Methods of Measuring Water Status
of Plants and Soils
John S. Boyer
University of Delaware
Lewes, DE 19958
Why Measure Plant or Soil Water Status?
Allows experimental conditions to be repeated
Allows comparison with other work
Gives information about mechanism
Lecture Covers
Isopiestic psychrometry
Pressure chamber
Pressure probe
Relative water content
Isopiestic Psychrometry
Westgate and Boyer. 1986. Crop Sci. 26, 947-951.
Isopiestic Psychrometry
Pro
Versatile (soil, most plant tissues, pollen, spores)
Small samples
Lab and field
Epidermis has little affect
Temperature has little effect
All component potentials can be obtained
No continuous liquid phase needed
Con
Not a “survey” instrument
Temperature must be uniform
Small signals
Pressure Chamber
Boyer. 1967. Plant Physiol. 42, 133-137.
Pressure Chamber
Pro
Excellent field method, also good in lab
Rapid
All component potentials can be obtained
Minimum temperature control
Con
High pressures dangerous
Seal can crush tissue
Tissue can dehydrate during measurement
Continuous liquid phase required
Pressure Probe
Tyerman and Steudle. 1982. Aust. J. Plant Physiol. 9, 461-480.
Pressure Probe
Pro
Cell-specific data
Direct turgor measurement
Membrane and wall properties can be obtained
Con
Cells must be penetrated
Restricted to lab
Continuous liquid phase required
Vibration must be minimized
Component potentials cannot be measured
Relative Water Content
VF − VD
RWC =
x100
VT − VD
VF = fresh wt
VT = turgid wt
VD = dry wt
Weatherley PE. 1950. New Phytologist 49, 81-97.
Conceptual Basis of Relative Water Content
Float leaf disc
on water
Component potentials
of fresh weight (VF)
Component potentials
of turgid weight (VT)
Weatherley PE. 1950. New Phytologist 49, 81-97.
Conceptual Basis of Relative Water Content
After Osmotic Adjustment Was Discovered
Float leaf disc
on water
Boyer, James, Munns. Unpublished data.
Leaf Fragments of Salinized Barley Lose
Protoplast Contents When Floated on Water
Boyer, James, Munns. Unpublished data.
Measuring
Osmotic
Adjustment
After
Rewatering
Jones and Turner. 1978. Plant Physiol. 61:124
Measuring Osmotic Adjustment
Using a Pressure Chamber
Boyer. 1967. Plant Physiol. 42, 213-217
Relative Water Content
Pro
Easiest, least expensive of methods
Field and lab
Gives water content of tissue
Con
Based on biological reference
Error from osmotic adjustment
Overall Summary
Potentials based on physical, constant reference.
RWC based on biological, variable reference.
Potentials indicate forces moving water.
Certain potentials affect metabolic activity
(turgor-cell enlargement, turgor-stomatal opening).
RWC indicates concentration of constituents.
RWC affects metabolic activity (cofactor
concentrations, metabolite concentrations)