The transport of photoassimilates in higher plants plays a crucial role in their growth and storage processes, primarily facilitated by the phloem system. This transport occurs through a mass flow mechanism driven by hydrostatic pressure differentials, which are influenced by osmotic water movement generated by solute transport across membranes. Among these solutes, sucrose is crucial in generating osmotic potential within sieve tubes. Reverse genetics has demonstrated the critical role of sucrose/proton symporters from the SUC/SUT family in phloem loading and transport processes. Additionally, aquaporins from the PIP2 subfamily have been detected along the phloem pathway. This suggests their involvement in facilitating water exchanges between sieve tubes and adjacent cells, potentially aiding in radial water exchanges between the xylem and phloem. In this study, we cultivated both PIP2 knock-out quintuple mutant and transgenic lines with defective SUC2 expression under two atmospheric CO2 conditions (ambient or elevated). This approach allowed us to observe the effects of elevated CO2 (eCO2) on photosynthesis, carbon fixation, and carbohydrate storage, and how these factors influence carbon allocation for plant growth and root development. The results, which showed increased root and shoot biomass under eCO2, were as expected. However, the observation of elevated leaf temperature in both PIP2 and SUC2-defective lines, regardless of the CO2 conditions, was not anticipated, indicating a defect in evapotranspiration. Despite this, under eCO2, both PIP2 mutant and SUC2-defective lines demonstrated more efficient translocation of carbon to the roots compared to wild-type plants. However, the consequences on root biomass and sugar accumulation in the roots varied between the lines. This study provides a new perspective on the complex interplay between carbon allocation and water flow regulation in plants, particularly under different atmospheric CO2 conditions. Further investigations are warranted to fully understand the integration of the processes coupling carbon allocation and water flows.