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Stoma in a tomato leaf shown via colorized scanning electron microscope image A stoma in horizontal cross section The underside of a leaf. In this species (Tradescantia zebrina) the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigments.
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions [1] that allows a plant to photosynthesize during the day, but only exchange gases at night.
Photosynthesis depends on the diffusion of carbon dioxide (CO 2) from the air through the stomata into the mesophyll tissues. Oxygen (O 2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by ...
Stomatal conductance is a function of the density, size and degree of opening of the stomata; with more open stomata allowing greater conductance, and consequently indicating that photosynthesis and transpiration rates are potentially higher. Therefore, stomatal opening and closing has a direct relationship to stomatal conductance.
[4] [6] During the following day, stomata are closed, malate is decarboxylated, and CO 2 is fixed by RuBisCO. This process alone is similar to that of C4 plants and yields characteristic C4 fractionation values of approximately -11‰. [6] However, in the afternoon, CAM plants may open their stomata and perform C3 photosynthesis. [6]
[29] [30] The most important benefit of CAM to the plant is the ability to leave most leaf stomata closed during the day. [31] This reduces water loss due to evapotranspiration. The stomata open at night to collect CO 2, which is stored as the four-carbon acid malate, and then used during photosynthesis during the day.
In full CAM, the stomata open only at night, when temperatures and water loss are lowest. CO 2 enters the plant and is captured in the form of organic acids stored inside cells (in vacuoles). The stomata remain closed throughout the day, and photosynthesis uses only this stored CO 2.
The pores or stomata of the epidermis open into substomatal chambers, which are connected to the intercellular air spaces between the spongy and palisade mesophyll cell, so that oxygen, carbon dioxide and water vapor can diffuse into and out of the leaf and access the mesophyll cells during respiration, photosynthesis and transpiration.