3. Limited-Transpiration Rate Under Elevated Atmospheric Vapor Pressure Deficit

The driving force of transpiration rate is the gradient in vapor pressure between the dry atmosphere and the wet interior of leaves, commonly referred to as the vapor pressure deficit (VPD). The other key variable is the resistance to vapor diffusion from the leaves attributed to the aperture of the stomata pores. Partial stomatal closure at modest VPD results in limited-transpiration rate and, hence, conservative use of water. The conservative use of water allows more water to be available to sustain physiological activity later in the growing species. However, in many crop species, the limited-transpiration rate is not commonly expressed in commercial genotypes. Only recently have specific genotypes been identified that express the trait, and these genotypes are now being exploited in breeding programs. These breeding efforts have led to commercial cultivars in maize and soybean resulting in increased yields under dry land conditions.