Abstract: Understanding how plants respond to drought and re-irrigation is crucial for the successful breeding of seedlings in artificial forests in semi-arid regions of China, as the frequency of high-intensity drought events has significantly increased in these areas. We conducted an experiment with four drought stress treatments, including no reduction in rainfall (control), 25% reduction in rainfall (mild stress), 50% reduction in rainfall (moderate stress), and 75% reduction in rainfall (severe stress). We subjected two-year-old Pinus tabuliformis seedlings to a long-term drought stress period of 20 months, followed by a high-intensity drought treatment (continuous 80 days with 100% rainfall reduction). Additionally, we administered re-irrigation treatments lasting 10 days on days 10, 40, 70, and 80 of the high-intensity drought period. We aimed to investigate the relationships among hydraulic conductivity, percentage loss of hydraulic conductivity, and the content of osmotic adjustment substances in new branches of P. tabuliformis, as well as their responses to drought. The results showed that under long-term drought stress, the water potential threshold at which hydraulic conductivity loss reached 50% for new branches under mild stress was -2.04 MPa, which was significantly lower than that of the other treatments. After 80 days of high-intensity drought, the proline content in new branches increased significantly by 19.9% to 226.0% compared to the initial value. Redundancy analysis showed that proline explained 40.4% of the variability in hydraulic function and was the primary osmotic adjustment substance during high-intensity drought. During re-irrigation, soluble sugars explained 29.4% of the variability in hydraulic function and were the main osmotic adjustment substances in this stage. Different factors accounted for the differences in hydraulic function of P. tabuliformis during drought and re-irrigation. Mild stress enhanced the embolism resistance under the long-term drought. New branches of P. tabuliformis maintained normal water transport by accumulating proline in response to high-intensity drought. During re-irrigation, soluble sugars were significantly positively correlated with hydraulic conductivity, which facilitated embolism repair and enhanced the recovery of hydraulic function.

Key words: long-term drought, high-intensity drought, re-irrigation, osmotic regulation, hydraulic function