Stomatal form and function in the light side of the leaf

Most plants have all or most of their stomata in the lower (abaxial) leaf side, whereas grasses and many other herbs, including the model plant Arabidopsis thaliana (Arabidopsis), have high stomatal numbers also on the upper (adaxial) leaf surface. Most studies on stomatal development and guard cell signalling have focused on abaxial stomata and very little is known on the formation and role of adaxial stomata in plants. Stomatal ratio describes the distribution of stomata between upper and lower leaf surfaces. How stomatal ratio is determined in plants remains unclear. Our recent work indicates that: i) mechanisms that govern stomatal development in the adaxial and abaxial epidermis in Arabidopsis are at least partly different, ii) adaxial and abaxial stomatal development and conductance respond differently to changes in environmental conditions, leading to changes in stomatal ratio and stomatal conductance ratio; and iii) stomatal ratio is positively related with yield in tomatoes. Thus, understanding the mechanisms of adaxial stomatal development and how stomatal ratio is adjusted in plants can help to breed crops with improved yield.