Shifts in leaf phosphorus fractions as dependent on plant age as an adaptive strategy for survival in the shade-tolerant Coptis chinensis

BACKGROUND AND AIMS: Phosphorus (P) allocation as dependent on a plant's age is a critical yet understudied factor in the adaptive strategies of shade-tolerant plants. We investigated how Coptis chinensis, a perennial medicinal plant species, allocates leaf P fractions and exhibits leaf structural modifications in response to the understorey light environment. METHODS: By analysing leaf P fractions, chlorophyll concentrations, photosynthetic parameters and leaf anatomical traits in juvenile (2- and 3-year-old) and adult (5-year-old) plants, we uncovered age-dependent shifts in leaf P fractions that balanced photoprotection and growth. KEY RESULTS: Leaves of juvenile plants allocated a greater proportion of P to nucleic acid P (PN), exhibited higher photoprotective pigment-to-chlorophyll ratios and maintained greater specific leaf area (SLA) with thinner tissue structures. The proportion of PN was positively correlated with photoprotective pigment-to-chlorophyll ratio, indicating that enhanced PN allocation contributes to light-stress resilience and early-stage growth. In contrast, as plants progressed to the adult stage leaves showed a marked shift towards higher allocation to metabolite P (PM), which correlated with higher photosynthetic rates, higher chlorophyll concentrations and increased Rubisco enzyme activity. Concurrent structural modifications, including thicker palisade mesophyll and reduced SLA, further supported enhanced photosynthetic performance. This developmental transition from PN-dominant to PM-dominant allocation thus reflects a physiological acclimation that sustains photosynthesis and improves photosynthetic P-use efficiency under shaded conditions. CONCLUSIONS: Our findings highlight a dynamic interplay between P allocation and structural acclimations in C. chinensis leaves, revealing an adaptive strategy that balances photoprotection and growth across different ages. By demonstrating how resource allocation shifts with age in a shade-adapted perennial, this study offers novel insights into the evolutionary strategies that underpin long-term survival and productivity in understorey environments.