Here Comes the Sun: Xanthophylls as Components of Plant Memory

It has been proposed that the de-epoxidation state of the xanthophyll cycle (AZ/VAZ) represents a mechanism of light memory storage in plants as it depends on the amount of light previously received by the leaf, later affecting the photosynthetic responses by modulating the rate of non-photochemical quenching (NPQ). In this study we examine what the time window is for which the cumulative photon dose (CumPFD) best explains AZ/VAZ. For this purpose, we developed a field experiment in a boreal forest in Finland on 17 species representing all major functional groups. In all species AZ/VAZ correlated more strongly with CumPFD than with instantaneous light intensity, supporting its role as a form of light memory. Besides AZ/VAZ was linked to photochemical efficiency, demonstrating that it influences current photosynthetic performance. However, the duration of this memory differed among species, with some (mosses, ferns, herbs and deciduous woody plants) where AZ/VAZ best reflected CumPFD of the previous 2-5 h (short-term memory) and others (mainly evergreen species) where AZ/VAZ integrated CumPFD received for 10-15 h (long-term memory). Overall, these results show that AZ/VAZ fulfills the concept of a memory mechanism. Additionally, they pinpoint that this form of storage is transient, varies with plant functional types, and generally vanishes within 24 h.