Stella Eggels, Sonja Blankenagel, Chris-Carolin Schön & Viktoriya Avramova

Theoretical and Applied Genetics June 2021; vol. 134:1663–1675

Key message

Carbon isotope discrimination is a promising trait for indirect screening for improved water use efficiency of C₄ crops.

Abstract

In the context of a changing climate, drought is one of the major factors limiting plant growth and yield. Hence, breeding efforts are directed toward improving water use efficiency (WUE) as a key factor in climate resilience and sustainability of crop production. As WUE is a complex trait and its evaluation is rather resource consuming, proxy traits, which are easier to screen and reliably reflect variation in WUE, are needed. In C₃ crops, a trait established to be indicative for WUE is the carbon isotopic composition (δ¹³C) of plant material, which reflects the preferential assimilation of the lighter carbon isotope ¹²C over ¹³C during photosynthesis. In C₄ crops, carbon fixation is more complex and δ¹³C thus depends on many more factors than in C₃ crops. Recent physiological and genetic studies indicate a correlation between δ¹³C and WUE also in C₄ crops, as well as a colocalization of quantitative trait loci for the two traits. Moreover, significant intraspecific variation as well as a medium to high heritability of δ¹³C has been shown in some of the main C₄ crops, such as maize, sorghum and sugarcane, indicating its potential for indirect selection and breeding. Further research on physiological, genetic and environmental components influencing δ¹³C is needed to support its application in improving WUE and making C₄ crops resilient to climate change.

See: https://link.springer.com/article/10.1007/s00122-020-03761-3

Figure 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 (WUE_plant) 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 (C_i) to the ambient CO₂ concentration (C_a; Yang et al. 2016). This ratio of C_i/C_a 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 (δ¹³C_air) 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