Question Video: Describing the Rate of Transpiration in Plants Biology

Video Transcript

The graph provided shows the relationship between the time of day and the rate of water flow through a plant. Between which hours was the rate of transpiration the greatest? (A) 2 AM and 8 AM, (B) 2 AM and 6 PM, (C) 10 AM and 2 PM, (D) 12 PM and 6 PM, or (E) 8 PM and 10 PM.

First, let’s work out what the graph is showing us. On the 𝑥-axis we have the time of day, from midnight through to 10 PM. On the 𝑦-axis, we have the rate of water flow through a plant. You may recall that a rate is how frequently something happens in a given period of time. So, the rate of water flow is how frequently water flows through the plant from the roots up to the leaves in a given period of time.

The flow of water through a plant is largely driven by transpiration pull. This is where water is lost from the leaves through the stomata, so more water is pulled up the xylem vessels of the plant to replace it. Therefore, the rate of water flow is equivalent to the rate of transpiration.

This question asks us for the hours between which the rate of transpiration was the greatest. We are therefore simply looking for the highest point of the curve, which is here. Of the time intervals given in the answer options, this peak falls between 12 PM and 6 PM.

So, the correct answer is (D). The rate of transpiration was the greatest between 12 PM and 6 PM.