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

Carbon isotope discrimination is a promising trait for indirect screening for improved water use efficiency of C crops. 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 C crops, a trait established to be indicative for WUE is the carbon isotopic composition (δC) of plant material, which reflects the preferential assimilation of the lighter carbon isotope C over C during photosynthesis. In C crops, carbon fixation is more complex and δC thus depends on many more factors than in C crops. Recent physiological and genetic studies indicate a correlation between δC and WUE also in C 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 δC has been shown in some of the main C crops, such as maize, sorghum and sugarcane, indicating its potential for indirect selection and breeding. Further research on physiological, genetic and environmental components influencing δC is needed to support its application in improving WUE and making C crops resilient to climate change.