CWunit. Three subsamples were collected from each site every month.
Fresh roots (0.5 g) of each sample were ground with a pre-chilled mortar
and pestle in an ice bathwith 2.5mLof 0.05mol·L−1phosphate buffer
(pH 7.8). The homogenate was centrifuged at 10,000 rpm for 10 min
at 4 °C. The supernatantwas separated for the following enzyme assays.
Superdismutase (SOD) activity was estimated by measuring the inhibition
of the photochemical reduction of nitrobluetetrazolium (NBT)
(Beauchamp and Fridovich, 1971). One unit of SOD was defined as the
amount of enzyme that produced 50% inhibition of NBT reduction. Peroxidase
(POD) activity was estimated according to Chance and Maehly
(1955) with some modifications. One unit of POD activity was defined
as an absorbance change of 0.01 units·min−1. The detailed methods
for the antioxidative enzymatic measurements can be found in SI.