Effects of Sunburn on Chlorophyll Fluorescence Induction Kinetics Characteristics in Tobacco Leaves

In order to explore the effects of sunburn on photosynthetic structure and function of tobacco leaves, and analyze its occurrence and response mechanisms, this study used flue-cured tobacco Cuibi No.1 as the experimental material. Sunburn was simulated by sudden strong light after shading and water spraying, mimicking abrupt sunny weather following prolonged overcast and rainy conditions. Rapid chlorophyll fluorescence induction kinetics analysis technology was used to explore the change of chlorophyll fluorescence characteristics in tobacco leaves during the important period from the occurrence of sunburn to short-term recovery (T0, normal growth; T1, high humidity and few light; T2, sudden strong light; T3, 12 h after sunburn; T4, 24 h after sunburn). The results showed that sunburn caused significant deformation of the OJIP curve during the T1-T4 period, with the curve tending to linearity by T4, accompanied by a notable decrease in the peak of fluorescence intensity and obvious variations in the O-P phase segments. During T2 period, F_m and QY_max decreased significantly, while F_o, V_J and M_o increased significantly. The increase of light energy utilization followed the order: DI_o/RC>ABS/RC>TR_o/RC>ET_o/RC. Additionally, Φ_P, Φ_E and ψ_o decreased significantly, whereas ψ_D increased significantly. A_c increased significantly, with the increase in D_d exceeding that of T_r. The environmental stress sensitivity parameter PI_ABS was significantly reduced by 70.40%. During the T3-T4 period, the related fluorescence parameters showed a trend of recovery or mitigation toward normal growth, but PI_ABS continued to decrease. This suggests that sunburn has caused serious irreversible damage to the photosynthetic structure of tobacco leaves. In the short-term recovery response process, chlorophyll fluorescence parameters showed obvious environmental adaptability changes: by reversibly inactivating reaction center, the absorption and quantum transfer of light energy are reduced to avoid the accumulation of excess excitation energy. Concurrently, the distribution ratio of light energy utilization was adjusted, and the heat dissipation was increased on the basis of reducing light energy capture, thereby enhancing resistance to sunburn damage.