Plant Physiology Methods: Whole Plants

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Wescor Thermocouple Psychrometer

The psychrometer can be used to measure total plant water potential (MPa) on fresh leaf tissue, or plant osmotic potential (MPa) using frozen/thawed tissue or expressed sap.

Samples (tissue or solutions) equilibrate with water vapor in the chamber atmosphere. Evaporation of a water film on the psychrometer thermocouple cools a thermocouple and generates a negative micro-voltage. The recorded micro-voltage is proportional to the water potential of the sample

1) Set-out the Wescor chambers in a temperature-stable area in the laboratory so their temperature can equilibrate. Minimise handling to avoid temperature effects.

2) Set-up the meter reader (HR-33T Dew Point Microvoltmeter).

turn the toggle-switch to "ON"

ensure the toggle-switch is set to "mV"

set the 'Function' knob to "SHORT

3) Collect your plant material from the greenhouse. The plant shoot is harvested and put in a plastic bag or wrapped on saranwrap to prevent desiccation. Speed is essential to reduce measurement bias.

4) For total leaf water potential, fresh leaf tissue is used

For osmotic potential, a fully-expanded leaf is taken from each plant, folded and put in a garlic press. In the press cells are crushed and broken; bean tends to be extruded as a puree, but corn will produce a clean sap sample. (an alternative is to rupture cell walls by freezing the sample with liquid N)

It is important to remove debris and sap from the rim and outside of the sample cup before it is put in the sample changer.

5) Use the mid-size sample chambers. Using the leaf punch, take several discs from a 'standard' part of the leaf. Place the discs into the sample well, slide the sample into the chamber, tighten the chamber knob (do not overtighten). Allow the sample 10-30 mins to equilibrate with the atmosphere of the chamber. Avoid handling the chamber to prevent temperature dis-equilibrium.

6) Plug the chamber wire into the meter reader

7) Calibrate the micro-voltmeter

The Wescor has different calibrations depending on the size of sample chamber used. For the medium-sized chambers:
push the 'p_v' button, and at the same time turn the 'p_v SET' knob so the needle reads 73.

8) turn the 'FUNCTION' knob to "READ"

9) turn the 'ZERO OFFSET' knob until the meter needle returns to zero. Use the 'coarse' and 'fine' knobs as necessary

10) turn the 'FUNCTION' knob to "COOL"

(this allows water to condense onto the thermocouple - 5 seconds is usually sufficiant)

11) turn the 'FUNCTION' knob to "DP" (dew point)

as water evaporates (cooling the thermocouple) the mV meter-needle will decrease fairly rapidly, but once evaporation reaches equilibrium with the humidity of the chamber the needle will stabilize - THIS IS THE mV TO RECORD. Once the thermocouple dries the mV meter-needle will return to zero.

12) repeat steps 8-11 to ensure the reading is stable
13) calculate the sample water potential (MPa) as : y = mV / -7.5

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The psychrometer can be used to measure total plant water potential (MPa) on fresh leaf tissue, or plant osmotic potential (MPa) using frozen/thawed tissue or expressed sap. Samples (tissue or solutions) equilibrate with water vapor in the chamber atmosphere. Evaporation of a water film on the psychrometer thermocouple cools a thermocouple and generates a negative micro-voltage. The recorded micro-voltage is proportional to the water potential of the sample
1) Set-out the Wescor chambers in a temperature-stable area in the laboratory so their temperature can equilibrate. Minimise handling to avoid temperature effects.
2) Set-up the meter reader (HR-33T Dew Point Microvoltmeter).		turn the toggle-switch to "ON" ensure the toggle-switch is set to "mV" set the 'Function' knob to "SHORT
	3) Collect your plant material from the greenhouse. The plant shoot is harvested and put in a plastic bag or wrapped on saranwrap to prevent desiccation. Speed is essential to reduce measurement bias.
	4) For total leaf water potential, fresh leaf tissue is used For osmotic potential, a fully-expanded leaf is taken from each plant, folded and put in a garlic press. In the press cells are crushed and broken; bean tends to be extruded as a puree, but corn will produce a clean sap sample. (an alternative is to rupture cell walls by freezing the sample with liquid N) It is important to remove debris and sap from the rim and outside of the sample cup before it is put in the sample changer.
5) Use the mid-size sample chambers. Using the leaf punch, take several discs from a 'standard' part of the leaf. Place the discs into the sample well, slide the sample into the chamber, tighten the chamber knob (do not overtighten). Allow the sample 10-30 mins to equilibrate with the atmosphere of the chamber. Avoid handling the chamber to prevent temperature dis-equilibrium.
6) Plug the chamber wire into the meter reader 7) Calibrate the micro-voltmeter The Wescor has different calibrations depending on the size of sample chamber used. For the medium-sized chambers: push the 'p_v' button, and at the same time turn the 'p_v SET' knob so the needle reads 73.
8) turn the 'FUNCTION' knob to "READ"		9) turn the 'ZERO OFFSET' knob until the meter needle returns to zero. Use the 'coarse' and 'fine' knobs as necessary
10) turn the 'FUNCTION' knob to "COOL" (this allows water to condense onto the thermocouple - 5 seconds is usually sufficiant)
11) turn the 'FUNCTION' knob to "DP" (dew point) as water evaporates (cooling the thermocouple) the mV meter-needle will decrease fairly rapidly, but once evaporation reaches equilibrium with the humidity of the chamber the needle will stabilize - THIS IS THE mV TO RECORD. Once the thermocouple dries the mV meter-needle will return to zero.
12) repeat steps 8-11 to ensure the reading is stable 13) calculate the sample water potential (MPa) as : y = mV / -7.5
\| HCS Home \| Index \| Calendar \| Resources \| Assignments \| Group rotation Copyright © The Ohio State University All rights reserved.