P>Many wetland plants produce aquatic adventitious roots from submerged stems. Aquatic roots can form chloroplasts, potentially producing endogenous carbon and oxygen. Here, aquatic root photosynthesis was evaluated in the wetland plant Meionectes brownii, which grows extensive stem-borne aquatic roots during submergence.Underwater photosynthetic light and CO2 response curves were determined for aquatic-adapted leaves, stems and aquatic roots of M. brownii. Oxygen microelectrode and 14CO(2)-uptake experiments determined shoot inputs of O-2 and photosynthate into aquatic roots.Aquatic adventitious roots contain a complete photosynthetic pathway. Underwater photosynthetic rates are similar to those of stems, with a maximum net photosynthetic rate (P-max) of 0.38 mu mol O-2 m-2 s-1; however, this is c. 30-fold lower than that of aquatic-adapted leaves. Under saturating light with 300 mmol m-3 dissolved CO2, aquatic roots fix carbon at 0.016 mu mol CO2 g-1 DM s-1. Illuminated aquatic roots do not rely on exogenous inputs of O-2.The photosynthetic ability of aquatic roots presumably offers an advantage to submerged M. brownii as aquatic roots, unlike sediment roots, need little O-2 and carbohydrate inputs from the shoot when illuminated.