Functional and developmental differences between adaxial and abaxial stomata in amphistomatous leaves

Stomatal pores mediate leaf CO₂ uptake and water loss. The number, size, and aperture of stomata determine plant gas-exchange potential and affect productivity and stress tolerance. To date, stomatal research has mostly focused on abaxial stomata (stomata on the lower leaf surface) to understand the mechanisms and regulation of stomatal development and stomatal opening and closure. Many plants are amphistomatous and produce stomata also on the adaxial (upper) leaf surface. Relatively, little is known of how adaxial stomata function, how they develop, how different stomatal distributions between leaf surfaces are achieved and how they affect plant physiology. Here, I discuss current knowledge on the function and making of adaxial stomata, and address the role and regulation of stomatal distribution between adaxial and abaxial leaf surfaces in amphistomatous plants. Evidence in the literature points toward independent adaxial and abaxial stomatal function and development, and indicates that stomatal distribution in leaves is an important functional trait.