Relationship between chloroplast structure and function and ultraweak luminescence emission of Cerasus humilis leaves...

Ultraweak luminescence (UWL) is a natural phenomenon characterized by very low intensity, occurring in living organisms. While it has intrigued scientists, the underlying mechanism of UWL remains unclear. To explore the luminescence mechanism, we investigated the unique Chinese fruit tree Cerasus humilis, subjecting it to weak light treatment and examining changes in its chloroplast structure and function (including chlorophyll content, photosystem activity, and photosynthesis), as well as variations in UWL intensity. The results show that: (1) Weak light reduces UWL intensity in Cerasus humilis leaves, with lower light radiation causing a greater reduction. (2) Weak light damages the chloroplast structure, decreasing its function, evidenced by increased Chl a, Chl b, and Chl a+b content, while carotenoid and the Chl a/b ratio decrease. Photosystem II activity is inhibited, and the photosystem I redox balance is disrupted. Key photosynthetic parameters (Fv/Fm, Fv/Fo, PI_ABS, RC/CSm) and gas exchange rates (Pn, Tr, Gs) decline, while Ci increases. (3) The UWL intensity in Cerasus humilis leaves was negatively correlated with Chl a and Chl b contents and positively correlated with carotenoid, Chl a/b ratio, photosystem activity, and photosynthetic performance. (4) The lower the light intensity and the longer the exposure, the greater the changes in UWL and chloroplast function, with correlations being stronger under weak light. In conclusion, weak light stress damages the chloroplasts of Cerasus humilis leaves, reducing UWL intensity. UWL intensity is closely related to chloroplasts and their functions, and chloroplasts may be one of the organelles responsible for UWL production. Therefore, UWL intensity can serve as an indicator of weak light stress damage in Cerasus humilis leaves.