Is intrinsic water use efficiency independent of leaf-to-air vapor pressure deficit?

Intrinsic water-use efficiency ( i WUE , the ratio between net photosynthetic rate and stomatal conductance to water vapor) is assumed to be insensitive to leaf-to-air vapor pressure deficit ( VPD l ) for some plant species. This assumption has led many researchers to accept that i WUE may be more useful for evaluating plants for drought-resistance than instantaneous water-use efficiency ( WUE , the ratio between net photosynthetic rate and transpiration rate), since the latter is known to depend on VPD l . However, the assumption that i WUE does not depend on VPD l may not be always correct because the stomatal aperture (ostiole area, A o ) and the resulting stomatal conductance ( gs w ) has been shown to respond to VPD l in some plant species. In this study, we present experimental data on leaf gas exchange and anatomical evaluations in three wild (non-commercial) and one commercial Phaseolus vulgaris L. cultivars, as well as a bibliographic review of the topic supporting that sensitivity of stomata to VPD l is species-specific. These results provide important implications for the interpretation of i WUE as a useful parameter to screen bean cultivars for identifying drought resistance forms.