Fig. 4.

Determination of the pH optimum of (A) the guaiacol-peroxidase activity and of (B) the NADH-peroxidase activity of four POD isoenzymes of the Mn-sensitive cowpea cultivar TVu 91. POD isoenzymes were eluted from BN gels that separated a mixture of AWF_H2O and AWF_NaCl extracted from the second oldest trifoliate leaf of Mn-treated (4 d) and ±Si-treated (as described in the Materials and methods) plants. Measurements were done in succinate buffer with pH values between 5.0 and 7.0 using 0.5 steps between the pH values. Measuring solution (0.1 M succinate buffer) for the determination of NADH-peroxidase activity consisted of 16 mM MnCl₂, 1.66 mM p-coumaric acid, and 0.22 mM NADH, measuring solution for guaiacol-peroxidase activity consisted of 18 mM guaiacol (in 90 mM succinate buffer) and 0.03% H₂O₂.