Question 1
Paper 2Hard16 marksSix similar, well-watered potted plants were exposed for 2 h to air at either 40% or 90% relative humidity (RH), with six plants in each condition. Light intensity, temperature and air movement were kept the same. Leaf water loss and xylem pressure potential in the upper stem were measured. Figure 1 shows the experimental arrangement and the individual measurements.
Using the six measurements in Figure 1B, calculate the mean leaf water loss under each RH condition and the percentage decrease from 40% RH to 90% RH.
Using Figure 1C, compare the mean upper-stem xylem pressure potential between the two RH conditions, and relate this comparison to leaf water loss.
Explain how water loss from cell walls in the leaf leads to movement of water from roots to leaves in the plant.
A student concludes: “Figure 1 proves that xylem tension alone is sufficient to maintain upward water transport; cohesion is unnecessary.”
Evaluate this conclusion using Figure 1 and your knowledge of water transport. Include one limitation of the evidence.