CPAC 08-

Question 1

Outline the practical method to measure the effect of environmental conditions on water uptake in a plant shoot

Answer 1

• set up potometer, fill capillary tube and rubber connector with water and insert leafy shoot into the rubber connector (do this step under water)
• clamp capillary tube into the stand. place bottom of capillary tube into beaker of water
• smear petroleum jelly around joint for airtight conditions
• leave for 5 minutes to allow bubble to be drawn up into capillary tube
• either measure the time taken for the bubble to move a certain distance or measure how far the bubble moves in a certain time
• convert this measurement into rate of transpiration by dividing volume of water taken up by time
• repeat experiment and change the abiotic factor e.g. placing lamp 10cm from light source

Question 2

What equipment is needed to measure water uptake in the shoot?

Answer 2

• large leafy shoot, end must be cut under water
• capillary tubing with short rubber connecting tube
• petroleum jelly
• fine marker pen
• ruler clamp stand
• 250cm3 and 500cm3 beaker
• stop clock
• fan, dark cupboard, light meter, plastic bag or incubator
• sharp scissors
• plastic cable ties

Question 3

Why is it necessary to form an airtight seal?

Answer 3

• to prevent air bubbles getting into the xylem which would block the transpiration stream
• water won’t be drawn up by the capillary tube and no air bubble movement possible
• air bubbles prevent cohesive forces and stop upward tension being transmitted through entire water column

Question 4

What happens if air comes into contact with exposed xylem vessels?

Answer 4

air will enter xylem in the shoot and airlock will form-prevents water uptake