Chlorophyll Fluorescence as a Tool to Monitor Plant Response to the Environment

Chlorophyll (Chi) fluorescence is an intriguing tool that can reveal information on plant performance and protective responses via rapid, non-intrusive measurements. Yet, if their limitations are not recognized, these measurements can be misleading and erroneous. A robust parameter obtained from Chi fluorescence in the field is the potential photochemical efficiency of (open) Photosystem II (PS II) units, either in darkness before sunrise (F_v/F_m, where F_v is variable Chi fluorescence, F_m – F₀, and F_m and F₀ are maximal and minimal fluorescence, respectively) or during exposure to light (F′_v/F′_m). Changes in this potential for photochemical utilization of light absorbed in PS II antennae can provide insight into changes in plants’ photosynthetic performance over the course of the day and between seasons. Decreases in F_v/F_m or F′_v/F′_m can be due to either increases in (the rate constant for) photoprotective energy dissipation or to decreases in (the rate constant for) photochemistry, and it is not possible to discern between these two possibilities without additional accompanying measures. Without a doubt, predawn depressions in PS II efficiency F_v/F_m suggest that the plant is experiencing stress, but the nature of the stress and the underlying mechanism responsible for the depression in PS II efficiency require additional investigation before any conclusions can be drawn. The determination of thermal energy dissipation activity from non-photochemical quenching (NPQ) is useful, but requires a control Fm value determined prior to sunrise that is fully relaxed. If the plant is under any stress at all, it is often impossible to obtain a fully relaxed F_m value. Photochemical quenching qp is sensitive to underestimation (overestimation of the degree to which PS II is reduced) at high levels of energy dissipation activity, and its utilization to estimate PS II electron transport rates should be viewed with caution.