Pore size regulates operating stomatal conductance, while stomatal densities drive the partitioning of conductance between leaf sides

Background and Aims Leaf gas exchange is influenced by stomatal size, density, distribution between the leaf adaxial and abaxial sides, as well as by pore dimensions. This study aims to quantify which of these traits mainly underlie genetic differences in operating stomatal conductance (g(s)) and addresses possible links between anatomical traits and regulation of pore width. Methods Stomatal responsiveness to desiccation, g(s)-related anatomical traits of each leaf side and estimated g(s) (based on these traits) were determined for 54 introgression lines (ILs) generated by introgressing segments of Solanum pennelli into the S. lycopersicum 'M82'. A quantitative trait locus (QTL) analysis for stomatal traits was also performed. Key Results A wide genetic variation in stomatal responsiveness to desiccation was observed, a large part of which was explained by stomatal length. Operating g(s) ranged over a factor of five between ILs. The pore area per stomatal area varied 8-fold among ILs (2-16 %), and was the main determinant of differences in operating g(s) between ILs. Operating g(s) was primarily positioned on the abaxial surface (60-83 %), due to higher abaxial stomatal density and, secondarily, to larger abaxial pore area. An analysis revealed 64 QTLs for stomatal traits in the ILs, most of which were in the direction of S. pennellii. Conclusions The data indicate that operating and maximum g(s) of non-stressed leaves maintained under stable conditions deviate considerably (by 45-91 %), because stomatal size inadequately reflects operating pore area (R-2 = 0.46). Furthermore, it was found that variation between ILs in both stomatal sensitivity to desiccation and operating g(s) is associated with features of individual stoma. In contrast, genotypic variation in g(s) partitioning depends on the distribution of stomata between the leaf adaxial and abaxial epidermis.