What causes the stomata to
open and close?
There are several hypotheses to explain the mechanism of stomatal
opening and closing. One of them is that during daytime, when carbon
dioxide is used for photosynthesis, solutes such as potassium ions
accumulate in the guard cells due to the lower carbon dioxide concentration.
This causes the influx of water into the guard cells by osmosis.
The thin outer walls of the guard cells bulge out and force the
thick inner walls into a crescent (·s¤ë§Î) shape. Stomata are open.
At night, when photosynthesis stops, this process is reversed and
the stomata close. View the animation in the following website to
have a more thorough understanding of the mechanism.
http://academic.kellogg.edu/herbrandsonc/bio111/animations/0021.swf
The stomata work by changing the turgor pressure in the guard
cells
Water or carbon
dioxide?
When stomata are open during daytime to allow the diffusion of carbon
dioxide into the plants, water is lost through the pores and this
can be life-threatening to plants living in the deserts. Cacti (¥P¤H´x)
overcome this problem by opening their stomata at night, when the
air temperature is much lower. The carbon dioxide taken in is stored
in the form of organic acids. When daylight comes, cacti keep their
stomata closed and carbon dioxide is released from the organic acids
for photosynthesis.
Plants living in the deserts open the stomata at night
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