The Scholander Pressure Chamber, aka “pressure bomb”, has become the standard for measuring the water potential of plants, whether as exposed or shaded leaves, or as internal stem potential at a point of measurement. Leaves under natural conditions can be measured; but if leaves are bagged in plastic bags with foil to stop transpiration and equilibrate with the internal water in the stems, the reading reflects the stem water potential.
Over many studies, it has been concluded that this method is a valid method that is particularly good for relative comparisons, such as one operator measuring several plants in different soils in a consistent manner. However, there as concerns about the accuracy (how close to the real value) and the precision (repeatability, especially amongst different operators).
Accuracy
We do not have a method that we consider absolutely accurate, so the accuracy is judged in relation to other methods that appear to be valid, such as the thermocouple psychrometer in controlled lab conditions. These two methods agree quite well though they both may have potential artifacts due to tissue excision and handling. But they are judged to be quite accurate when done carefully.
Precision
Procedures and Operator Variations - There is more concern about variation in readings due to operator variation. Individual operators differ somewhat in the process of cutting the leaf off the plant, possibly trimming the cut surface if uneven, enclosing the leaf in a plastic bag or not, the handling and time taken from cutting to beginning of pressurization, the rate of pressurization and the visual decision that the water has reached the cut surface.
Below is a comparison of several experienced research operators attempting to measure the exposed leaf water potential in the same apple trees (Landsberg et al., 1975).
Many years of observation of this method with many operators has shown that there can be quite a lot of variation due to operator variation. A recent paper by Levin (2019) evaluating operator procedures concluded that “the effect of the operator stood out as the one of the most important factors affecting Ψleaf and Ψstem readings.” So, although field measurements with the pressure chamber are very valuable, clearly they are not absolute values.
Variations in the Plant – We know that there are significant variations of water potential in plants at any given time. When sampling exposed leaves for leaf water potentials, the exposure of the leaf to sunlight can vary with the angle of the leaf giving varying energy loads and transpiration rates that affect water potential. Shaded leaves transpire less and will have a less negative potential. In some cases the immediate prior history of exposure can affect the stomatal opening, transpiration and thus water potential.
Gradients of stem water potential in plants occur due to the hydraulic resistance along the transpiration stream from roots to leaves. Since many woody perennials have fairly high hydraulic resistance, the stem and leaf water potentials may decrease by up to 0.1 MPa (1 bar) per meter of vascular distance along the root-to-leaf pathway under high transpiration conditions.
The actual potential depends on the stem potential at any point and an additional drop in potential due to transpiration of exposed leaves. So, with variable exposures and leaf activity, a range of water potentials can be found on the same plant. The stem potential at the base of the plant, however, integrates all the stem and exposed leaf potentials.
Variations Due to Environment and Time of Day – During the morning and later afternoon, the water potential in perennials changes quite rapidly as seen below (from Jones et al, 1985). Also when clouds come and go, strongly changing the radiation load, exposed leaf potentials can change rapidly. The stem water potential does not change as rapidly, possibly due to some capacitance of water in the tissues. Consequently, the speed of response to rapid changes are expected to differ between the stem potential in the main trunk versus the water potential of exposed leaves.
In summary, there should be a good relationship between stem potentials measured by the microtensiometer and either stem or leaf potentials measured by pressure chambers. But there are many reasons to see some variation between them as they are not precisely the same measurement.
References
Jones, H.G., A.N. Lakso and J.P. Syvertsen. 1985. Physiological control of water status in temperate and subtropical fruit trees. Horticultural Reviews 7:301-344.
Landsberg, J.J., C. L. Beadle, P. V. Biscoe, et al. 1975. Diurnal Energy, Water and CO2 Exchanges in an Apple (Malus pumila) Orchard. J. Applied Ecology 12:659-684
Levin, A.D. 2019. Re-evaluating pressure chamber methods of water status determination in
field-grown grapevine (Vitis spp.). Agric. Water Mgt. 221:422–429.