Stomata have daily rhythms of opening and closing. Stomatal aperture width and the concentration of potassium ions in guard cells were measured in the leaves of basil plants, Ocimum basilicum. The basil plants were grown in conditions of natural light, with optimum temperature and soil moisture.

Figure 3.1 shows the results obtained between the times of 06:00 and 17:00. Error bars have been included.

Figure 3.1, a graph of mean stomatal aperture width in micrometres (left axis, solid line, filled circles) and concentration of potassium ions in guard cells in arbitrary units (right axis, dashed line, filled squares) against time of day from 04:00 to 20:00, with dark shown before 06:00 and after 18:00; stomatal aperture width rises from about 1 $\mu$m at 06:00 to a peak of about 6.5 $\mu$m at 11:00, then falls to about 3 $\mu$m by 17:00, while potassium ion concentration follows a similar but slightly earlier rise and fall, also peaking at about 11:00 and dropping to its lowest value by 17:00; error bars are shown on each point.

3(a)(i)Plant Responses Growth RegulatorsHard4 marks

With reference to Figure 3.1, describe the changes that occur in mean stomatal aperture width and suggest reasons why these changes benefit basil plants.

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3(a)(ii)Plant Responses Growth RegulatorsMedium3 marks

Figure 3.1 shows there is a correlation between mean stomatal aperture width and the concentration of potassium ions in guard cells. Explain how a change in one of these factors may cause a change in the other factor.

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3(b)Plant Responses Growth RegulatorsMedium3 marks

Researchers observed a daily rhythm in the synthesis of abscisic acid (ABA) in plants. Suggest whether ABA concentration is more likely to increase when light intensity is increasing or is more likely to increase when light intensity is decreasing. Give reasons for your answer.

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