Cold priming improves chilling resistance in wheat seedlings: Changing of photosystem II imprints during recovery from priming

Chilling stress oriented from frequent extreme events has seriously depressed growth and productivity of wheat. Cold priming can effectively improve cold tolerance of crops. However, it is still unclear about the changes of stress memory imprints during recovery from priming and the potential function of the imprints on improved cold tolerance. Here we subjected wheat plants to cold priming (10 degrees C/6 degrees C, 1 d) at the trifoliate stage, and then to chilling stresses (2 degrees C/0 degrees C, 1 d) at 3, 6, 9, 12, 15 and 18 days of recovery from priming, respectively. Compared with non-primed plants (NL), primed plants (PL) showed the most obvious resistance phenotype at 9 days after priming. Its terminal spreading leaves exhibit temporally specific physiological memory and persistent a high photochemical quenching capacity of photosystem II after priming. In addition, it appeared faster photosynthetic performance in the post-stress rewarming phase during 3-12 days after priming. Further analysis of different leaf positions revealed that priming increased light energy absorption by the augmented antennal pigments content and synthesis of light-harvesting proteins, and a higher percentage of primary and secondary quinone electron acceptors (QAQB) active reaction centers (RCs) which enhanced energy capture and transfer performance in primed control plants (PC). Meanwhile, the increased antenna pigments in PC indirectly affected PL. Compared with NL, priming increased electron flux into the inter-system by alleviating the degradation of antenna pigments and increased Photosystem II Subunit S (PsbS) gene expression to promote the occurrence of non-photochemical quenching (NPQ), lowered the excitation pressure per RC, and increased the percentage of QA-RCs, it finally improved the light energy absorption and utilization capacity in PL. In summary, priming -induced memory is mainly present in primed leaves (3rd and 4th leaves during R6-R15) and diminishes with spatial and temporal changes, it exists even no obvious difference in phenotype. This study contributes to a deeper understanding of the long-term effects of fluctuating temperatures on the plant.