Causal Mediation Mechanism of Endogenous Hormones in Seedling Growth Response of Picea abies and Picea crassifolia to Post-Sunset Supplemental Light Durations

Post-sunset supplemental light promotes Picea seedling stem elongation, but the underlying hormonal regulation mechanisms on interspecific differences in spruce growth response to photoperiod remain unclear. This study aimed to clarify the physiological mechanism underlying the response of two Picea species to different supplemental light durations. Three-year-old seedlings of P. abies and P. crassifolia were subjected to 0 (CK), 4, 8, and 12 h of post-sunset supplemental light treatments for two growing seasons, with growth characteristics and endogenous hormone contents analyzed. The results showed that species and the interaction between species and photoperiod were the principal factors driving phenotypic divergence in spruce growth traits. Supplemental light treatments significantly promoted sustained growth of P. abies, with 4 h treatment being optimal. This treatment also resulted in the highest levels of gibberellins (GAs) and zeatin riboside (ZR), as well as the highest ratios of ZR/GAs. For P. crassifolia, supplemental light treatment promoted dry matter accumulation (8 h treatment being optimal) but had no significant effect on other growth traits, most endogenous hormones (ZR, IAA), and their ratios across treatments. Correlation and causal inference mediation analysis suggest that ZR and the ZR/IAA ratio could be the main factors driving shoot elongation. Thus, the findings provide a valuable insight for optimizing species-specific supplemental light regimes for seedling production in nurseries.