Greater aperture counteracts effects of reduced stomatal density on water use efficiency: a case study on sugarcane and meta-analysis.

Stomata regulate CO₂ and water vapor exchange between leaves and the atmosphere. Stomata are a target for engineering to improve crop intrinsic water use efficiency (iWUE). One example is by expressing genes that lower stomatal density (SD) and reduce stomatal conductance (g _sw). However, the quantitative relationship between reduced SD, g _sw, and the mechanisms underlying it is poorly understood. We addressed this knowledge gap using low-SD sugarcane (Saccharum spp. hybrid) as a case study alongside a meta-analysis of data from 10 species. Transgenic expression of EPIDERMAL PATTERNING FACTOR 2 from Sorghum bicolor (SbEPF2) in sugarcane reduced SD by 26–38% but did not affect g _sw compared with the wild type. Further, no changes occurred in stomatal complex size or proxies for photosynthetic capacity. Measurements of gas exchange at low CO₂ concentrations that promote complete stomatal opening to normalize aperture size between genotypes were combined with modeling of maximum g _sw from anatomical data. These data suggest that increased stomatal aperture is the only possible explanation for maintaining g _sw when SD is reduced. Meta-analysis across C₃ dicots, C₃ monocots, and C₄ monocots revealed that engineered reductions in SD are strongly correlated with lower g _sw (r ²=0.60–0.98), but this response is damped relative to the change in anatomy. [ABSTRACT FROM AUTHOR]