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Guard cells open and close in response to which set of cues?
ALight, leaf water status and carbon dioxide level
BLight, soil nitrogen level and phloem sugar level
CWind speed, soil pH and internal starch reserves
DWind speed, leaf age and internal calcium stores
Answer & Solution
Correct answer: A. Light, leaf water status and carbon dioxide level
1. Regulation of transpiration is achieved mainly by opening and closing the stomata.
2. Each stoma is flanked by two guard cells that respond to environmental cues.
3. Those cues are light intensity and quality, leaf water status, and carbon dioxide concentrations.
4. Stomata must open to let carbon dioxide in for photosynthesis, yet opening also loses water vapour.
5. So the plant must balance efficient photosynthesis against water loss, using exactly those signals.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.5 Transport of Water and Solutes in Plants_
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