Leaf gas exchange is influenced by stomatal size, density, distribution between the leaf adaxial and abaxial sides, as well as by pore dimensions. Fanourakis et al. introgress segments of Solanum pennelli into a S. lycopersicum cultivar to generate 54 introgression lines (Ils), which they study in order to determine which traits underlie genetic differences in operating stomatal conductance (gs). They find a wide genetic variation in stomatal responsiveness to desiccation, a large part of which is explained by stomatal length. The pore area per stomatal area varies eight-fold among the ILs, and is the main determinant of differences in operating gs. Differences between operating gs based on pore dimensions and maximum gs based on stomatal dimensions are large, suggesting that stomata utilize only a small portion of their operating capacity.