The key to stomatal openings are the guard cells which surround the stomata themselves. These are two kidney shaped cells, one either side of the pore (stoma). When these are fully turgid they createan opening in the middle, which is the stomata. The stomata allows water and gases to enter and exit to the intercellular air spaces, which are just behind the stomata. From there, these pass into the plant mesophyll cells and are taken by osmosis and diffusion to parts where they are needed for metabolic processes.
For the stoma to open, the guard cells need to be turgid i.e. full of water in their cytoplasm. This is affected by several influences. The first is the availability of water. If the guard cells lose their water, they become flaccid and close the stomatal pore. Water must be available in a constant supply but the cells can withstand a certain amount of water loss. When water is no longer available, the cells wilt – this is the permanent wilting point from which a plant cannot recover because the cytoplasm in guard cells and other cells breaks down.
The air around the guard cells will be humid or dry and this will affect the stomatal opening too. Water diffusing into humid air does so at a slower rate than into dry air because the difference in concentration needs to be less outside the stomatal opening than inside for diffusion to occur.
A guard cell has a unique ability to draw in water to remain turgid because, unlike most lower epidermal cells, (where most stomata are found) ,the guard cells have lots of chloroplasts which photosynthesise and create sugars in the cell sap. This increases the concentration of the cell sap and so the cell draws water from adjacent cells by a special diffusion called osmosis.
Temperature also affects the stomatal opening and in hotter weather, the stoma remain open for longer than in cool periods. Ths helps cool the plant by allowing a constant flow of water vapour out of the plant. This is the transpiration stream.
Wind will affect the humidity of the outside air and so influence stomatal opening. As air is moved away and replaced by drier air, the rate of diffusion increases and the stomatal openings remain wider.
Chemical actions also affect stomatal opening. If a plant is taking in concentrated toxins, the stomatal openings are reduced because their chemical balance and therefore their processes of photosynthesis and respiration are reduced, causing the plant to wilt and eventually die. This is how many weed killers and herbicides work.
At night, stomatal openings are smaller because the guard cells are not photosynthesising so their sap contains less sugars and they therefore draw in less water from the surrounding cells and become flaccid. This closes the stomata but not fuly as gases are still needed to enter and leave the plant intercellular air spaces for respiration – which continues both day and night.
The stoma are the plant’s entry and exit points for oxygen, carbon dioxide and water vapour and so form a very important part of the plant’s physiology.
Without stoma, transpiration could not occur, plants woud lack water and carbon dioxide required for photosynthesis and the plants could become hot and die.
