Influence of photoperiod on endogenous phytohormone levels and cadmium-related gene expression in Sedum alfredii

To determine whether photoperiod influences integrated physiological and molecular mechanisms underlying cadmium (Cd) uptake and tolerance in Sedum alfredii, plants were exposed to varying day lengths (2-24 h). Distinct photoperiod-dependent trends emerged: very short photoperiods primarily stimulated stress-related hormone signaling and early-stage flavonoid synthesis, while an intermediate photoperiod (~10 h) concurrently enhanced growth-promoting hormones, jasmonate signaling, and antioxidant defenses. This optimal photoperiod elicited a coordinated peak in phytohormonal responses, antioxidant enzyme activities, and Cd transporter gene expression. Canonical correspondence analysis identified three major co-regulatory modules integrating hormonal signaling, secondary metabolism, and heavy-metal transport. These modules included an auxin - IAA oxidase network, an ABA - phenolic metabolism axis linked to key metal transporter genes (HMA and ZIP families), and a cytokinin - flavonoid pathway associated with additional Cd transporters. The convergence of these modules underscores a systemic regulatory mechanism balancing plant growth, defense responses, and heavy-metal management. These findings provide a mechanistic understanding of how photoperiodic signals modulate endogenous hormone networks and associated molecular processes to optimize Cd accumulation and tolerance. This study thus identifies photoperiod as a pivotal environmental cue that could inform strategies to enhance S. alfredii's effectiveness in phytoremediation of Cd-contaminated soils.