Leaf structure, physiology and transcriptome response of ancient Populus szechuanica cuttings in southwest China under different drought stresses

Against the background of global climate change, increasingly severe drought stress exerts a significant impact on plant growth and yield. This study aimed to clarify the leaf anatomical structure, physiology and biochemistry and transcriptome-level metabolic adaptation mechanisms of ancient P. szechuanica to environmental stress. We selected cuttings of ancient P. szechuanica with a diameter of breast-height (DBH) ≥ 1 m and a tree age of 300–500 years as experimental materials. Natural drought stress was applied to investigate the responses of leaf anatomical structure, physiological and biochemical traits, and transcriptome-level metabolic processes of ancient P. szechuanica under drought stress. The results showed the following changes in leaf anatomical structure under drought stress (compared with the control group, the same below): leaf thickness, pith length and palisade tissue thickness decreased by 49.60 %, 20.1 % and 28.68 %, respectively. The thickness of upper and lower epidermis and spongy tissue first increased and then decreased, with final reductions of 53.13 %, 54.26 % and 50.30 %, respectively. Stomatal length and width also decreased, by 15.67 % and 24.26 % respectively. For physiological and biochemical traits, with the prolongation of drought stress, the soluble sugar content decreased significantly by 7.49 %, while the soluble protein content increased significantly by 44 %.At the transcriptome level, significant differentially expressed genes (DEGs) were screened at different drought stages: 3,353 upregulated and 3,161 downregulated DEGs on the day 4 of drought, 5,208 up-regulated and 9,560 down-regulated DEGs on the day 8, and 15,659 up-regulated and 14,870 down-regulated DEGs on the day 12. These DEGs mediated the drought stress response of P. szechuanica via positive up- or down-regulation.