Rising atmospheric [CO ₂] causes global warming but may also benefit photosynthesis and yield of C3 crops such as rice. Previous research showed that positive effects depend on a cultivar’s sink-source ratio as sink limitation incurs acclimation of photosynthesis to elevated [CO ₂] (e-CO ₂). To enable breeding for e-CO ₂ response, predictive, easily measurable proxy traits under ambient [CO ₂] are needed. The local source-sink ratio (LSSR: flag leaf/panicle size) is a potential proxy trait, proposed by a previous study. We evaluated this and similar trait indices for two diverse rice cultivar samples under e-CO ₂ vs ambient level in controlled environments. The significant negative effect of genotypic LSSR on maximum photosynthesis ( A _max) under e-CO ₂, and a similar but weaker effect on the grain yield response, was confirmed. However, LSSR observed was more predictive under e-CO ₂ than ambient, rendering this proxy trait impractical for field-based selection. This difference was due to the phenotypic plasticity of LSSR between [CO ₂] levels in our populations. Variants of LSSR incorporating SPAD leaf chlorophyll content and panicle sink capacity improved LSSR predictive power under ambient [CO ₂] for A _max. We conclude that genotypic sink-source ratio is an important physiological determinant of [CO ₂] response, but proxy traits need to be further refined and field-validated to become useful selection or phenotyping tools for improved e-CO ₂ response of rice.