Exogenous hydroxyapatite and silicon nanoparticles mitigate salinity and lead (Pb) stress in coneflower (Echinacea angustifolia) by adjusting growth and biochemical attributes

Heavy metals and salinity represent two of the most eminent challenges facing agriculture. Recently, there has been a lot of interest in silicon nanoparticles (Si NPs) and hydroxyapatite nanoparticles (HAP NPs) for managing abiotic stress. Their comparative application in salinity and lead (Pb) stress, however, has not received much attention. Therefore, the current study was carried out to determine the effects of HAP and Si NPs on mitigating Pb and salinity stress in coneflower ( Echinacea angustifolia ) through physiological and biochemical attributes. The plants were sprayed with HAP and Si NPs at 200 mg L −1 and exposed to Pb (600 mg kg −1 of soil) and salinity (100 mM NaCl). The results showed that Pb and salinity respectively lowered shoot weight (29 % and 36 %), root weight (20 % and 23 %), chlorophyll (Chl) a+b (31 % and 35 %), relative water content (RWC, 16 % and 19 %), total phenolic content (TPC, 5 % and 14 %), total flavonoid content (TFC, 11 % and 25 %), essential oil (EO) content (14 % and 26 %), EO yield (39 % and 52 %), but increased malondialdehyde (MDA, 43 % and 66 %) and proline (11 % and 18 %) in plants without NPs application. The use of Si and HAP NPs modulated Pb and salinity as their combination increased shoot weight (30 %), root weight (13 %), Chl a+b (17 %), RWC (15 %), proline (40 %), TPC (17 %), TFC (37 %), EO content (29 %), EO yield (68 %), but lowered MDA (24 %) in salt-exposed plants. The heat map showed that EO yield exhibited the greatest variability among the traits. The use of combined HAP and Si NPs suggests the potential of this combined treatment to mitigate salinity and Pb stress. • Pb and salinity reduce growth, chlorophyll, relative water content, and essential oil yield in Echinacea angustifolia. • Salinity was more deleterious than Pb toxicity. • The combined effects of silicon and hydroxyapatite nanoparticles under Pb and salinity stress got the best effect. • Silicon and hydroxyapatite nanoparticles enhanced secondary metabolite production while reducing malondialdehyde. • Essential oil yield was the most sensitive trait, highlighting an effective tool for determining plant changes.