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

To determine whether photoperiod influences integrated physiological and molecular mechanisms underlying cadmium (Cd) uptake and tolerance in <i>Sedum alfredii</i>, 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 (<i>HMA</i> and <i>ZIP</i> 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 <i>S. alfredii</i>'s effectiveness in phytoremediation of Cd-contaminated soils.