Effect of elevated carbon dioxide on the growth, development, and nutrient composition of C₃ and C₄ functional groups

The concentration of CO₂ ([CO₂]) in the atmosphere has been rapidly increasing over recent decades and is projected to reach 1000 ppm (parts per million) by 2100. This rise in [CO₂] has the potential to impact plant growth and development. Different functional groups of plants have been suggested to respond differently to increases in [CO₂]. In this study, we investigated five crop species belonging to two functional groups represented by one plant species, C₃ dicot (cotton), C₃ monocot (wheat), nodule-forming and nitrogen-fixing C₃ dicot (soybean), C₄ monocot (sorghum), and a C₄ dicot (Amaranthus). We hypothesized that the C₄ functional groups would exhibit a saturated response at current [CO₂] levels, while C₃ plants will continuously respond positively to increased [CO₂]. The five plant species were grown in sunlit plant growth chambers under six [CO₂] levels ranging from 320 ppm to 820 ppm in 100 ppm increments from planting till 34 days. C₃ species, mainly cotton and soybean, exhibited significant increases in leaf area (74%), shoot dry weight (87%), and total biomass under elevated [CO₂], while C₄ species showed minimal response. Root weight and root-by-shoot ratio of all the crops except cotton (69% increase in root weight) were unaffected by increases in [CO₂]. The overall micro- and macro-nutrient composition of leaves decreased under elevated [CO₂], with notable exceptions in zinc for Amaranthus and copper for wheat. The results showed that C₃ plants, mainly dicots, are more responsive to increases in [CO₂], likely due to their photosynthetic mechanism, while C₄ plants showed limited or no response. The study highlights the varying responses of different functional groups to increasing [CO₂] and emphasizes potential challenges for the nutrient quality of crops in the future.