Assessing the photoprotective effectiveness of non-photochemical chlorophyll fluorescence quenching: A new approach

The photoprotective nature of non-photochemical quenching (NPQ) has not been effectively quantified and the major reason is the inability to quantitatively separate NPQ that acts directly to prevent photoinhibition of photosystem II (PSII). Here we describe a technique in which we use the values of the PSII yield and qP measured in the dark following illumination. We expressed the quantum yield of PSII (ΦPSII) via NPQ as: ΦPSII = qP × (Fv/Fo) / (1 + Fv/Fo + NPQ). We then tested this theoretical relationship using Arabidopsis thaliana plants that had been exposed to gradually increasing irradiance. The values of qP in the dark immediately after the illumination period (here denoted qPd) were determined using a previously described technique for Fo′ calculation: Fo′calc. = 1 / (1/Fo − 1/Fm − 1/Fm′). We found that in every case the actual ΦPSII deviated from theoretical values at the same point that qPd deviated from a value of 1.0. In an increasing series of irradiance levels, WT leaves tolerated 1000 μmol m− 2 s− 1 of light before qPd declined. Leaves treated with the uncoupler nigericin, leaves of the mutant lacking PsbS protein and leaves overexpressing PsbS showed a qPd reduction at 100, 600 and 2000 μmol m− 2 s− 1 respectively, each at an increasing value of NPQ. Therefore we suggest that this simple and timely technique will be instrumental for identifying photoprotective NPQ (pNPQ) and that it is more appropriate than the qE component. Its applications should be broad: for example it will be useful in physiology-based studies to define the optimal level of nonphotochemical quenching for plant protection and productivity.

► Effectiveness of non-photochemical fluorescence quenching (NPQ) is assessed.
► The method measures dark levels of PSII yield and qP after illumination.
► The relationships between PSII yield/qP and NPQ display the onset of photoinhibition.
► The level of protective NPQ (pNPQ) when reaction centres remain intact is determined.
► The maximum light intensity which is “safe” for plants to grow at can be evaluated.