Fig. 4. Cell wall peroxidase activities in control non-inoculated (empty bars) or Azo- spirillum-inoculated (filled bars) cucumber hypocotyl basal or apical segments. (A) Total peroxidase activity assayed with guaiacol as a substrate, (B) ferulic acid peroxi- dase activity assayed against ferulic acid, and (C) NADH oxidase activity. Activities were determined by measuring the initial rate of the decrease of absorbance at corre- sponding maxima. Reactions were followed spectrophotometrically. Total peroxidase activity was measured at 470 nm for 5 min. Blanks were measured without H2O2 added. Results are expressed as units of total peroxidase. g DW 1. One U 1⁄4 mmol of tetraguaiacol min 1. Ferulic acid peroxidase activity was measured at 310 nm. One unit of Ferulic acid peroxidase was defined as a decrease of A310 min 1. NADH oxidation by peroxidases was measured at 340 nm for 4 min with 50 mM NADH at pH 6.5. Results are expressed as units of NADH peroxidase. g DW1. One U 1⁄4 DA340 min 1. Results are shown as mean S.D. (P < 0.05), obtained from three replicates of 1.2 g each. Columns denoted by different letters differ significantly at P < 0.05.
If elongation was a pure enzyme-catalyzed process, an Azospir- illum augmented peroxidase activity could be hypothesized. However, as it was stated [29], segment elongation is an enzyme mediated process and a ROS induced mechanism. It is known that cellular expansion zones have a greater ROS production [34,35].