Figure 3 Typical chlorophyll fluorescence emission trace for a dark-adapted leaf made with a “Kautsky”-type fluorometer. Interpretation: A is the point illuminated by a pulse of non-saturating actinic5 light; B is the chlorophyll emission when all reaction centers are open, i.e. unquenched where NPQ = 0; C is the point illuminated by a pulse of saturating light; D is the emission peak, E is the emission approaching steady state; F is the emission when all reaction centers have relaxed. F0 = non-electron transport-stimulating emission ‘background’ eliciting the minimum fluorescence value; Fm is the maximum fluorescence; Fv is variable fluorescence = Fm – F0; F′ (F-prime), steady state fluorescence in light. If the plant is under stress, e.g. from lack of moisture or exposure to pathogens, herbicide, or extreme temperature, the emission trace might look like the slowly relaxing upper green dotted line.