Seasonal and structural variability of pyrrolizidine alkaloids in Heliotropium sarmentosum (Lam.) Craven

Heliotropium sarmentosum (Lam.) Craven (Tournefortia sarmentosa Lam.), distributed across Southeast Asia and Australia, has been traditionally used as an herbal medicine. Although studies have reported its detoxicant, antioxidant and anti-inflammatory properties, research on its toxic pyrrolizidine alkaloids (PAs) remains limited. This study analyzes PA content in H. sarmentosum across seasons, plant organs and growth stages. A total of eighteen PAs were identified and quantified, with sixteen reported for the first time in this species. 9-Angeloylretronecine was identified as the predominant PA compound by ¹H NMR analysis. The total content of 1,2-unsaturated PAs ranged from 13.4 to 57.9 μg/g dried weight in whole plant samples, varying significantly by season, organ, and plant age. Across all seasons, roots consistently exhibited high PA level (160.5–251.5 μg/g), while leaves maintained low level (0–9.9 μg/g). Notably, PA levels in fruits increased from autumn to winter, peaking at 186.3 μg/g, surpassing even root concentrations (160.5 μg/g) in the same season. The organ-specific variation in PA composition suggests that H. sarmentosum may employ dynamic accumulation strategies to defend different organs against herbivores and insects. This study enhances our understanding of PA composition and seasonal variation in H. sarmentosum, providing critical insights for its medicinal use and toxicity assessment.