Hi Klaus Kremer,

In my experience, midday water potential better reflects the maximum water stress to which plants are subjected. If you are interested in knowing the level of stress that can trigger cavitation or mortality, I would measure midday water potential. Otherwise, if you are trying to relate plant stress to soil conditions, it might be better (or more accurate) to just measure it at pre-dawn.

Dear Klaus: The pre-down water potential informs you on the soil water potential for a plant, on a given day. The stress extent that the plant is going to experience will also depend on the water being demanded from air, that is VPD. In other words, for a given soil water content, the plant water status might be more or less favorable depending on the leaf to air VPD. Therefore, the midday water potential should be a better choice for assessing water stress.

Dear Klaus

Midday water potential can better represent plant water status. If we assume a steady state, midday water potential is a function of the predawn potential (as you correctly point out, they are correlated), but also of leaf to air DPV, stomatal conductance, hydraulic conductance, leaf area, sapwood area, plant height (or path lenght water), among others.

The following article is a review made by Martínez-Villalta on this topic.

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.12912

Good luck!

Hello Klaus,

Plant water potential (PWP) is dynamic. It depends on the potentials of water in the different segments of the soil-plant-atmosphere system, the flux of water through the system and the resistances (or conductances) of the segments. The Ohm's Law analogy has been much used in analyses. When the flux is very small the system approaches equilibrium (the time required for this must also be considered). At the end of the (long) dark period PWPis close to the soil water potential in the root zone from where the water is taken. So a measure of PWP `pre-dawn' is a very useful indicator of the relevant (to the plant) water status of the soil. In the light, the water flux increases but often varies considerably due to the meteorological conditions, and the PWP reflects this and the soil water potential and the resistance of the soil, plus the root and stem resistances. The difference between pre-dawn and midday may be rather small with small flux and moist soil but get considerably greater and more variable as the soil dries and the atmospheric demand increases. But they will correlate.

If you are interested in establishing the relation between PWP and plant processes both measures are important.

PWP will relate to changes in cellular turgor, and hence be linked to changes in leaf/organ expansion - this occurs at night as well as during the day so probably a measure of pre-dawn and midday PWP would be useful. Photosynthesis during the day depends on (amongst may things) stomatal conductance, and this relates to PWP then. So, experience shows that measuring both pre-dawn and during the day is important. See: The effects of drought on barley: soil and plant water relations W. Day, D. W. Lawlor and B. J. Legg (1981) J. Agri Sci 96;, 61-77. https://doi.org/10.1017/S0021859600031889.

Hope this helps. Regards

David Lawlor

Whether predawn works better or midday, needs to be correlated with a given parameter , since water potential is highly dynamic over different time intervals in a day....We observed pre-dawn water potential acts as an effective parameter of monitoring water stress, however , from practical field application point of view , its so difficult to operationalise ....