Abstract
Regulated heat dissipation under excessive light comprises a complexity of mechanisms, whereby the supramolecular light-harvesting pigment–protein complex (LHC) shifts state from light harvesting towards heat dissipation, quenching the excess of photo-induced excitation energy in a non-photochemical way. Based on whole-leaf spectroscopy measuring upward and downward spectral radiance fluxes, we studied spectrally contiguous (hyperspectral) transient time series of absorbance A(λ,t) and passively induced chlorophyll fluorescence F(λ,t) dynamics of intact leaves in the visible and near-infrared wavelengths (VIS–NIR, 400–800 nm) after sudden strong natural-like illumination exposure. Besides light avoidance mechanism, we observed on absorbance signatures, calculated from simultaneous reflectance R(λ,t) and transmittance T(λ,t) measurements as A(λ,t) = 1 − R(λ,t) − T(λ,t), major dynamic events with specific onsets and kinetical behaviour. A consistent well-known fast carotenoid absorbance feature (500–570 nm) appears within the first seconds to minutes, seen from both the reflected (backscattered) and transmitted (forward scattered) radiance differences. Simultaneous fast Chl features are observed, either as an increased or decreased scattering behaviour during quick light adjustment consistent with re-organizations of the membrane. The carotenoid absorbance feature shows up simultaneously with a major F decrease and corresponds to the xanthophyll conversion, as quick response to the proton gradient build-up. After xanthophyll conversion (t = 3 min), a kinetically slower but major and smooth absorbance increase was occasionally observed from the transmitted radiance measurements as wide peaks in the green (~ 550 nm) and the near-infrared (~ 750 nm) wavelengths, involving no further F quenching. Surprisingly, in relation to the response to high light, this broad and consistent VIS–NIR feature indicates a slowly induced absorbance increase with a sigmoid kinetical behaviour. In analogy to sub-leaf-level observations, we suggest that this mechanism can be explained by a structure-induced low-energy-shifted energy redistribution involving both Car and Chl. These findings might pave the way towards a further non-invasive spectral investigation of antenna conformations and their relations with energy quenching at the intact leaf level, which is, in combination with F measurements, of a high importance for assessing plant photosynthesis in vivo and in addition from remote observations.
Highlights
Photosynthetic light-harvesting complexes (Lhcs) are sophisticated multichromophoric assemblies used to regulate and concentrate photo-excitations under wide-ranging incident irradiances for delivery to the reaction centres (Scholes et al 2011)
There is still debate on how and where the ΔpH-dependent and ΔpH-independent mechanisms take place in the Lhcs associated with the two photosystems (PSI and photosystem II (PSII)), forming the supramolecular light-harvesting pigment–protein complexes (PSI-LHCI and PSII-LHCII)
We show examples in which slow chloroplast motion was only slightly or not observed at all, as it is not part of the regulated photoprotection and it complicates the detection of other features, which may be of interest
Summary
Photosynthetic light-harvesting complexes (Lhcs) are sophisticated multichromophoric assemblies used to regulate and concentrate photo-excitations under wide-ranging incident irradiances for delivery to the reaction centres (Scholes et al 2011). To protect themselves and the reaction centres from a potentially harmful solar irradiance excess, several regulated photoprotection mechanisms are activated at different time scales at the level of these complexes, balancing. Photosynthesis Research (2019) 142:283–305 out the given energy supply This regulated lowering of the excitation pressure on the reaction centres decreases photochemical quenching as energy is non-photochemically quenched inside the leaves by various physical and chemical signals (Demmig-Adams and Adams III 1992). The fast or so-called energy-dependent quenching (qE) mechanism has been assigned to thermal deactivation of singlet excited chlorophyll (1Chl*) in the antenna of photosystem II (PSII), lowering the quantum yield of fluorescence (F) (Niyogi 1999). Different mechanisms contribute to non-photochemical energy quenching (NPQ) in the shortterm, qE is typically presented as the dominating form of controlled energy dissipation in leaves under most natural conditions (Holzwarth et al 2009). Questions still remain on how these mechanisms are triggered, and how they occur in vivo for different plant species as they are typically indirectly observed both in vivo and in vitro through fluorescence dynamics and accompanying shifts
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