The activity of glutathione reductase in the plant leaves was determined according to protocols by Cakmak and Marschner (1992) and Çakmak et al. (1994). A 0.7-mL portion of 50 mM phosphate buffer solution (pH 7.6, also containing 0.1 mM of Na-EDTA) was mixed with 0.1 mL of a 0.5 mM oxide glutathione (GSSG) solution, 0.1 mL of a 0.12 mM NADPH solution and 0.1 mL of the enzyme extract. The total volume of the mixture was adjusted to 1 mL, and its absorbance at 340 nm was measured.