Figure 1. (A) The ratio of constant fluorescence and the peak of variable fluorescence (O/P) and (B) chlorophyll content in flag leaves from 12 cultivars of winter wheat under heat stress conditions. Chlorophyll a fluorescence and chlorophyll contents were measured on the same flag leaves after 0, 8, 10, 12, 14, and 16 d of exposure to heat stress. Increases in O/P indicate damage to thylakoid membranes, the greater the damage the lower the tolerance to heat stress (Ristic and Cass, 1993). Plotted data are from Experiment 1. Bars indicate ±standard errors; n = 5. Similar results were observed in a duplicate experiment.