Transpiration is fastest when humidity is low and temperature is high, but in some plants it seems to increase in response to light as well. During one 12-hour period when cloud cover and light intensity varied frequently, a scientist studying a certain crop plant recorded the data in the table (top right). (The transpiration rates are grams of water per square meter of leaf area per hour.)
Do these data support the hypothesis that the plants transpire more when the light is more intense?
If so, is the effect independent of temperature and humidity?
Explain your answer. (Hint: Look for overall trends in each column, and then compare pairs of data within each column and between columns.)

A plant cell with a Ψ_S of −0.65MPa maintains a constant volume when bathed in a solution that has a Ψ_S of −0.30MPa and is in an open container. The cell has a

a. Ψ_P of +0.65MPa

b. Ψ of −0.65MPa

c. Ψ_P of +0.35MPa

d. Ψ_P of 0 MPa

Draw a plant cell in pure water. Add dots to indicate solutes inside the cell. Now add more dots to indicate an increase in solute potential inside the cell. Add an arrow showing the net direction of water movement in response. Add arrows showing the direction of wall pressure and turgor pressure in response to water movement. Repeat the same exercise, but this time, add solutes to the solution outside the cell at a concentration that is greater than inside the cell.

Compared with a cell with few aquaporin proteins in its membrane, a cell containing many aquaporin proteins will

a. Have a faster rate of osmosis

b. Have a lower water potential

c. Have a higher water potential

d. Accumulate water by active transport

A mutant plant lacking the ability to pump protons out of leaf companion cells will be unable to do which of the following?

a. Initiate transpiration

b. Load sucrose into sieve-tube elements

c. Carry out photosynthesis

d. Transport water through the xylem

Which of the following would tend to increase transpiration?

a. Spiny leaves

b. Sunken stomata

c. A thicker cuticle

d. Higher stomatal density

Your friend claims that phloem always carries sugars down a plant. What, if anything, is wrong with that statement?

Consider a tree that is 50 m tall and is transpiring roughly 90 liters of water each day. Approximately how many calories will the tree use to transpire this quantity of water?