Static and dynamic acclimation mechanisms to extreme light intensities in Hedera helix (Ivy) plants

Under natural conditions, plants face the need to acclimate to widely varying light intensities to optimize photosynthetic efficiency and minimize photodamage. Studying the mechanisms underlying these acclimation processes is essential for understanding plant productivity and resilience under fluctuating environmental conditions. This study aimed to investigate static and dynamic acclimation mechanisms in Hedera helix (Ivy) plants under two extreme light conditions spanning the range of their adaptive abilities, deep shade (LL, ~5 μmol photons m⁻² s⁻¹) to full sunlight (HL, ~2000 μmol photons m⁻² s⁻¹), focusing on their structural and functional acclimation. LL and HL plants were examined for their leaf structure, chlorophyll and carotenoid contents, and photosynthetic protein levels. Dynamic responses were evaluated through chlorophyll fluorescence spectroscopy, measuring the effective photosynthetic unit size (σ) and the capacity for non-photochemical quenching (NPQ). HL plants exhibited a ~ 78% lower chlorophyll contents as compared to LL and increased chlorophyll a/b ratios. The carotenoid content of HL plants was ~94% lower, while the PsbS content increased fivefold. These results may indicate a smaller HL effective antenna size. However, σ fast fluorescence kinetics analysis indicated the opposite. NPQ analysis demonstrated that both compositions of the photosynthetic systems supported the ability to quench access energy. HL plants had a large dynamic range for NPQ and faster on/off kinetics. Our finding suggests massive changes in the organization of the photosynthetic apparatus. These modifications preserve a large dynamic range for reacting to light intensity under both conditions.