Validation of the F
threshold test for viability. Drought-affected Columbia (◇), Landsberg (□) and C24 (Δ) plants were rehydrated after their Fv/Fm levels were observed to fall below 0.750. Filled symbols represent plants that recovered within 3 days of rehydration, while empty symbols represent plants that failed to evidence signs of recovery following watering. The 33% threshold for a typical average control Fv/Fm of 0.800 is shown as a dotted line. (a) Fv/Fm measurements of individual specimens immediately prior to rehydration. For each line, n = 20. (b) Change in Fv/Fm of drought-treated plants following rehydration. Columbia, Landsberg and C24 plants were rewatered after 14, 15 and 16 days' drought respectively, as indicated by arrows. For each line, n = 6. The data presented in Figures 3a and 3b were obtained from separate experiments.