Abstract: 【Objective】This study analyzed the characteristics of chlorophyll fluorescence imaging of the 2^nd (mature) and 4^th (newly born) leaves of tomato seedlings in response to chilling stress, aiming to probe the mechanism by which leaves with different ages adapted to chilling stress in tomato seedlings.【Method】In this study, Solanum lycopersicum L. cv Zhongshu No.4 was used as the research material. The tomato seedlings in the 4-leaf stage were treated at chilling (8℃, 200 μmol·m^-2·s^-1) for 15 d and then recovered at normal temperature (26℃ day/20℃ night, 500 μmol·m^-2·s^-1) for 1 d. Chlorophyll fluorescence imaging of the whole seedlings was measured at different stages of treatment, and the characteristics of chlorophyll fluorescence imaging of the 2^nd and 4^th leaves were compared. 【Result】The results showed that the relative area of photosynthetically active regions (R_AP) on the 4^th leaves decreased slowly during the first 5 d of the chilling stress, while the R_AP on the 2^nd leaves and the whole plants decreased steadily during the whole chilling treatment. The R_AP recovered completely after 1 d of recovery. Under chilling treatment, the relative area of fluorescence active regions (R_AF) for quantum yield of regulatory energy dissipation (Y(NPQ)), quantum yield of nonregulatory energy dissipation (Y(NO)), and nonphotochemical quenching (NPQ) showed similar changes as that of R_AP. However, the R_AF of effective PSII quantum yield (Y(II)) and coefficient of photochemical quenching (qP) were significantly lower than R_AP. Maximum PSII quantum yield (Fv/Fm), Y(NPQ), and NPQ decreased while Y(NO) increased sharply in the photosynthetically active region in tomato seedlings during the first 5 d of the chilling treatment. In the following days, the Fv/Fm remained unchanged, whereas Y(NPQ) and NPQ increased and Y(NO) decreased. However, Y(II) declined sharply 1 d after chilling stress and then remained unchanged. Interestingly, qP in the photosynthetic active region of the 4^th leaves decreased only slightly after 1 d of chilling treatment, and then maintained higher than that before treatment. However, qP of the 2^nd leaves increased significantly on the 5^th day and then decreased rapidly. Overall, the Fv/Fm, Y(II), Y(NPQ) and qP in the 4^th leaves were higher than those in the 2^nd leaves, whereas the Y(NO) were relatively lower in the 4^th leaves.【Conclusion】The study found that tomato seedlings responded to chilling stress by decreasing the area of photosynthetically active regions. The regulatory nonphotochemical quenching, which plays a central role in photoprotection, was inhibited in the early stage, but gradually increased during the later stage of chilling treatment. Mature leaves adapted to chilling stress likely by decreasing the size of photosynthetically active regions, while higher capacities of PSII photochemistry and thermal dissipation were maintained in the newly born leaves in response to chilling stress. Protection of the shoot apex and the newly born leaves may be the priority of tomato seedlings in response to chilling stress. For newly born leaves, chilling induced the closure of some active PSII reaction centers, but improved the operating efficiency of the remaining active PSII reaction centers, which is potentially beneficial for the recovery of photosynthetic activity during the recovery period.

Key words: tomato (Solanum lycopersicum L.), chilling stress, chlorophyll fluorescence imaging, photoinhibition, leaf age