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Stem Hygrometer
References

Dixon, M.A. and Tyree, M.T., 1984, A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb, Plant, Cell and Environment, vol 7 no. pp. 693-697

A simple stem hygrometer for attachment to a bared section of sapwood or a cross-sectional cut end of a shoot is described. Two welded chromelconstantan thermocouples inside the chamber, one touching the sample and the other in the chamber air, allowed measurement of and correction for the temperature gradient between the sample and the dewpoint measuring junction. The instrument was attached to the cut end of an apical shoot if Thuja occidentalis L. protruding from a Scholander-Hammel pressure bomb. Cut-end water potential (Ψ_hyg), measured using the stem hygrometer, was compared to the xylem pressure potential (Ψ_xp) while the latter was manipulated in the pressure bomb. After an initial equilibrium time of 3-4 hours, hygrometer equilibrium values were achieved within 1.5-4.0 min of changing the Ψ_xp in the pressure bomb. The half-time (t1/2) for vapour pressure equilibration was 15-40 s. Stable temperature gradients between the sample and dewpoint measuring junction of 0.01-0.1 ^oC were measured. Correcting Ψ_hyg for the temperature gradient resulted in excellent agreement with Ψ_xp.

Lee, D.R., Dixon, M.A., et al., 1989, Simultaneous measurements of tomato fruit and stem water potentials using in situ stem hygrometers, Canadian Journal of Botany, vol 67 no. pp. 2352-2355

Simultaneous measurements were made of the water potentials of the stem and fruit of intact tomato plants (Lycopersicon esculentum Mill. var. Heinz 2653) using in situ temperature-corrected stem psychrometers. Water potential of the fruit remained consistently lower than the water potential of the stem except when the plant had been subjected to prolonged water stress. Stem water potential recovered quickly with rewatering, increased by approximately 0.5 MPa in 1 h, but the water potential of the fruit remained consistently near -1.0 MPa. The results indicate a significant resistance to water flow between the stem and the fruit and a substantial hydraulic capacitance represented by the volume of the fruit. Diurnal changes in dimensions of tomato fruit were also measured. Fruit diameter expanded at night and contracted during the day even when the ware potential gradient favoured flow towards the fruit. This indicates that bidirectional flow (to and from the fruit) is not responsible for the observed diurnal changes in the fruit dimensions.

Vogt, U.T., 2001, Hydraulic vulnerability, vessel refilling, and seasonal courses of stem water potential of Sorbus aucuparia L. and Sambucus nigra L., Journal of Experimental Botany, vol 52 no. 360, pp. 1527-1536

Differences in the seasonal variation in stem water potential between the two shrub species Sorbus aucuparia and Sambucus nigra were related with their vulnerability to xylem cavitation. It was also demonstrated indirectly that the two species differ in the extent to which they reverse cavitation. Seasonal variation in stem water potential was investigated during three growing seasons with in situ stem psychrometers. Sorbus experienced wide water potential variations and reached a minimum of -4.2 MPa during drought. Under the same microclimatic conditions, Sambucus experienced consistant stem water potentials with a minimum of -1.7 MPa. The relationship between percentage loss in hydraulic conductivity (PLC) and water potential (hydraulic vulnerability curve) of the two species differed in shape: a flat curve with nearly total water loss of conductivity at -6 MPa was found for Sorbus. Sambucus showed a steep vulnerability curve with 90% loss conductivity at -2.2 MPa. Thus Sambucus is extremely vulnerable to cavitation, but Sorbus is an almost invulnerable species. This different cavitaion resistance adjusted the ranges of field stem water potential that the species experienced. Finally, seasonal courses of naturally occurring (native) embolism were compared with calculated PLC sources. This comparison indicates that Sorbus did not refill embolized xylem vessels whereas Sambucus reversed embolism. It was concluded that species which are highly vulnerable to cavitation and drought-induced embolism need refilling of embolized vessels as well as isohydric water potential patterns as two strategies of survival.

Comstock, J.P., 2000, Correction of thermocouple psychrometer readings for the interaction of temperature and actual water potential, Crop Science, vol 40 no. pp. 709-712

The standard method of temperature correction for a thermocouple psychrometer only yields satisfactory results doe a limited range of water potential and temperature. Contrary to assumptions in standard practices, there is an interaction between the actual water potential under measurement and the temperature correction. Fortunately, the errors associated with ignoring this interaction are often small, and data are presented here that permit an assessment of whether the error is within acceptable limits under specific experimental conditions. More elaborate algorithms are given that can be used more robustly across a wide range of measurement conditions if needed. The temperature responses of two commonly used commercial psychrometers were examined, and it was found that this interaction resulted in model-specific correction algorithms. More specifically, it was found that the frequently made assumption of a correction factor that changes linearly with temperature is a satisfactory approximation across a range of at least 15 to 35^oC. However, it was also found that the slope describing how this correction factor changes with temperature itself changes as a function of the actual water potential being measured. The details and magnitude of this effect were model specific.

Fisher, R.A., Williams, M., et al., 2006, Evidence from Amazonian forests is consistent with isohydric control of leaf water potential, Plant Cell & Environment, vol 29 no. pp. 151-165

Climate modelling studies predict that the rainforests of the Eastern Amazon basin are likely to experience reductions in rainfall of up to 50% over the next 50�100 years. Efforts to predict the effects of changing climate, especially drought stress, on forest gas exchange are currently limited by uncertainty about the mechanism that controls stomatal closure in response to low soil moisture. At a through-fall exclusion experiment in Eastern Amazonia where water was experimentally excluded from the soil, we tested the hypothesis that plants are isohydric, that is, when water is scarce, the stomata act to prevent leaf water potential from dropping below a critical threshold level. We made diurnal measurements of leaf water potential (Ψ₁), stomatal conductance (g_s), sap flow and stem water potential (Ψ_stem) in the wet and dry seasons. We compared the data with the predictions of the soil�plant�atmosphere (SPA) model, which embeds the isohydric hypothesis within its stomatal conductance algorithm. The model inputs for meteorology, leaf area index (LAI), soil water potential and soil-to-leaf hydraulic resistance (R) were altered between seasons in accordance with measured values. No optimization parameters were used to adjust the model. This �mechanistic� model of stomatal function was able to explain the individual tree-level seasonal changes in water relations (r²=0.85, 0.90 and 0.58 for Ψ₁, sap flow and g_s, respectively). The model indicated that the measured increase in R was the dominant cause of restricted water use during the dry season, resulting in a modelled restriction of sap flow four times greater than that caused by reduced soil water potential. Higher resistance during the dry season resulted from an increase in below-ground resistance (including root and soil-to-root resistance) to water flow.

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