Pre-existing embolism causes reverse vulnerability segmentation in Populus

Area of Science:

• Plant Physiology
• Xylem Hydraulics
• Drought Resistance

Background:

• Vulnerability segmentation in xylem protects against drought-induced embolism.
• Classical segmentation: leaves more vulnerable than stems.
• Reverse segmentation (leaves more resistant) has been suggested in Populus.

Purpose of the Study:

• Investigate mechanisms of leaf-stem hydraulic segmentation in Populus species.
• Determine conditions leading to reverse vulnerability segmentation.
• Clarify if reverse segmentation is an adaptive trait.

Main Methods:

• Optical vulnerability method to assess embolism resistance.
• Comparison of field-grown and glasshouse-grown Populus species (P. trichocarpa, P. deltoides, P. grandidentata).
• Micro-computed tomography and hydraulic conductivity measurements.

Main Results:

• Reverse vulnerability segmentation occurred only in older, field-grown stems.
• Field-grown older stems showed significant native embolism (>25% loss of conductivity).
• No segmentation was observed in glasshouse-grown or newly formed stems.

Conclusions:

• Reverse segmentation in Populus is not due to inherent xylem properties.
• Accumulated native embolism, likely from freeze-thaw cycles or stem age, causes reverse segmentation.
• Reverse segmentation in Populus appears to be an environmental byproduct, not a specific adaptation.