CO2, which is needed for photosynthesis, enters leaves via tiny pores called stomata. Stomata also permit the loss of water by transpiration. Guard cells open or close stomata in response to a variety of environmental cues. Plants control their total numbers of stomata by shedding leaves or by altering the density of stomata on new leaves.
learning outcomes
You should be able to:
Analyze reasons why stomata do not always remain open, considering the processes of both photosynthesis and water conservation.
Analyze the importance of the proton pump and K+ ions in the actions of stomata, and list environmental changes that trigger the opening and closing of stomata.
What is the role of K+ ions in the functioning of guard cells?
In sunlight, an H+ pump in the cell membrane of guard cells pumps H+ out of the cells. This sets up an electrochemical gradient across the guard cell membrane and K+ enters the cells. This in turn sets up an osmotic gradient and water enters the guard cells, causing them to become turgid and a gap to appear between adjacent guard cells. This gap is an open stoma. In darkness, the proton pump becomes less active, K+ diffuses passively out of the cell, and the stoma closes.
Describe how water availability can affect stomatal function.
On a hot, dry day, mesophyll cells lose water and this results in stomatal closure. Stomata remain open on cooler, humid days.
Stomata are normally open during daylight hours, allowing photosynthesis—