Facultative CAM plants such as Mesembryanthemum crystallinum (ice plant) possess
ID: 82950 • Letter: F
Question
Facultative CAM plants such as Mesembryanthemum crystallinum (ice plant) possess C3 metabolism when unstressed but develop CAM under water or salt stress. When ice plants shift from C3 metabolism to CAM, their stomata remain closed during the day and open at night. Recent studies have shown that the stomatal response of ice plants in the C3 mode depends solely on the guard cell response to blue light. Recent evidence for a possible role of the xanthophyll, zeaxanthin, in blue light photoreception of guard cells led to the question of whether changes in the regulation of the xanthophyll cycle in guard cells parallel the shift from diurnal to nocturnal stomatal opening associated with CAM induction. In the present study, light-dependent stomatal opening and the operation of the xanthophyll cycle were characterized in guard cells isolated from ice plants shifting from C3 metabolism to CAM. Stomata in epidermis detached from leaves with C3 metabolism opened in response to white light and blue light, but they did not open in response to red light. Guard cells from these leaves showed light-dependent conversion of violaxanthin to zeaxanthin. Induction of CAM by NaCl abolished both white light- and blue light-stimulated stomatal opening and light-dependent zeaxanthin formation. When guard cells isolated from leaves with CAM were treated with 100 mM ascorbate (pH 5.0) for 1 h in darkness, guard cell zeaxanthin content increased at rates equal to or higher than those stimulated by light in guard cells from leaves in the C3 mode.
a. Based on the steps of stomatal opening in C3 plants that you just reviewed in question #4., why would the “abolishment” of light dependent zeaxanthin described here result in stomatal closure? (4 pts)
a. How do you explain the fact that the ascorbate treatment resulted in the production of zeaxanthin even in the dark? (4 pts)
b. Would you expect the stomates on the leaves treated with ascorbate to open or close and why? (4 pts)
2 pts. Extra Credit: In the excerpt above they mention that they used NaCl (salt) to for the “induction of CAM”. In other words they were creating drought conditions so that the plants would use CAM carbon fixation rather than C3 carbon fixation. Why would the addition of salt to the soil help create dry conditions for the plants?
Explanation / Answer
1) Zeaxanthin has its role in the opening of stomata as a photoreceptor and when its synthesis/formation is interrupted, it resulted in the closure of stomata.
2)Ascorbate treatment must have worked as an alternative chemosynthesis of zeaxanthin, resulted in the removal of the epoxy group from violaxanthin.
3) ascorbic acid treatment lowers the pH and encourages the de-epoxidation of Violaxanthin to zeaxanthin, thus resulting in the closure of stomata.
4)Adding salt to the soil will make the plant lose its water due to osmosis, thereby creating a dry condition for the plant.
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