The carbon isotopic signature of C₄ crops and its applicability in breeding for climate resilience

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.