Stable carbon isotope ratios (δ¹³C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ¹³C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C₃ plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO₂ concentrations (c_i/c_a), coordination between stomatal conductance and photosynthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C₄ plants, variation in bundle‐sheath leakiness to CO₂ can either damp or amplify the effects of c_i/c_a on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night‐time CO₂ fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

Contents Summary 950 I. Introduction 950 II. Carbon isotope discrimination 951 III. The C₃ photosynthetic pathway 951 IV. The C₄ photosynthetic pathway 957 V. Crassulacean acid metabolism 959 VI. Conclusion 961 Acknowledgements 961 Reference 961