Measurement of water potential using thermocouple hygrometers.

Abstract

Theory predicts that the time dependent voltage curve of a thermocouple psychrometer where there is no change in output voltage with time during the evaporation cycle defines the wet bulb temperature T[w] corresponding to the water potential. In practice, a change in voltage with time does occur and it is convenient to define the voltage corresponding to the water potential as the maximum point of- inflection voltage. A predictive model based on calibration data at a few tempertures is used to obtain the psychrometer calibration slope at any temperature. Use of this model indicates that psychrometers differ from each other and therefore must be individually calibrated if accuracy better than ±5 % in the measurement of water potential is required. Dewpoint hygrometers are shown to be less temperature sensitive than psychrometers and have the added advantage of a voltage sensitivity nearly twice that of psychrometers, typically -7,0 x 10¯³ μV/kPa compared to -3,7 x 10¯³ μV/kPa at 25 °C. The accurate temperature correction of hygrometer calibration curve slopes is a necessity if field measurements are undertaken using either psychrometric or dewpoint techniques. In the case of thermocouple psychrometers, two temperature correction models are proposed, each based on measurement of the thermojunction radius and calculation of the theoretical voltage sensitivity to changes in water potential. The first model relies on calibration at a single temperature and the second at two temperatures. Both these models were more accurate than the temperature correction models currently in use for four leaf psychrometers calibrated over a range of temperatures (15 to 38°C). The model based on calibration at two temperatures is superior to that based on only one calibration. The model proposed for dewpoint hygrometers is similar to that for psychrometers. It is based on the theoretical voltage sensitivity to changes in water potential. Comparison with empirical data from three dewpoint hygrometers calibrated at four different temperatures indicates that these instruments need only be calibrated at, say 25°C, if the calibration slopes are corrected for temperature. A model is presented for the calculation of the error in measured thermocouple hygrometric water potential for individual hygrometers used in the dewpoint or psychrometric mode. The model is based on calculation of the relative standard error in measured thermocouple psychrometric water potential as a function of temperature. Sources of error in the psychrometric mode were in calibration of the instrument as a function of water potential and temperature and in voltage (due to electronic noise and zero offsets) and temperature measurement in the field. Total error increased as temperature decreased, approaching a value usually determined by the shape of the thermocouple junction, electronic noise (at low voltages less than 1 μV) and errors in temperature measurement. At higher temperatures, error was a combination of calibration errors, electronic noise and zero offset voltage. Field calibration data for a number of leaf psychrometers contained total errors that ranged between 6 (at a °C) and 2 %(at 45 °C) for the better psychrometers and between 11 (at 0° C) and 5 % (at 45 C) for the worst assuming that the zero offset was 0,5 μV. Zero offset values were less than 0,7 μV at all times. The dewpoint errors arose from calibration of the dewpoint hygrometer as a function of water potential, extrapolation of the calibration slope to other temperatures, setting the dewpoint coefficient and errors in voltage and temperature measurement. The total error also increased as temperature decreased, because of the differences in temperature sensitivity between dewpoint and psychrometric calibration constants. Consequently, the major source of error in the dewpoint mode arose from the difficulty in determining the dewpoint coefficient. This error, which is temperature dependent, contains three subcomponent errors; the temperature dependence, random variation associated with determining the temperature dependence and error in setting the correct value. Calibration and extrapolation errors were smaller than those of the psychrometric technique. Typically, the error in a dewpoint measurement varied between about 6 and 2 % for the best hygrometer and between 10 and 3 % for the worst for temperatures between 0 and 45 °C respectively. At low temperatures, the dewpoint technique often has no advantage over the psychrometric technique, in terms of measurement errors. In a comparative laboratory study, leaf water potentials were measured using the Scholander pressure chamber, psychrometers and hydraulic press. Newly mature trifoliates cut from field grown soybean (Glycine max (L) Merr. cv. Dribi) were turgidified and, after different degrees of dehydration, leaf water potential measured. One leaflet from the trifoliate was used for the thermocouple psychrometer and another for the press while the central leaflet with its petiolule was retained for use in the pressure chamber. Significant correlations between measurements using these instruments were obtained but the slopes for hydraulic press vs psychrometer measurement curve and hydraulic press vs pressure chamber were 0,742 and 0,775 respectively. Plots of pressure-volume curves indicate that the point of incipient plasmolysis was the same (statistically) for the thermocouple psychrometer and the pressure chamber, but much larger for the hydnaulic press. The above-mentioned differences between the three instruments emphasize the need for calib rating the endpoint defined us i ng the press against one or more of the standard techniques, and, limi ting the use of the press to one person. Cuticular resistance to water vapour diffusion between the substomatal cavity and the sensing psychrometer junction is a problem unique to leaf psychrometry and dewpoint hygrometry; this resistance is not encountered in soil or solution psychrometry. The cuticular resistance may introduce error in the leaf water potential measurement. The effect of abraiding the cuticle of Citrus jambhiri to reduce its resistance, on the measured leaf water potential was investigated. Psychrometric measurements of leaf water potential were compared with simultaneous measurements on nearby leaves using the Scholander pressure chamber, in a field situation. Leaf surface damage, due to abrasion, was investigated using scanning electron microscopy. Thermocouple psychrometers are the only instruments which can measure the in situ water potential of intact leaves, and which may be suitable for continuous, non-destructive monitoring of water potential. Unfortunately, their usefulness is limited by a number of difficulties, among them fluctuating temperatures and temperature gradients within the psychrometer, sealing of the psychrometer chamber to the leaf, shading of the leaf by the psychrometer and resistance to water vapour diffusion by the cuticle when the stomates are closed. Using Citrus jambhiri, several psychrometer designs and operational modifications were tested. In situ psychrometric measurements compared favourably with simultaneous Scholander pressure chamber measurements on neighbouring leaves, corrected for the osmotic potential and the apparent effect of "xylem tension relaxation" following petiole excision. It is generally assumed that enclosure of a leaf by an in situ thermocouple psychrometer substantially modifies the leaf environment, possibly altering leaf water potential, the quantity to be measured. Furthermore, the time response of leaf psychrometers to sudden leaf water potential changes has not been tested under field conditions. In a laboratory investigation, we found good linear correlation between in situ leaf psychrometer (sealed over abraided area) and Scholander pressure chamber measurements (using adjacent leaves) of leaf water potential, 2 to 200 minutes after excision of citrus leaves. A field investigation involved psychrometric measurement prior to petiole excision, and 1 min after excision, simultaneous pressure chamber measurements on adjacent citrus leaves immediately prior to the time of excision and then on the psychrometer leaf about 2 min after excision. Statistical comparisons indicated that within the first two minutes after excision, psychrometer measurements compared favourably with pressure chamber measurements. There was no evidence for a psychrometer leaf water potential time lag. For the high evaporative demand conditions, water potential decreased after excision by as much as 700 kPa in the first minute. Psychrometer field measurements indicated that within the first 5 min of leaf petiole excision, the decrease in leaf water potential with time was linear but that within the first 15 s, there was a temporary increase of the order of a few tens of kilopascal. The thermocouple psychrometer can be used to measure dynamic changes in leaf water potential non-destructively, with an accuracy that compares favourably with that of the pressure chamber. Using in situ thermocouple leaf hygrometers (dewpoint and psychrometric techniques employed) attached to Citrus jambhiri leaves, an increase in measured water potential immediately following petiole excision was observed. The increase ranged between 20 to 80 kPa and occurred 30 s after petiole excision and 100 s after midrib excisions. No relationship between the actual leaf water potential and the increase in water potential due to excision, was found.