The effect of Mn deficiency on plant growth and activities of superoxide dismutase (SOD) was studied in hydroponically-grown seedlings of transgenic tobacco (Nicotiana tabacum L.) engineered to overexpress FeSOD in chloroplasts or MnSOD in chloroplasts or mitochondria. In comparison to the non-transgenic parental line, the activity of MnSOD in the lines overproducing MnSOD was 1.6-fold greater, and the activity of FeSOD in the FeSOD-overproducing lines was 3.2-fold greater, regardless of the Mn treatment (deficient or sufficient). The MnSOD activities decreased due to Mn deficiency, while activities of FeSOD and Cu/ZnSOD remained unaffected 25 d after transplanting (DAT). With an increased duration of the Mn deficiency stress (45 DAT), FeSOD activity decreased, and that of MnSOD continued to decrease, while Cu/ZnSOD activity simultaneously increased. Under Mn sufficiency, non-transgenic parental plants had greater shoot biomass than the transgenics; however, when subjected to Mn deficiency stress, non-transgenic parents suffered a proportionally greater growth reduction than transgenic lines. Thus, overproduction of MnSOD in chloroplasts may provide protection from oxidative stress caused by Mn deficiency. (C) 1999 Annals of Botany Company.