Measurement of leaf xylem water potential and transpiration during soil drying using a root pressure chamber system

Simultaneous measurements of leaf water potential (psi(leaf)) and transpiration rate (E) for varying soil moistures (theta) is necessary to identify the hydraulic constrains on transpiration as the soil dries. However, continuous and accurate measurements of psi(leaf) in intact plants remain challenging. This work aims at describing a root pressure chamber system which allows high resolution measurements of the relationship between psi(leaf) and E during soil drying. The root pressure chamber is equipped with a cuvette where E is changed by altering the light intensity and air humidity. The belowground part of a plant, typically grown in soil-filled columns, is enclosed in a pressure chamber and the shoots in the cuvette. The chamber and the cuvette are connected to a main controller where different parameters are regulated and monitored psi(leaf) is determined by applying the pneumatic pressure necessary to bring the xylem sap to the cut of a leaf. Sensitivity of different parameters was conducted with a series of E and theta using pearl millet as a model plant psi(leaf) was measured at increasing and decreasing E for different theta. The results show that the method was sensitive to 0.1 bar (10 kPa) and the measurements of psi(leaf) were accurate in this range. The temporal resolution of the measurements of psi(leaf) was in the order of 5-10 min. E measured by the root pressure chamber system and by the balance matched well. In summary, the method allows accurate measurements of the psi(leaf) (E) relation in intact plants.