Outline4 marksStandard
Scientists investigated stomatal aperture in two plant species, X and Y, under well-watered and water-stressed conditions. Stomatal aperture (width in μm) was measured at three times of day. The results are shown in the table.
| Time | Species X, well-watered | Species X, water-stressed | Species Y, well-watered | Species Y, water-stressed |
|---|---|---|---|---|
| 06:00 | 2.5 | 1.8 | 3.0 | 2.8 |
| 12:00 | 5.0 | 0.5 | 5.5 | 4.0 |
| 18:00 | 4.0 | 0.8 | 4.5 | 3.5 |
Outline the mechanism by which stomata close in response to water stress, as seen in the data for Species X. Include the roles of abscisic acid (ABA) and calcium ions in your answer.