Photosystem II photoinhibition and photoprotection in a lycophyte, Selaginella martensii

Abstract

The Lycophyte Selaginella martensii efficiently acclimates to diverse light environments, from deep shade to full sunlight. The plant does not modulate the abundance of the Light Harvesting Complex II, mostly found as a free trimer, and does not alter the maximum capacity of thermal dissipation (NPQ). Nevertheless, the photoprotection is expected to be modulatable upon long‐term light acclimation to preserve the photosystems (PSII, PSI). The effects of long‐term light acclimation on PSII photoprotection were investigated using the chlorophyll fluorometric method known as “photochemical quenching measured in the dark” (qPd). Singularly high‐qPd values at relatively low irradiance suggest a heterogeneous antenna system (PSII antenna uncoupling). The extent of antenna uncoupling largely depends on the light regime, reaching the highest value in sun‐acclimated plants. In parallel, the photoprotective NPQ (pNPQ) increased from deep‐shade to high‐light grown plants. It is proposed that the differences in the long‐term modulation in the photoprotective capacity are proportional to the amount of uncoupled LHCII. In deep‐shade plants, the inconsistency between invariable maximum NPQ and lower pNPQ is attributed to the thermal dissipation occurring in the PSII core.

Long-Term Alleviation of the Functional Phenotype in Chlorophyll-Deficient Wheat and Impact on Productivity: A Semi-Field Phenotyping Experiment

“…However, NPQ MAX , as a technical parameter is complex in origin, and includes not only pH-dependent de-excitation processes (“high-energy quenching”, qE), but also other components, e.g., related to the plastid redox state or the integrity of photosystems [ 38 , 52 , 53 , 54 , 55 , 56 , 57 ]. Not all NPQ components have a primary photoprotective meaning, and high levels of NPQ may not correspond to high levels of PSII photoprotection [ 58 , 59 ]. Interestingly, a comparison of NPQ MAX and pNPQ makes it clear that the former may underestimate the severity of the photosynthetic-regulation defect, particularly in the “intermediate” mutants 7A and 7B, and also 4B ( Figure 3 ).…”

“…The leaves were then dark-acclimated for 30 min, and analysed using a Junior-PAM (Heinz Walz, Effeltrich, Germany) fluorometer. Measurements were performed as described by Colpo et al [ 59 ], with some modifications. Minimum and maximum fluorescence ( F 0 and F M , respectively), were measured before starting the light-response-curve experiment.…”

“…NPQ was calculated as ( F M −F M ’ )/ F M ’ , and qP as ( F M ’−F )/( F M ’−F 0 ’ ), where F is the fluorescence level recorded just before applying the saturation pulse [ 32 ]. Photoprotective thermal dissipation (pNPQ) was calculated by applying the qP d protocol, as described by Ruban and Murchie [ 39 ], with modifications as detailed by Colpo et al [ 59 ]. The theoretical fundament of the “qP d model” is based on the comparison between actual F’ 0 and the theoretical F’ 0 value, calculated according to Oxborough and Baker ( F’ 0 calc ) [ 69 ].…”

“…In the model, the threshold used to assess the occurrence of PSII photoinhibition was set at qP d = 0.98; the NPQ associated with the last qP d > 0.98 is pNPQ [ 39 ]. In the case where qP d assumes values above 1 because of significant PSII-antenna uncoupling, the determination of pNPQ is still possible, with suitable corrections [ 59 , 70 ].…”

Two Congeneric Shrubs from the Atacama Desert Show Different Physiological Strategies That Improve Water Use Efficiency under a Simulated Heat Wave

“…Here, the values obtained for NPQ components qE, qP and qL reveal different mechanisms in the Atriplex species to deal with light energy after HW (Figure 1D−F). Specifically, the higher values of q E in A. clivicola suggest that HW would affect the capability of the photochemical apparatus to use light energy in photochemistry and must activate the components that rapidly relax high-energy state quenching in order to drain the excess of light energy and avoid potential photoinhibition and photo-oxidative stress [43][44][45]. On the other hand, HW did not alter the photosystem functioning of A. deserticola, which maintained qP and qL values without significant variations compared to those observed before HW and were comparatively higher than those of A. clivicola after HW.…”