Pollination <i>versus</i> carnivory: functional distribution of naphthoquinones in <i>Nepenthes khasiana</i>

Nepenthes, a major genus of carnivorous plants, secrete floral and extrafloral nectars (FN and EFN) in their flowers and pitchers, respectively. Recently we demonstrated Nepenthes khasiana EFN as a sugar mix with minimal nitrogenous metabolites and vitamin C. N. khasiana EFN showed insect toxicity due to its strong acetylcholinesterase (AChE) inhibition, and the active principle has been characterized as the naphthoquinone derivative, (+)-isoshinanolone. These findings prompted us to study the chemical composition and toxicity of N. khasiana FN. N. khasiana FN was analyzed for its sugars, amino acids, proteins, fatty acids and vitamin C by HPTLC-densitometry, UFLC, GC-MS and biochemical assays. C:N ratio and naphthoquinones were analyzed by CHNS analyzer, headspace-GC-MS, and GC-FID, respectively. Toxicity of FN and the naphthoquinone droserone, found in its pitcher fluid, was evaluated using the AChE inhibition assay. N. khasiana FN showed a similar chemical pattern as its EFN, viz., sugar (Glc-Fru-Suc)-mix with minimal amino acids, vitamin C, and moderate protein and fatty acid contents. It showed fairly high C:N ratio. The naphthoquinone, plumbagin, was the major volatile constituent in N. khasiana flowers. Crucially, the AChE inhibitors, (+)-isoshinanolone and plumbagin, were absent in N. khasiana FN, which showed no AChE inhibition. Droserone, however, exhibited strong AChE inhibitory activity. The chemical profiles of EFN and FN reflect a nitrogen-limited metabolism in N. khasiana. Notably, the functional allocation of neurotoxic naphthoquinones - abundant in EFN, absent in FN, and present in the pitcher fluid - reflects an adaptive strategy that optimizes both nutritional gain through prey capture and reproductive success through pollination.