Carbon isotopes and water use efficiency in C4 plants

Drought is a major agricultural problem worldwide. Therefore, selection for increased water use efficiency (WUE) in food and biofuel crop species will be an important trait in plant breeding programs. The leaf carbon isotopic composition (delta C-13(leaf)) has been suggested to serve as a rapid and effective high throughput phenotyping method for WUE in both C-3 and C-4 species. This is because WUE, leaf carbon discrimination (Delta C-13(leaf)), and delta C-13(leaf) are correlated through their relationships with intercellular to ambient CO2 partial pressures (C-i/C-a). However, in C-4 plants, changing environmental conditions may influence photosynthetic efficiency (bundle-sheath leakiness) and post-photosynthetic fractionation that will potentially alter the relationship between delta C-13(leaf) and C-i/C-a. Here we discuss how these factors influence the relationship between delta C-13(leaf) and WUE, and the potential of using delta C-13(leaf) as a meaningful proxy for WUE.