Fabian G, Scholz1,3, Guillermo Goldstein2,3, Sandra J. Bucci1,3
1Grupo de Estudios Biofísicos y Ecofisiológicos, Universidad Nacional de la Patagonia SJB, Comodoro Rivadavia, Argentina
2Laboratorio de Ecología Funcional, Universidad de Buenos Aires, Argentina
The purpose of using in situ stem psychrometers is to estimate the diurnal changes of stem water potentials of intact stems in living plants. Other approaches have been used such as removing sapwood samples with a 5-mm increment borer and immediately placing them in the caps of thermocouple psychrometer chambers which can be measured with a dew point microvoltmeter. Another non in situ approach is to measure leaf water potentials of bagged leaves hydraulically connected to the stem. These approaches even though valid cannot record daily dynamics continuously.
Using in situ stem psychrometers makes possible the study of stem water potentials in intact plants. This technic allows precise and continuous measurements at daily, seasonal or annual temporal scale, when used appropriately. Stem water potential (YSTEM) is generally higher than leaf water potential and is related to stomatal regulation, hydraulic conductivity and reliance on stem hydraulic capacitance. Consequently, determination of YSTEM is of high functional significance. Data from in situ stem psychrometers (Dixon and Tyree, 1984) are recorded with a datalogger at regular intervals (e.g. 10-min). Care has to be taken to attach the psychrometers to the stem after the underlying bark was removed very gently. It is extremely important to minimize temperature gradients when installing the psychrometers on the most shaded portion of the trunk and it is important to invest time and effort to get an appropriate thermal insulation of the sensor. To allow for dissipation of residual thermal gradients, the values obtained during the first day after installations should not be used.
The procedure described here is based on in situ stem psychrometers (Plant Water Status Instruments, Guelph, Ontario, Canada) recorded with a CR-7 datalogger with specific electronic cards (CR-7, Campbell Scientific, Logan, UT, USA). Currently similar stem psychrometers are manufactured by ICT International, Armidale, NSW, Australia, which have incorporated a dedicated datalogger (Dixon and Downey, 2013).
The stem psychrometer must be calibrated before each installation. The calibration is done with salt solutions of known osmolality (e.g. 0,1; 0,3; 0,5; 0,7; 0,9 molal NaCl, see Lang, 1967; Vogt, 2001) under isotherm conditions at a controlled temperature of e.g. 25 oC to generate a slope and intercept for each individual psychrometer. This linear regression will be used to transform the voltage output from each measurement into water potentials. The stem psychrometer incorporates two very small welded chromel-constantan thermocouples in series, inside a chamber, made of fine wire of 25 mm diameter. To determine the stem water potential the sensor measures, psycrometrically, the relative humidity inside the chamber between the cavity on the device and the exposed sapwood. One of the thermocouples touches the sample surface while the other is in the chamber air. The chamber thermocouple functions alternately as a dry psychrometric thermometer and, after Peltier cooling, as a wet psychrometric thermometer. Temperature readings of the sample thermocouple allow measuring of, and correction for any error resulting from temperature gradients between the sample and the chamber (Vogt and Losch, 1999).
Units, terms, definitions
Stem water potential is measured in MPa.
The sensor is attached to the base of the trunks as well as on large stems or branches.
1) The bark must be removed carefully until the sapwood with an area similar to the contact area of the sensor is exposed. A Forstner bit can be used because it bores a precisely, perfect flat surface at the bottom of the hole. The hole drilled in this way enable a vapor seal between the chamber and the xylem. A Forstner bit consists of two cylindrical cutting surfaces around the perimeter of the drill. A centering point is used to start the hole before the two cutting edges shear the xylem tissue at the edge of the bore. A 26 mm bit should be used allowing for ease of insertion which leaves a 0.5 mm gap around the perimeter of the psychrometer chamber.
2) Carefully put the longer thermocouple in position to do contact with the xylem tissue and insert the sensor in the drilled hole. The chamber is held firmly in place by the bark of the tree and simplifies the attachment and clamping requirements. The bark provides a crucial layer of insulation around the body of the psychrometer.
3) The sensor is then sealed with vacuum silicone grease, fixed by clamps and tape and over-sealed with silicone sealant. Using silicon grease and silicone sealant on the back of the sensor, the chamber is held in place and insures at the same time a good seal to avoid exchange of atmospheric air with the air inside the chamber.
4) Finally the sensor is carefully isolated using several layers of foam material and aluminum foil on a large portion of the stem.
It must be taken into account that the sensor must be always attached to the most shaded portion of the studied trunk, stem or branch and, if possible, shielded by leaves to avoid receiving direct solar radiation.
5) Connect the psychrometer to the data logger
With the appropriate use of in situ stem psychrometers it is possible to obtain data as observed in the next figure (See also e.g. Fig 3 Vogt, 2001)
Stem sapwood water potential measured using in situ psychrometers (Plant Water Status Instruments, Guelph, Ontario, Canada) in savanna trees (Scholz et al. 2007 Plant Cell and Environment 30: 236-248). The insets show leaf (closed symbols) and sapwood (open symbols) water potential changes from 0600 to 1700 h. The data points for sapwood water potential were taken from the daily course measurements at the time when leaf water potential was measured.
Programming for CR7 upon request
Notes and troubleshooting tips
- It is extremely important to clean the psychrometer chamber before calibration. The cleaning process involves using an organic solvent such as Chloroform or an electronics contact cleaner. (see Dixon and Downey, 2013)
- Thermocouples are easily broken if handled incorrectly. It is necessary to be very careful during handling.
- All data were obtained by the authors by installing the psychrometers on big stems with tick bark where the psychrometers remained installed at least 1 cm into the bark before entering in contact with the study tissue (stem xylem). This kind of installation (when possible), together with robust thermal protection of sensors, samples (foam, aluminum foil) and datalogger (which was buried half a meter below soil surface) contribute to get appropriate conditions to ensure optimal thermal conditions (thermal equilibrium between sensors, electronic devices and studied trees)
Links to resources and suppliers
Campbell Scientific: http://www.campbellsci.com
Dixon MA and Tyree MT (1984). A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb. Plant, Cell & Environment 7: 693‐697.
Dixon M and Downey A (2013). PSY1 Stem Psychrometer Manual. Ver 4.4. pp1-162. ICT International
Lang ARG (1967). Osmotic coefficients and water potentials of sodium chloride solutions from 0 to 40 ºC Australian Journal of Chemestry 20, 2017-2023
Scholz FG, Bucci SJ, Goldstein G, Meinzer FC, Franco AC, Miralles-Wilhelm F. (2007). Biophysical properties and functional significance of stem water storage tissues in neo-tropical savanna trees. Plant, Cell & Environment 30: 236-248
Vogt UK and Losch R (1999) Stem water potential and leaf conductance: a comparison of Sorbus aucuparia and Sambucus nigra. Phys Chem Earth (B) Vol 24 No 1-2 121-123
Vogt, UK (2001) Hydraulic vulnerability, vessel refilling and seasonal courses of stem water potentials of Sorbus aucuparia L. and Sambucus nigra L. Journal of Experimental Botany Vol 52 No 360, pp. 1527-1536