Effects of shading stress on endogenous hormone levels in Eleutherococcus giraldii: hormonal dynamics and growth strategy analysis.

Background: Eleutherococcus giraldii (E. giraldii) is a quintessential traditional Chinese medicinal plant with significant developmental potential. Its growth and development are highly responsive to environmental factors, particularly light conditions. However, the endogenous hormonal changes in E. giraldii under shading stress remain unclear, and its adaptive growth strategies require further investigation. Methods: Experimental groups with varying light transmittance (29.12%, 39.68%, and 100%) were established using shade nets, comprising moderate shading, light shading, and control groups. The endogenous hormone contents in apical and lateral leaves of E. giraldii were quantified using the enzyme-linked immunosorbent assay (ELISA). Statistical analysis and graphical presentations were performed using IBM SPSS Statistics 27 and Origin 2022 software. Results: Shading treatment consistently promoted the accumulation of isopentenyl adenine nucleoside (iPA), zeatin riboside (ZR), gibberellic acid 3 (GA3), and abscisic acid (ABA) in both apical and lateral leaves of E. giraldii, while exhibiting differential effects on indoleacetic acid (IAA)-enhancing its content in lateral leaves but reducing it in apical leaves. Quantitative analysis revealed that moderate shading induced: (1) maximal increases in iPA, ZR, and GA3 levels in apical leaves (P < 0.05); (2) maximal increases in ABA, iPA, ZR, and GA3 concentrations as well as the ABA/ZR ratio in lateral leaves; (3) maximal reductions in the ABA/iPA and ABA/GA3 ratios in apical leaves along with the ABA/GA3 ratio in lateral leaves; (4) decreased ABA/IAA and ABA/ZR in apical leaves coupled with increased ABA/iPA in lateral leaves. Under light shading conditions, the most substantial changes included: (1) greatest ABA enhancement in apical leaves and IAA accumulation in lateral leaves; (2) most pronounced declines in IAA content (apical leaves) and ABA/IAA ratio (lateral leaves); (3) elevated ABA/IAA and ABA/ZR ratios in apical leaves with concurrent reduction of ABA/iPA in lateral leaves. Importantly, comprehensive correlation analysis demonstrated positive correlations among all examined hormones (ABA, IAA, iPA, ZR, and GA3) in both leaf types, indicating systemic hormonal coordination during shade adaptation. Conclusion: Shading stress significantly restructured endogenous hormone profiles and their homeostasis in E. giraldii. Under moderate shading conditions, E. giraldii likely adopted a conservative strategy characterized by "apical leaf growth promotion coupled with lateral leaf growth restriction", whereas mild shading induced an expansive strategy featuring "apical leaf growth inhibition coordinated with moderate lateral leaf expansion". These findings provide a theoretical foundation for optimizing cultivation protocols and offer new insights into phytohormonal dynamics in shrubs under light limitation.