Foliar potential expresses the force with which water is retained by the leaves. The measurement of the foliar potential allows to evaluate the water status of the plant, to identify when the plant enters a condition of water stress and therefore to intervene with irrigation. In practice we can say that when the plant has reached water stress we will have a certain value of foliar potential, which varies from crop to crop (from species to species).
Figure 1 – Sholander pressure chamber
For the measurement of the foliar potential we can use the so-called Sholander chamber, it is a pressure chamber equipped with a special pressure gauge (unit of measurement in bar). During the measurements this chamber is connected to a cylinder containing inert liquid gas at high pressure (generally it is nitrogen, not oxygen because it damages the plant tissues). The measurement procedure involves the following steps:
- removal of the shoots containing a certain number of leaves (as in the case of the olive tree) or the single leaf (as in the case of the vine), making a clean transversal cut (possibly with the use of a scalpel)
- the sprout (or the leaf in the case of the vine) is quickly led inside the chamber (to prevent the water from evaporating from the cut part)
- a portion of the sprout is passed through a rubber or silicone stopper which in turn is inserted into a special metal cap with threading so that the sprout is sealed inside the chamber and the severed part is outside the chamber and observable by the operator
- through a system of valves the gas is introduced into the chamber (by monitoring the progressive increase in pressure through the appropriate pressure gauge)
- at the same time the severed part of the shoot is observed, possibly with the aid of a magnifying glass
- the observation is continued until the instant in which liquid begins to come out of the xylem vessels
- the pressure value at this instant expresses the leaf potential.
In general, several measurements are taken throughout the day at regular intervals, usually every one or two hours, to obtain complete monitoring. Just before dawn, the plant has the maximum water content and the minimum foliar potential of the day, while in the hottest hours (around 13:00) the plant has the minimum water content and the maximum foliar potential.
