Effects of elevated CO<sub>2</sub> and cadmium stress on vegetable quality and cadmium accumulation

The aim of this study was to determine the effects of elevated CO₂ (eCO₂) on the quality and cadmium (Cd) accumulation of vegetables grown in soil with a background Cd level or in Cd-contaminated soil. We used four types of vegetable: pak choi (<i>Brassica rapa</i> L.), water spinach (<i>Ipomoea aquatica</i> Forssk.), cherry radish (<i>Raphanus raphanistrum</i> subsp. <i>sativus</i> (L.) Domin) and pepper (<i>Capsicum annuum</i> L.). Cd stress significantly reduced vegetable vitamin C and reducing sugar content; however, under eCO₂, vitamin C and reducing sugar content levels were significantly higher than they were under ambient CO₂ (aCO₂) levels. The nitrate content of pak choi, cherry radish and pepper was reduced under eCO₂, and the effect was more pronounced among plants grown under Cd stress. The Cd content of cherry radish and pepper roots grown under eCO₂ and Cd stress was significantly reduced (29.2% and 18.5%, respectively) compared with plants grown under aCO₂ conditions and Cd stress. The Cd transfer coefficient of pak choi and water spinach grown under eCO₂ and Cd stress was significantly lower (22.7% and 25.2%, respectively) than under aCO₂ conditions. Our data suggest that growing vegetables under eCO₂ is beneficial, especially when grown in Cd-contaminated soil, because Cd accumulation is reduced and vegetable quality is improved.