WATER POTENTIAL PROBLEMS

 

IN LIVING CELLS THE:

 

   S IS ALWAYS NEGATIVE

 

   P IS ALWAYS POSITIVE IN LIVING

   CELLS

 

  Y = P + S

 

PURE WATER HAS AN S OF ZERO

SO THAT P MUST ALSO BE ZERO SINCE THE WATER POTENTIAL OF PURE WATER IS ZERO

 

WHEN A SOLUTE IS ADDED TO PURE WATER, S BECOMES NEGATIVE

 

THE PRESSURE POTENTIAL (P) IS ALWAYS POSITIVE UNLESS THE CELL IS DEAD

 

WATER POTENTIAL CAN BE POSITIVE, NEGATIVE OR ZERO, IN A LIVING CELL

 

 

FIGURE THE FOLLOWING WATER POTENTIALS GIVEN THE FOLLOWING VALUES FOR S AND P:

 

   1.   S = -0.5

      P =  0.2

 

 

 

   2.   S = -0.1

      P =  0.1

 

 

 

 

SOLVE FOR THE MISSING VALUES

FOR THE PROBLEMS PRESENTED BELOW.

 

1. A PLANT CELL IS FOUND TO HAVE    A WATER POTENTIAL OF -0.3. THE

   PRESSURE POTENTIAL IS FOUND TO

   BE O.7.  WHAT IS THE SOLUTE

   POTENTIAL?

 

 

2.   SOIL WATER IS FOUND TO HAVE A

   WATER POTENTIAL OF -5.  A ROOT

   CELL OF A PLANT GROWING IN

   THIS SOIL HAS A PRESSURE

   POTENTIAL OF 4 AND WATER IS

   SEEN TO MOVE FROM THE SOIL

   INTO THE ROOT.  WHAT IS THE

   GREATEST THE SOLUTE POTENTIAL

   CAN BE TO SUPPORT THESE

   OBSERVATIONS?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.   THE CELLS AT THE SURFACE OF A

   ROOT HAVE A WATER POTENTIAL OF

   -1.  THE CELLS CLOSER TO THE

   STELE HAVE A SOLUTE POTENTIAL

   OF -3.  WATER MOVES FROM THE

   INSIDE TO THE OUTSIDE OF THE

   ROOT.  WHAT IS THE GREATEST

   THE PRESSURE POTENTIAL CAN BE

   TO SUPPORT THIS OBSERVATION?

 

 

 

 

 

 

 

4. IF SOLUTE POTENTIAL IS -0.2

 

 

   a.   WHAT PRESSURE POTENTIAL

      CAN GIVE A NEGATIVE   ?

 

   b.   WHAT PRESSURE POTENTIAL

      CAN GIVE A POSITIVE   ?

 

 

WATER MOVEMENT IN CELLS AND TISSUES

 

WHEN THE PRESSURE IN A CELL IS ZERO THE CONDITION IS CALLED

INCIPIENT PLASMOLYSIS

 

AT INCIPIENT PLASMOLYSIS, THE PROTOPLAST JUST FILLS THE CELL AND NO PRESSURE IS EXERTED AGAINST THE CELL WALL

 

IF THE PRESSURE POTENTIAL IS ZERO, WHAT IS THE RELATIONSHIP BETWEEN WATER POTENTIAL AND SOLUTE POTENTIAL?

 

WHEN CELLS ARE PUT IN HYPOTONIC SOLUTIONS, WATER MOVES INTO THE CELL, INCREASING THE TURGOR PRESSURE.

 

 

 

 

 

WHEN CELLS ARE PUT IN HYPERTONIC SOLUTIONS, WATER MOVES OUT OF THE CELL, DECREASING THE TURGOR PRESSURE. THE PROTOPLAST PULLS AWAY FROM THE CELL WALL.  THIS IS CALLED PLASMOLYSIS

 

MEASURING WATER POTENTIAL AND ITS COMPONENTS

 

1.   TISSUE WEIGHT CHANGE METHOD

 

   PRINCIPLE:EQUILIBRATE    PREWEIGHED SAMPLES IN    SOLUTIONS OF KNOWN OSMOTIC    POTENTIAL

 

   DETERMINE WHICH SOLUTION HAS A    WATER POTENTIAL CLOSEST TO THE    TISSUE SAMPLE

 

      a.   CUT TISSUE SAMPLES

      b.   WEIGH TISSUE SAMPLES

     

 

 

      c.   PLACE IN SOLUTIONS OF

         VARYING OSMOTIC                      POTENTIAL FOR 1-2 HOURS

      d.   WEIGH TISSUE SAMPLES

      e.   CALCULATE WEIGHT GAIN OR

         LOSS

      f.   GRAPH WEIGHT CHANGE

         AGAINST OSMOIC (OR

         WATER) POTENTIAL

 

 

 

 

 

 

 

      g.   WATER POTENTIAL IS WHERE

         THE LINE CROSSES THE X

         AXIS

 

 

 

 

 

 

 

2.   THERMOCOUPLE PSYCHOMETRY

 

   a.   MEASURED WITH A VAPOR

      PRESSURE OSMOMETER