Energy dissipation
NPQ exhibited a decreasing tendency following a slow increase (Fig. 3B). The slow NPQ development followed a single exponential function and as a consequence fast-activated qE component was absent, while the NPQmaxshowed a low value (Fig. 3C). Moreover, proteome data showed that the luminal pH sensor PsbS protein, which was required for qE, remained unchanged in response to light exposure (Table S1). Slightly upregulated zeaxanthin epoxidase (ZEP) and violaxanthin de-epoxidase (VDE) levels indicated a low level of xanthophyll cycle-dependent energy dissipation (Table S1). The two key enzymes of chlororespiration, post-illumination chlorophyll fluorescence increase and ubiquinol oxidase, remained unchanged. Similarly, D-glycerate 3-kinase, the core enzyme in photorespiration pathway, both ascorbate peroxidase (APX) and superoxide dismutase, the key enzymes of the Mehler reaction, and malate dehydrogenase, the key enzyme of malic acid synthesis, remained unchanged (Table S1). These results suggested that alternative electron flows associated with energy dissipation were not significantly activated.
Antioxidant system defense
Both the chlorophyll synthesis pathway and the early light-induced protein were not significantly induced by light exposure (Fig. 2A and Table S1), suggesting that there was no accumulation of free chlorophyll which would have acted as a generator of reactive oxygen species. Although Delta-aminolevulinic acid dehydratase, the enzyme committed to tetrapyrrole biosynthesis, and geranylgeranyl diphosphate reductase, which provides phytol, were upregulated, the terminal key enzyme NADPH-protochlorophyllide oxidoreductase of chlorophyll synthesis was downregulated (Fig. 2A). The expression of the stromal antioxidant enzymes glutathione peroxidase (GPX) and APX remained unchanged (Table S1), further displaying that the antioxidant ability was not significantly enhanced following light exposure.