Abstract: [Objective] The ability of maintaining hydraulic structure and carbon balance is a key factor for plants to affect the survival and recovery from drought under drought stress. The main purposes of this study were to explore the interaction of hydraulic failure and carbon starvation on black locust(Robinia pseudoacacia) seedlings by monitoring the changes of hydraulic traits and nonstructural carbohydrates (NSC) reserves during drought and rewatering periods, with a particularly interest in the seedling's performances during the rewatering period after experiencing severe drought stress to explore the major factors limiting leaf recovery from drought. Results can illustrate the interaction pattern of hydraulic traits and carbon metabolism during drought and rewatering periods, meanwhile provide research idea and experimental evidence for revealing physiological mechanism of decline and die-off for black locust in arid and semi-arid region.[Method] We measured predawn and midday twig water potential, midday branch percentage loss of conductivity (PLC), net photosynthetic rate, stomatal conductance, leaf area of individual plant, and NSC concentration of branches and roots after drought-induced severe defoliation and with having regenerated leaves after rewatering. Stem vulnerability curve after severe drought was determined. Leaf area, specific leaf weight and net photosynthetic rate of leaves resprouting on stem base and top after rewatering were compared. Predawn and midday twig water potential, and midday PLC of branches resprouting on stem base and top after rewatering were also compared. [Result] At the time of drought-induced complete defoliation, predawn and midday twig water potential were -3.01 MPa and -3.73 MPa respectively, and they were significantly lower than those of control. The midday PLC defoliated seedlings reached to 91%, and was significantly higher compared to control. At the time of more than 90% leaves defoliation, leaf net photosynthetic rate was negative with near-zero of stomatal conductance. Midday branch PLC was 81% in 30 days after rewatering, still significantly higher than that of control. P₅₀ (the water potential causing a 50% conductivity loss) was -1.09 MPa for branches subjected to drought and rewatering, which was 0.37 MPa higher compared to control. Cavitation resistance in branch segments was significantly reduced by drought. NSC concentration in branches and roots markedly declined during drought and rewatering periods, and NSC concentration dropped more severe during rewatering. Total leaf area per individual resprouting plant was only 38% of the control. Leaves resprouted mainly on stem base and top, but leaf area, net photosynthetic rate and stomatal conductance of leaves on stem base was significantly greater than those of leaves on the top. Predawn and midday water potential of twigs resprouting on stem base was also significantly higher than that of twigs resprouting on the top.[Conclusion] Severe drought lowered water potential, increased xylem embolism, and constrained water transport, then reduced photosynthetic rate resulting in negative carbon balances. Severe drought induced cavitation fatigue, and the lowered NSC reserves limited resprouting ability during rewatering period. Reduced NSC reserves may also have impact on instant xylem cavitation repair. The interaction of hydraulic failure and carbon starvation exacerbated the effect of drought on black locust seedlings.

Key words: Robinia pseudoacacia, drought, rewatering, hydraulic structure, nonstructural carbohydrates (NSC), foliage recovery