A field experiment was conducted in a free-air CO₂ enrichment (FACE) facility to investigate the effect of elevated atmospheric CO₂ on growth and physiology of green gram (Vigna radiata (L.) R.Wilczek) and associated weed species (Euphorbia geniculata Ortega and Commelina diffusa Burm.f.). Physiological and reproductive behaviour and interaction of the crop and two weed species under elevated CO₂ was also studied. Plants were grown under ambient (390 ± 5 ppmv) and elevated (550 ± 50 ppmv) CO₂. The results showed that growth, photosynthesis and carbonic anhydrase activity increased in all the test species. Stomatal conductance and transpiration decreased in V. radiata (5.1% and 30.5%, respectively) and C. diffusa (19% and 13.7%) but increased in E. geniculata (6.5% and 27.6%), suggesting a unique adaptive potential of E. geniculata at elevated CO₂. Higher accumulation of reactive oxygen species (hydrogen peroxide and superoxide) was noticed at elevated CO₂ in V. radiata than in E. geniculata and C. diffusa. Potential of E. geniculata to maintain redox homeostasis in its original state may provide an advantage over two other species in adaptation to climate change. Isoenzyme patterns of superoxide dismutase and stronger activity of antioxidant enzymes suggest species-specific differential regulation and induction of new isoforms under elevated CO₂. Enrichment of atmospheric CO₂ at a competitive density of weeds lowered the yield (12.12%) and quality of green gram seed, with diminished protein content (16.14% at ambient CO₂ to 15.42% at elevated CO₂) and enhanced carbohydrate content (3.11%). From the study, it may be concluded that a rise in atmospheric CO₂ concentration affects plant performance in a species-specific manner. Among the three species, E. geniculata emerged as most responsive to elevated CO₂, showing higher transpiration and stomatal conductance and a stronger antioxidant defence system in a higher CO₂ atmosphere. At elevated CO₂, weed–crop interaction altered in favour of weeds leading to considerable yield loss of green gram seed.