Plant scientists use carbon isotope discrimination (Δ) to express the relative abundances in plant carbon of the isotopes carbon-12 (¹²C) and carbon-13 (¹³C). These stable isotopes are present in carbon dioxide (CO₂) in ambient air. They are incorporated by photosynthetic CO₂ assimilation into plant organic matter, but the two isotopes do not appear in plants in the same proportions as in the air. Carbon isotope discrimination by plants has been studied extensively in agricultural and native species (such as wheat, rice, and crested wheatgrass), often together with plant water-use efficiency and yield.