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Description
Introduction.
The measurement of xylem water potential in plants has always been a necessity in the study and modelling of the soil-plant-atmosphere continuum, which drives and explain the water flux through plants and crops. Until now, the available methods for the measurement of water potential were discontinuous, destructive, and labor-intensive. The recent appearance of innovative xylem microtensiometers opens new interesting perspectives in this field of study. Nevertheless, the functioning principle of these new sensors relies on the assumption of a constant equilibrium in water potential between the sensor porous-membrane interface and the surrounding unmodified and conductive xylem. This is physically impossible, as a coupling medium of finite water conductivity is necessarily present bridging the living xylem and the sensor membrane. This work describes simple experiments aimed to assess the response of these new microtensiometers to quick changes in the water potential of living trees.
Materials and Methods.
Xylem microtensiometers (FloraPulse Co., Davis, CA, USA) were installed in the trunk of two 5-year-old olive trees in the summer of 2022 in Córdoba, Spain, and read at 1-min intervals. The trees were planted in 50-L pots; one of the trees was chosen because it showed strong stomatal oscillations. The soil in the pots was kept at field capacity (soil water potential near zero) by automatic drip irrigation. An advanced sap-flow system (Testi and Villalobos, 2009) was also installed in each tree, monitoring sap velocity at 5-min intervals. On 20 September the canopy of the non-oscillating tree was wetted by a sprayer pump, to stop transpiration abruptly, completely, and continuously during an hour.
Results:
1) Oscillations: the oscillating tree showed sap flux oscillations up to 1.9 L/h of amplitude (vs. maximum flux of 2.6 L/h at the peak), with periods between 45 and 65 min. The microtensiometers were able to show oscillations in the xylem water potential of around 5 bars in amplitude, with the same period as the sap oscillations.
2) Wetting experiment: the wetting of the canopy stopped the transpiration; the sap flow decreased abruptly close to zero in few minutes, reflecting only the refilling of the tree capacitance. Before the wetting (which began at 11:00, solar time) the microtensiometer was showing decreasing xylem water potential, and marked -7.4 bars. The xylem water potential inverted its naturally decreasing trend between 3 and 4 minutes after the wetting started, and after 60 minutes of zero transpiration the xylem water potential was -5.70 bars. The water potential returned to the decreasing trend around 10 minutes after the canopy wetting was stopped, i.e. the time needed for the canopy to dry up completely.
Conclusions
The microtensiometers installed in living trees showed a remarkable small response time at least qualitatively, i.e. they detect impulsive changes in the soil-plant-atmosphere continuum in terms of minutes. Nevertheless, the time required for reaching the new equilibrium seems much longer. Thus, the quantitative response after abrupt changes in the water status may well require specific corrections to use these new sensors in tracing changes in the soil-plant-atmosphere continuum occurring in less than a day. For applications involving slower physiological processes — for example monitoring water status in trees for irrigation scheduling — corrections are probably unnecessary.
Testi L, Villalobos FJ (2009) New approach for measuring low sap velocities in trees. Agric For Meteorol 149: 730-734. https://doi.org/10.1016/j.agrformet.2008.10.015
| Keywords | Microtensiometers; soil-plant-atmosphere continuum; xylem water potential; sap-flow; transpiration |
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