Objective The aim of this study was to explore the changes and responses of hydraulic characteristics of plants under different levels of drought stress, and their ability to repair embolism after rehydration, thereby providing theoretical hydraulic evidences for plants to adapt to drought conditions and their ability to recover after drought.
Method 84K poplar (Populus alba × P. glandulosa) cuttings were potted and subjected to a progressive drought. According to the changes of plant morphological characteristics, water was controlled to four stages, i.e. the cessation of leaf expansion, whole plant wilting, 50% leaf mortality and 100% leaf mortality. Then, all plants of each stage were rewatered until new leaves appeared. After water control and rehydration treatment, hydraulic characteristics, such as xylem water potential, leaf water potential, vulnerability to xylem cavitation, stem hydraulic conductivity, the percentage loss of xylem conductance, and xylem anatomy such as the vessel diameter, contact fraction and vessel implosion resistance were measured at each stage.
Result With the intensification of drought stress, leaf water potential and xylem water potential decreased, and the embolism increased, but xylem vulnerability to cavitation increased. Also, compared with the control group, the vessel diameter decreased significantly, and the contact fraction and vessel implosion resistance increased significantly. At the stage of 50% leaf mortality, the percentage loss of hydraulic conductivity (PLC) of stem was 44%, and reached 65% at the stage of 100% leaf mortality. After rehydration for 10−24 days, the xylem water potential and leaf water potential in each stage returned to the level of control group, and the stem hydraulic conductivity increased with the PLC decreased. When the PLC of stem restored to 28%−37%, three new leaves spread out. After the plants with 100% leaf mortality were rehydrated until the emergence of new leaves, the PLC of stem was significantly reduced, but the plant diameter did not increase, which showed no new conduits participated in the water transportation during the rehydration, indicating that the restoration of stem hydraulic conductivity may result from embolism repair.
Conclusion Drought stress can adversely affect the hydraulic characteristics of plants, but plants can also adapt to the environment by changing the structure of their xylem. There is a certain trade-off between the plant’s ability to maintain leaf survival and transport water, and the leaves may be sacrificed to maintain the water conductivity of stem under drought stress. However, when drought stress is released, the hydraulic characteristics of the plants can also be restored. Even if plants without survival leaves can resume growth after rehydration, leaf death cannot be used as an indicator of plant death. There is a strong correlation between the recovery of plant growth and the restoration of stem hydraulic conductivity. Embolism repair may be the main reason for the restoration of stem hydraulic conductivity.
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