Seed priming with nitrogen-doped carbon dots enhances production, reduces pungency, and improves the postharvest quality and storability of onion bulbs

Seed nanopriming is an emerging agricultural technique with the potential to enhance seed germination, growth, and yield while providing resistance to biotic and abiotic stressors through the induction of secondary metabolite biosynthesis, which plays a dual role in plant protection and health promotion. Despite its numerous benefits, concerns about potential toxicity associated with the use of metal nanoparticles are often raised. The use of carbon dots as a green priming agent can potentially eliminate or minimize these concerns, but its efficacy on crop production and postharvest attributes is largely unexplored. This study investigates the effects of seed priming with nitrogen-doped carbon dots (N-CDs) on production of onions, an economically vital crop in Texas. N-CDs were synthesized using a green approach employing phenylalanine (180 mM) and citric acid (270 mM) in a 95:5 ratio. The resulting N-CDs were used as priming agents for two commercial yellow onion varieties (Legends and Pecos) and two Texas A&M-developed varieties (TAM53Y (yellow) and TAM23W (white)). The analysis included germination percentage, seedling growth, yield, and postharvest attributes such as pungency, ascorbic acid, flavonoids, and shelf-life. N-CD treatment significantly (P≤0.05) enhanced germination, seedling growth, and yield across all cultivars, especially increasing these parameters by over 1.7-fold in Pecos and TAM53Y varieties. Pungency was reduced in all cultivars with N-CD treatment (ranging from 1.3-1.5-folds), significantly in Legend. Additionally, N-CD priming increased levels of ascorbic acid (9.5–10.8 mg/100 g FW) compared to control (7.7 mg/100 g FW) and flavonoids, especially quercetin, quercetin-3,4-di-glucoside, and quercetin-4-O-glucoside with varied titers among cultivars. Higher levels (~1.8-fold) of quercetin and quercetin-3,4-di-glucoside were observed in TAM53Y after seed priming. Moreover, levels of quercetin 4-O-glucoside were 1.5-fold higher than unprimed controls in TAM23W and 1.4-fold higher in TAM53Y. Delayed ethylene evolution and respiration rates were observed in N-CD-primed onion bulbs, indicating extended shelf life, most prominently in the TAM53Y cultivar compared to the control. Overall, N-CD priming shows promise for enhancing onion production, enriching health-promoting compounds, and extending shelf life, with potential applications in other crops.