Quick conversions and de novo synthesis within the entire α- and β-carotenoid branches during non-steady-state light transients.

Understanding the rapid adjustments of plants to high-light exposure remains challenging, as multiple excitation and de-excitation pathways are simultaneously activated. In this study, we examined carotenoid pigment conversions at the second-scale in three tree species in parallel with high temporal resolution (<1 s) in vivo fluorescence and absorption spectroscopy. Our results reveal that both β-branch (violaxanthin, antheraxanthin, zeaxanthin) and α-branch (lutein, lutein epoxide) xanthophylls exhibit remarkably fast and oscillating pool dynamics within the first 20 seconds of illumination, reaching even maximal values in that timeframe. Prompt (0-20 s) conversion of the lutein is observed at the expense of both lutein epoxide and α-carotene in certain species, while accumulation of antheraxanthin and zeaxanthin is seen both prompt (0-20 s) and slower (>30 s). Interestingly, mirror trends between whole α- and β-branch carotenoids seem to indicate balancing trends, involving dynamic precursor shifts between α- and β-carotenes. Further, we observe that quick xanthophyll changes match the kinetic trends of fitted Gaussian-modeled absorbance peaks (approx. at 520, 535, 560 nm) within the early seconds. These quick changes in photon absorption are followed by slower-triggered non-photochemical de-excitation through a particular xanthophyll, seen from the dominant 535-nm peak, and likely attributed to antheraxanthin or zeaxanthin. The quick xanthophylls conversion redistributing the excessive excitation energy while quenching fluorescence (EET phase) is shown as one of the first responses to excessive light, before regulated energy dissipation as heat is initiated. These observations invite to interpret the non-steady state conditions and their parametrization more carefully, considering different photoprotective strategies across species.