The reaction mixture absorbance was measured by the Spectrum SP 2000UV Spectrophotometer at 560 nm and one unit of SOD activity (UA) was defined as the amount of enzyme required to cause a 50% reduction in the NBT photo-reduction rate. Results were expressed as UA/mg P (protein) (Beauchamp & Fridovich, 1971; Giannopolitis & Ries, 1977).