Abstract:

Objective: Under the background of global warming, there is a lack of research on the hydraulics and carbon metabolism of Betula albo-sinensis (red birch), and the mechanism of death and lethal threshold due to persistent drought stress are still unclear. This study aims to explore the hydraulics and carbon metabolism characteristics of B. albo-sinensis under drought stress, revealing its lethal threshold and death mechanism, which is necessary for clarifying the survival and adaptive capacity of B. albo-sinensis in the context of global warming. It also provides theoretical reference for red birch seedling cultivation and forest management. Method: The present study was carried out on four-year-old B. albo-sinensis seedlings subjected to continuous drought stress, which was classified into four stages of stress according to the appearance of the leaf blades and physiological status (Stage Ⅰ, reaching the point of leaf turgor loss; Stage Ⅱ, leaf blade drooping prolapse; Stage Ⅲ, leaves starting to dry up; Stage Ⅳ, leaves completely yellow). After reaching each stage, the seedlings were rehydrated for 1 week, 2 weeks, and 4 weeks. During the period of drought stress and at specific stages of rehydration, the soil volumetric water content, predawn leaf water potential, midday water potential, leaf specific conductance, stem specific hydraulic conductance, stem hydraulic conductivity loss, and soluble sugars and starch contents in roots, stems and leaves were measured, and the total amount of non-structural carbohydrates (NSC) was calculated. Result: 1) The results showed that at four stages of drought stress, leaf predawn water potential decreased to –0.732, –1.32, –1.712, –2.23 MPa, respectively. In the final stage of stress, the stem specific hydraulic conductivity and leaf hydraulic conductivity decreased by 94.5% and 99.1%, respectively, and the loss of hydraulic conductivity was more than 88%. The soluble sugar content in each organ rose, starch content decreased, the soluble sugar to starch ratio increased, and the total root NSC decreased to 62.6%. There was a significant correlation between the soluble sugar and starch contents and the loss of hydraulic conductivity values (P<0.01). 2) After rehydration, the soluble sugar content in various organs of seedlings that experienced stage Ⅰ, Ⅱ and Ⅲ drought stress was significantly higher than that of the control group (P<0.05), and the water transport capacity of B. albo-sinensis seedlings was able to be restored. However the hydraulic traits of B. albo-sinensis seedlings that experienced stage Ⅳ stress could not be restored, and the NSC content of roots and stems was lower than the level of the control group. 3) The loss values of stem hydraulic conductivity of B. albo-sinensis seedlings at the four stages were 15.57% ± 0.61%, 49.49%± 5.03%, 77%± 2.52%, and 96.4%± 1.11%, and the mortality rates were 0, 0, 33.3%, and 100%, respectively. Conclusion: There is a coupling relationship between the hydraulic traits and NSC of B. albo-sinensis seedlings during drought stress, and B. albo-sinensis seedlings cope with the impact of sustained drought stress on water transport by dynamically regulating the carbon allocation pattern of each organ and converse starch into soluble sugars. After rehydration, the recovery degree of the hydraulic traits and NSC of B. albo-sinensis seedlings is affected by the degree of the previous drought stress. Hydraulic failure and carbon starvation co-occur during plant death, and B. albo-sinensis seedlings are already at risk of death at a value of 77% loss of stem hydraulic conductivity.

Key words: drought stress, non-structural carbohydrates, hydraulic properties, mechanisms of mortality, lethal thresholds