Abstract: Elucidating how plant species respond to variable light conditions is essential to understanding the ecological adaptation to heterogeneous environments. Plant performance and gene regulatory network underpinning the adaptation have been well documented in heliophytic species. However, it remains largely unclear how the sciophytic plants respond to distinct light conditions. We measured phenotypic and transcriptomic features of four sciophytic (Fatsia japonica, Metapanax delavayi, Heptapleurum arboricola, and Heptapleurum delavayi) and one heliophytic woody species (Tetrapanax papyrifer) of the Araliaceae family under distinct light conditions. Our phenotypic comparisons demonstrate that the four sciophytic species maintain similar photosynthesis efficiency between high light and low light conditions. However, a significantly decreased photosynthesis rate was observed under the low light conditions of the heliophytic species compared with the high light conditions. In addition, our leaf anatomical analyses revealed that, while all five species showed different anatomical structures under distinct light conditions, the sciophytic species possessed a lower degree of phenotypic plasticity relative to the heliophytic species. Further comparisons of the transcriptome profiling showed that differentially expressed genes identified in the five species were functionally related to photosynthesis, secondary metabolites, and other basic metabolisms. In particular, differential regulation of the photosynthesis-related and photomorphogenesis-related genes were potentially correlated with the phenotypic responses to the distinct light conditions of the five species. Our study provides evolutionary and ecological perspectives on how the heliophytic and sciophytic woody species respond to shade and sunlight environments.

Key words: Araliaceae, divergent adaptation, evolution, phenotypic plasticity, shade avoidance, shade tolerance