A new method for in-situ measurements of stem hydraulic conductance in intact plants

Background/Question/Methods

Hydraulic conductivity of plant stems is a key trait of the hydraulic architecture of plants, with high hydraulic conductivity typically being associated with high rates of stomatal conductance, transpiration, and photosynthesis.  In addition, measurements of hydraulic stem conductance are essential for standard methods in plant hydraulics research to document the abundance of embolisms in xylem.  This is needed for xylem vulnerability curves, i.e. the relationship between stem water potential and the percent loss of conductance, and for documenting embolism formation and repair in plants.  The main drawback of hydraulic methods to measure embolisms has been that they are destructive and labor-intensive.  Stems have to be cut and hydraulic conductance measured, then stems have to be hydrated to remove embolisms, and then re-measured to determine maximum hydraulic conductance.  Measurements of xylem vulnerability curves are especially labor-intensive and require very large numbers of replicates for the bench-drying technique or very long hours in the lab to measure just a few replicates with the air injection or centrifuge methods.  The objective of this research was to develop an in-situ system to log hydraulic stem conductance in intact plants in the field for an extended period of time (days to weeks).

Results/Conclusions

Conductance is defined by the flow equation: Conductance equals flow divided by the pressure difference driving the flow.  The new in-situ hydraulic conductance measurement system consists of two temperature-corrected stem psychrometers for determining the pressure differential and a sap flow gage that uses the stem heat balance method for measuring volumetric sap flow.  One stem psychrometer is mounted either on a leaf or on a terminal branch and the other stem psychrometer is mounted near the base of the plant, with the sap flow gage mounted on a suitable stem between the two psychrometers.  An additional sap flow gage for measurement of sap velocity, using the heat ratio method, can be mounted on the same stem to give additional information about relationships between embolism formation and sap velocity.  The new in-situ system was tested successfully on large, potted and free-growing plants of Malosma laurina (Anacardiaceae), which is a chaparral shrub from southern California.  Hydraulic conductance declined with decreasing water potentials, indicating the formation of xylem embolisms.  By imposing a drying cycle on the plants, in-situ xylem vulnerability curves could be created.  The new method has great potential for advancing plant hydraulics research in the field.