MEASURING WATER POTENTIAL BY THERMOCOUPLE PSYCHOMETRY

 

PSYCHOMETRY REFERS TO THE MEASUREMENTS OF THE DIFFERENCE IN TEMPERATURE BETWEEN AN ATMOSPHERE AND A FREELY EVAPORATING MOIST SURFACE IN THAT ATMOSPHERE

 

THE PSYCHOMETER MEASURES SMALL DIFFERENCES IN VAPOR PRESSURE.

 

A THERMOCOUPLE IS FORMED WHERE WIRES OF TWO DIFFERENT METALS ARE JOINED (A JUNCTION)

 

IF BOTH ENDS OF THE TWO WIRES ARE JOINED, A CLOSED CIRCUIT WILL BE FORMED.

 

   1.   IF THE TWO JUNCTIONS ARE

      HELD AT DIFFERENT TEMP-

      ERATURES, AN ELECTRIC

      CURRENT WILL FLOW THROUGH

      THE CIRCUIT.

 

   2.   THE MAGNITUDE OF THE

      CURRENT IS A MEASURE OF THE

      DIFFERENCE IN TEMPERATURE

      BETWEEN THE TWO JUNCTIONS.

 

THERMOCOUPLE PSYCHOMETRY DEPENDS ON THE PRINCIPLE THAT WATER VAPOR AT EQUILIBRIUM WITH PLANT TISSUE WILL HAVE THE SAME WATER POTENTIAL AS THE TISSUE.

 

WATER POTENTIAL OF TISSUE CAN BE MEASURED BY MEASURING THE VAPOR PRESSURE OF THE CHAMBER.

 

BASIC TECHNIQUE

 

1.   SEAL PLANT MATERIAL IN A SMALL    CHAMBER WITH A THERMOCOUPLE    JUNCTION, A SILVER RING AND A    DROP OF WATER.

 

2.   THE CHAMBER QUICKLY SATURATES    WITH WATER VAPOR

 

   a.   SINCE THE WATER POTENTIAL       OF THE TISSUE IS MORE          NEGATIVE THAN THE WATER

      POTENTIAL OF THE PURE DROP

      OF WATER, WATER VAPOR MOVES       FROM THE DROP INTO THE            TISSUE.

 

   b.   AS THE WATER EVAPORATES             FROM THE JUNCTION, THE            TEMPERATURE DROPS.  THIS       DROP IN TEMPERATURE IS          COMPARED TO A KNOWN AMBIENT       TEMPERATURE AND VAPOR             PRESSURE CALCULATED.

 

PRESSURE MEASUREMENTS

 

1.   PRESSURE BOMB - MEASURES

   NEGATIVE HYDROSTATIC PRESSURE    NORMALLY PRESENT IN XYLEM

   VESSELS

 

   a.   SHOOT IS CUT OFF A PLANT

 

 

   b.   WATER IN XYLEM PULLS AWAY

      FROM THE CUT SURFACE.

 

   c.   SHOOT ENCLOSED IN A CHAMBER

      (PRESSURE  BOMB).

 

   d.   PRESSURE APPLIED TO RAISE

      WATER BACK TO CUT SURFACE.

 

 

 

 

 

 

 

 

 

 

2.   MICROMANOMETER

 

   a.   MEASURES TURGOR PRESSURE IN

      CELLS

 

   b.   INSERTED DIRECTLY INTO A

      CELL

 

      1.   TIP INSERTED INTO A CELL

 

      2.   OIL IN MANOMETER DIS-

         PLACED

 

      3.   PRESSURE APPLIED TO

         RETURN OIL TO ITS

         ORIGINAL POSITION

 

      4.   THE AMOUNT OF PRESSURE

         APPLIED IS RECORDED

 

 

 

 

 

 

 

WATER RELATIONS OF THE WHOLE

PLANT

 

SOIL -->  PLANT --> ATMOSPHERE

 

    TRANSPIRATION

 

ONLY 5% OF WATER TAKEN IN BY THE ROOTS IS USED BY THE PLANT.  THE OTHER 95% IS LOST TO THE ATMOSPHERE

 

EXAMPLES - MAIZE PLANT TAKES IN 200L OF WATER IN ITS LIFETIME

 

TRANSPIRATION - LOSS OF WATER FROM A PLANT IN THE FORM OF WATER VAPOR.

 

90% OF WATER LOST BY A PLANT LOST THROUGH THE LEAVES.

 

   a.   LEAVES COVERED WITH A

      CUTICLE TO MINIMIZE WATER

      LOSS

 

      1.   MADE UP OF CUTIN AND

         WAXES

 

         - CUTIN - MADE OF

            HYDROXYLATED FATTY

            ACIDS

 

         -   WAXES - LONG CHAIN

            SATURATED HYDRO-

            CARBONS

 

   b.   INTEGRITY OF EPIDERMIS AND

      CUTICLE INTERRUPTED BY

      PORES CALLED STOMATA.

 

      STOMATA CONSIST OF:

 

      1.   STOMA - OPENING

 

      2.   TWO GUARD CELLS -

 

         REGULATE THE SIZE OF THE

            STOMA

         CONTAIN CHLOROPLASTS

  

      3.   SURROUNDED BY SUBSIDIARY

         CELLS