Microtensiometers Accurately Measure Stem Water Potential in Woody Perennials

Abstract

Stem water potential (Ψ_stem) is considered to be the standard measure of plant water status. However, it is measured with the pressure chamber (PC), an equipment that can neither provide continuous information nor be automated, limiting its use. Recent developments of microtensiometers (MT; FloraPulse sensors), which can continuously measure water tension in woody tissue of the trunk of the tree, can potentially highlight the dynamic nature of plant water relations. Thus, this study aimed to validate and assess the usefulness of the MT by comparing the Ψ_stem provided by MT with those same measurements from the PC. Here, two irrigation treatments (a control and a deficit treatment) were applied in a pear (Pyrus communis L.) orchard in Washington State (USA) to capture the full range of water potentials in this environment. Discrete measurements of leaf gas exchange, canopy temperature and Ψ_stem measured with PC and MT were made every two hours for four days from dawn to sunset. There were strong linear relationships between the Ψ_stem-MT and Ψ_stem-PC (R² > 0.8) and with vapor pressure deficit (R² > 0.7). However, Ψ_stem-MT was more variable and lower than Ψ_stem-PC when Ψ_stem-MT was below -1.5 MPa, especially during the evening. Minimum Ψ_stem-MT occurred later in the afternoon compared to Ψ_stem-PC. Ψ_stem showed similar sensitivity and coefficients of variation for both PC and MT acquired data. Overall, the promising results achieved indicated the potential for MT to be used to continuously assess tree water status.

Keywords: gas exchange; irrigation scheduling; pear; precision agriculture; pressure chamber; sensors; tree water status; vapor pressure deficit; water stress indicators.

Figure 1

Diurnal patterns of mean volumetric soil water content (SWC) and vapor pressure deficit (VPD) for four representative days (12 June (A), 2 July (B), 31 July (C), 11 August (D)) during 2021 season.

Figure 2

Diurnal patterns of stem water potential measured with the pressure chamber (PC) (A,C,E,G) or with the microtensiometer (MT) (B,D,F,H) for control (CTL, blue) and deficit-irrigated (DI, red) trees on four representative days (12 June, 2 July, 31 July, 11 August) during the 2021 growing season (N = 6). Asterisks indicate statistically significant differences between irrigation treatments according to ANOVA (p ≤ 0.05).

Figure 3

Relationship between stem water potential measured with the pressure chamber and the microtensiometers for control trees (CTL, blue) and deficit irrigated trees (DI, red) in the morning (A), midday (B), afternoon (C), and evening (D) of four representative days (12 June (circle), 2 July (square), 31 July (triangle), 11 August (diamond)) during 2021 season. (*** p < 0.001).

Figure 4

The linear relationship between stem water potential measured with the microtensiometers and the difference between the pressure chamber and the microtensiometers for control trees (CTL, blue) and deficit irrigated trees (DI, red) in the morning (A), midday (B), afternoon (C), and evening (D) of four representative days (12 June (circle), 2 July (square), 31 July (triangle), 11 August (diamond)) during 2021 season. (* p < 0.05, *** p < 0.001).

Figure 5

Relationship between the stem water potential measured with the microtensiometers (MT, blue) and the pressure chamber (PC, white) with the vapor pressure deficit (VPD) for control trees (CTL) in the morning (A), midday (B), afternoon (C), and evening (D) of four representative days (12 June (circle), 2 July (square), 31 July (triangle), 11 August (diamond)) during 2021 season. (*** p < 0.001).

Figure 6

Stem water potential mean value and distribution according to a 0.1 MPa scale and a water stress scale (0 absence of water stress, 5 severe water stress) for stem water potential values measured during the experiment with the pressure chamber (PC) and the microtensiometers (MT).

Figure 7

Diurnal course of photosynthetic photon flux density (PPFD), net photosynthesis (P_n) (A,D,G,J), stomatal conductance (G_s) (B,E,H,K), canopy temperature (Tc), air temperature (Ta), and the difference between Tc and Ta (Tc-Ta) (C,F,I,L) for control (CTL, blue) trees and deficit-irrigated (DI, red) trees on four representative days (12 June, 2 July, 31 July, 11 August) during the 2021 season (N = 6). Asterisks indicate statistically significant differences between irrigation treatments according to ANOVA (p ≤ 0.05).

Figure 8

Relationship between the stem water potential measured with the microtensiometers (MT) and the pressure chamber (PC) with the stomatal conductance for control trees (CTL, blue) and deficit-irrigated trees (DI, red) of four representative days (12 June (circle), 2 July (square), 31 July (triangle), 11 August (diamond)) (A) and three representative days (12 June (circle), 2 July (square), 11 August (diamond)) from 10:30 to 15:30 h during 2021 season (B). (*** p < 0.001).

Figure 9

Relationship between stem water potential measured with the microtensiometers (MT) and the pressure chamber (PC), and canopy temperature (A) and canopy-to-air temperature (B) for control trees (CTL, blue) and deficit-irrigated trees (DI, red) of three representative days (12 June (circle), 2 July (square), 11 August (diamond)) from 10:30 to 15:30 h during 2021 season. (*** p < 0.001).

Figure 10

Microtensiometer installation scheme (A) and real sensor installed into the tree (B).