2.5. Antioxidant enzyme assays
Leaves of both cultivars were sampled on the 0, 7and 14 days
after stress treatments were used for enzyme analysis. Leaves were
frozen in liquid nitrogen immediately after harvesting and stored
at 20 C until enzyme assays. 1 g leaves homogenized in 3 ml
0.05 M Na phosphate buffer (pH 7.8) including 1 mM EDTA and 2%
(w/v) PVPP. The homogenatewere centrifuged at 13,000g for 40 min
at 4 C. Supernatant was used for enzyme activity and protein
content assays. All assays were done at 4 C. Total soluble protein
contents of the enzyme extracts were determined according to
Bradford [27] using BSA as a standard. All spectrophotometric analyses
were conducted on a Shimadzu (UV-1600) spectrophotometer.
Superoxide dismutase (SOD; EC 1.15.1.1) activity assaywas based
on the method of Beauchamp and Fridovich [28] which measures
the inhibition in the photochemical reduction of nitroblue tetrazolium
(NBT) spectrophotometrically at 560 nm. One unit of
enzyme activity was defined as the quantity of SOD required to
produce a 50% inhibition of reduction of NBT and the specific
enzyme activity was expressed as units mg1 protein. The reaction
mixture contained 50 mM Na phosphate buffer (pH 7.8), 33 mM
NBT, 10 mM L-methionine, 0.66 mM EDTA and 0.0033 mM riboflavin.
Reactions were carried out at 25 C, under light intensity of
about 300 mmol1 m1 s1 through 10 min.