The relationship between chlorophyll metabolism and ultraweak luminescence of leaves under salt stress in Cerasus humilis

【Objective】Ultraweak luminescence (UWL) is a natural luminescence phenomenon in all living organisms. However, the understanding of the mechanism of luminescence is still limited. In order to analyze the excitation mechanism of UWL in plants, this study investigated the changes in UWL intensity, chlorophyll metabolism, and chlorophyll content in leaves of Cerasus humilis under salt stress, and carried out correlation analysis. The purpose was to reveal the relationship between plant physiological status and UWL, with focus on chlorophyll metabolism so as to provide understandings related to physiology of UWL emission in plants.【Methods】The potted seedlings of biennial C. humilis were taken as the materials in this study. The seedlings were subjected to mild and severe stress treatments with 400 mmol·L-1 and 800 mmol·L-1 NaCl, respectively. Each potted seedling was irrigated with 400 mL of salt solution at different concentrations, and irrigation with the same amount of water was taken as the control. The UWL intensity, main precursor substances of chlorophyll (ALA, PBG, UroⅢ, CopⅢ, ProtoⅨ, Mg-protoⅨ, and Pchl), main chlorophyll synthetases (ALAD and MgCH), enzymes related to chlorophyll degradation, and chlorophyll contents (Chla, Chlb, and Chla+b) in the leaves of C. humilis were measured every 2 days. The correlation between the indexes of chlorophyll metabolism and UWL was analyzed. About 10-20 mature leaves were selected from the base of the branches for measurement of the UWL intensity using an UWL test system (BPCL-2- SH, Beijing). Take 5 leavesfrom each treatment, and take samples from the top, middle, and bottom three parts of each leaf to measure UWL. Then, take a sufficient amount of leaves that were washed with distilled water and dried. After removal of the main leaf veins, they were quickly frozen with liquid nitrogen and stored at -80 ℃ to measure chlorophyll metabolism and contents. The tests were repeated three times, each with three biological repeats.【Results】(1) With the extension of stress time, the UWL intensity of C. humilis leaves under different levels of salt stress showed a decreasing trend, which was 40.80% and 83.26% lower in mild and severe stresses than that before the stress, respectively. During the whole stress period, UWL in severe stress treatment decreased more rapidly compared with that in the mild stress treatment, and at the end of the experiment, it decreased by 35.93% and 81.88%, compared with the control respectively. (2) With the extension of stress time, the contents of seven chlorophyll synthesis precursors (ALA, PBG, Uro Ⅲ, Cop Ⅲ, Proto Ⅸ, Mg-proto Ⅸ, and Pchl) in C. humilis leaves under different salt stress treatments showed a decreasing trend. Among them, Cop Ⅲ decreased fastest under mild stress with a reduction of 57.97% compared with that before stress, and Pchl decreased fastest under severe stress with a reduction of 67.17% compared with that before stress. The contents of the main synthetase (ALAD and MgCH) and the degrading enzymes chlorophyllase (Chlase) showed an increasing trend. Under mild and severe stress treatments, ALAD increased the fastest and was 1.59 times and 1.27 times higher than before stress, respectively. Chlorophyll (Chla, Chlb, and Chla+b) contents showed a decreasing trend, among which Chlb decreased fastest under mild stress with a reduction of 39.47% compared with that before stress, and Chla decreased fastest under severe stress with a reduction of 76.66%. At the same time, the severe stress led to greater changes in chlorophyll metabolism indicators than mild stress. (3) Correlation analysis showed that under the two salt stress treatments, the UWL intensity of leaves was significantly positively correlated with the contents of chlorophylls and their precursors, and was significantly negatively correlated with the content of chlorophyll metabolism enzymes. Under mild stress, the intensity of UWL was significantly positively correlated with the chlorophyll precursors, ALA, PBG, and Mg-proto Ⅸ, as well as Chla and Chla+b, but significantly negatively correlated with metabolism-related enzymes, ALAD and Chlase. Under severe stress, UWL intensity was significantly positively correlated with chlorophyll precursor PBG, Uro Ⅲ, and Cop Ⅲ, as well as Chla, Chlb and Chla+b, but negatively correlated with metabolic enzyme Chlase.【Conclusion】Salt stress blocked the chlorophyll synthesis process of leaves in C. humilis, enhanced the degradation enzymes, and decreased the chlorophyll content. The UWL intensity decreased with the decrease in chlorophyll synthesis precursors, chlorophyll enzyme activity, and chlorophyll content. The above changes in chlorophyll metabolism may lead to changes in leaf UWL intensity. Therefore, the UWL of leaves in C. humilis is closely related to chlorophyll metabolism. Salt stress led to the decrease in UWL emission, and the decrease becomes faster under more severe salt stresses.