-o) of symptom trees was significantly higher than that of symptomless trees (P < 0.05). While the non-photochemical quenching coefficient (NPQ) of 'Wen185' symptom trees was significantly higher than that of its symptomless counterpart (control) (P < 0.05), the NPQ of 'Xinxin2' symptom trees attained the same level as that of the control in the afternoon, with no significant difference. With the prolonged time of the stress and increased severity of the disease, the F-o of each symptom tree was higher than that of the control (P < 0.05). The maximum fluorescence (F-m), maximum fluorescence under light (F-m'), maximum photochemical efficiency of photosystem II (F-v/F-m), and the potential activity of PSII (F-v/F-o) of symptom trees were all significantly lower than the corresponding values of the control (P < 0.05). The decline rate of 'Wen185' symptom trees was smaller than that of 'Xinxin2' plants. CONCLUSION: Leaf scorch damaged the photosynthetic mechanism in leaves of 'Wen185' and 'Xinxin2', reduced the electron transfer efficiency, destroyed the PSII reaction center, and even lost the potential activity, resulting in different degrees of photoinhibition in leaves of symptomatic walnut varieties. Compared to 'Xinxin2', 'Wen185' showed greater adaptability to leaf scorch and improved self-regulation, whereas 'Xinxin2' was more seriously affected by the disease.