Fig. 4. Activity of glutathione reductase (units gfw^–1) in cotton tissues from early planting (DOY 113) in 2003 and late plantings (DOY 133) in 2003 and 2004 at Lubbock, TX. A unit of activity is the amount of enzyme that will catalyze the reduction of 1 µmol of GSSG per min at 25°C. Each data point represents a minimum of three samples. Error bars indicate standard error of the mean.