Effects of drought and re-irrigation on osmotic regulator and hydraulic function of <i>Pinus tabuliformis</i>.

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 <i>Pinus tabuliformis</i> 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 <i>P</i>. <i>tabuliformis</i>, 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 <i>P</i>. <i>tabuliformis</i> during drought and re-irrigation. Mild stress enhanced the embolism resistance under the long-term drought. New branches of <i>P</i>. <i>tabuliformis</i> 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.