Physiology and transcriptomics reveal that hybridization improves the tolerance of poplar photosynthetic function to salt stress

Key message Significant improvement in salt tolerance of poplar hybrids with different salt 2 tolerant varieties. Abstract Salt stress is a global environmental factor that limits plant growth and productivity. Poplar has the characteristics of fast growth and fine texture, and the adaptability of hybrid plants to environmental stress is better than that of their parents. In this study, Populus simonii ‘CAFDF’ (♀), P. nigra ‘CAFDM’ (♂), P. simonii × P. nigra CV. ‘CAFDS2’ (DS2), and Populus simonii × P. nigra CV. ‘CAFDS3’ (DS3) were used to reveal the response and adaptation mechanism of the photosynthetic function of hybrid poplar leaves to salt stress by physiological combined with transcriptome technology. The results showed that NaCl stress significantly reduced the chlorophyll content of the leaves of both parents, but the reduction of chlorophyll in hybrid offspring under salt stress was lower than that in parents. A Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of differentially expressed genes (DEGs) in both hybrid plants and biparental progenies under NaCl stress was enriched for photosynthesis-antenna proteins, photosynthesis, and carbon fixation in photosynthetic organisms. Under salt stress treatment, the ability of leaves in DS2 and DS3 plants to capture solar energy and transfer their excitement energy to the reaction center were higher than that of their parental plants. NaCl stress caused damage to photosystem II (PSII) and photosystem I (PSI) in poplar leaves, and their activities decreased. The hybrid offspring improved the integrity of PSII receptor side electron transfer and donor side oxygen-evolving complex (OEC) compared with the two parents. The expression of other coding genes of the PSII reaction center, as well as coding genes for PSI reaction center and ATP synthetase in the hybrid offspring under NaCl stress are higher than that of the parents, which reduces damage to the electron transfer chain and the energy supply required for maintaining plant growth and development. The cyclic electron flow dependent on PGR5 and NDH pathways (PGR5-CEF and NDH-CEF) of hybrids and female parents were significantly higher than those of male parents. Hybrid progeny and the maternal parent alleviated the damage of salt stress on PSII and PSI by promoting CEF. The hybrid plants protect the photosynthetic apparatus by regulating energy dissipation, while the PSII of the parents plants was damaged due to serious photooxidation. Under salt stress, the net photosynthetic rate ( P n ) in hybrid plants increased, and the non-stomatal factors, which was also confirmed by the Calvin cycle.