Fig. 1.
Associations between water use efficiency (WUE) and the carbon isotopic composition of C4 plant material. Negative effects are depicted by light orange arrows, positive effects are depicted by dark green arrows. The WUE of a plant (WUEplant) can be assessed by the destructive measurement of biomass in relation to the sum of water transpired by the plant. The biomass, which the plant accumulates, depends on assimilation rate and respiration, while the water transpired by the plant depends on the stomatal conductance, as well as night time transpiration and the vapor pressure deficit of the air over its lifetime. The intrinsic WUE (iWUE) is defined as the ratio of assimilation rate over stomatal conductance of a leaf section at a specific time and is by definition related to the ratio of the intercellular CO2 concentration (Ci) to the ambient CO2 concentration (Ca; Yang et al. 2016). This ratio of Ci/Ca is theoretically negatively correlated to the discrimination against the 13C isotope during assimilation (∆13C), when the influence of leakiness is stable below 0.37 as it was observed, e.g., in Henderson et al. (1992). The isotopic composition of tissues like leaves and grains (δ13C) is an indirect and integrated measure for ∆13C, when the isotopic composition of the air (δ13Cair) is accounted for. Post-photosynthetic fractionations influence δ13C further as these fractionations lead to distinct isotopic signatures of different plant compounds, which through their relative contribution to the composition of a tissue determine its δ13C