Quenching partitioning through light-modulated chlorophyll fluorescence: A quantitative analysis to assess the fate of the absorbed light in the field

Plants use a small part of the total absorbed light energy for net carboxylation, while the remaining amount is dissipated via alternative pathways involving thermal processes, fluorescence and non-carboxylation photochemistry in order to limit the formation of reactive oxygen species (ROS) and other photooxidative risks. The commonly used analysis of the Photosystem II (PSII) fluorescence signals gives qualitative information about absorbed light energy management by plants, but it is difficult to appreciate the relative contribution of each pathway in energy partitioning.This study reports the application of quenching partitioning through a chlorophyll fluorescence approach performed on peach leaves subjected to three different light intensities for four durations of exposure in absence of recovery from photo-damage. This methodology was compared with the P₇₀₀ redox kinetic method for determining the functional PSII fraction in leaves. In the absence of recovery processes the active PSII concentration decayed with an increase in photon exposure (the product of irradiance and the time of exposure), following an exponential pattern according to the reciprocity law. The photoprotective thermal dissipation (Φ_NPQ) was proportional to irradiance up to 30 min of photoinhibitory treatment. Afterwards Φ_NPQ was limited by the increasing competition for the absorbed energy re-emitted by the inactive PSII (Φ_NF). Φ_NF increased with the photon exposure dissipating up to 70% of the total incoming energy. The energy funnelled to photochemistry (Φ_PSII) decreased with increasing exposure time or intensity, becoming zero after 120 min of photoinhibitory treatment at the maximum irradiance (2100 μmol photon m⁻² s⁻¹). The relation between the fraction of energy dissipated by the inactive PSII (derived from the quenching partitioning) and the inactive PSII fraction (measured with the P₇₀₀ redox kinetic method) was linear.The quenching partitioning through light-modulated chlorophyll fluorescence is a useful tool to analyse plant energy management and gives also a reasonable estimation of the active PSII fraction. This methodology can easily be used in the field as measurements are rapid, non-destructive and detection devices are portable.