Strigolactone (GR24) Application Positively Regulates Photosynthetic Attributes, Stress-Related Metabolites and Antioxidant Enzymatic Activities of Ornamental Sunflower (Helianthus annuus cv. Vincent’s Choice) under Salinity Stress

Strigolactones, a new group of phytohormones, are reported to improve plant tolerance to multiple abiotic stresses. A pot experiment was conducted to investigate the impact of synthetic strigolactone (GR24 at 0.001, 0.01 and 0.1 mg L−1) application on ornamental sunflowers (Helianthus annuus cv. Vincent’s Choice) grown under salt stress (150 mM NaCl). Salt stress was applied after 14 days, and SL was applied 25 days seed sowing. The results showed that amongst various GR24 concentrations, 0.01 mg L−1 proved to be superior, as it enhanced the photosynthetic rate (9.29%), transpiration rate (0.76%), stomatal conductance (77.5%), total soluble protein (0.55%) and K+ (14.63% in roots; 14.87% in shoots) and Ca2+ (12.63% in roots; 11.48% in shoots) contents under control conditions. Similarly, the leaf turgor potential (Ψp), osmotic potential (Ψs) and free proline, glycinebetaine (GB), superoxide dismutase (SOD), catalase (CAT) and peroxide (POD) contents increased by 58.17, 89.95, 159.04, 101.54, 74.42, 175.68 and 53.62%, respectively, under salt stress conditions. The leaf water potential (Ψw) decreased (−0.14%) and the malondialdehyde (MDA) content increased (16.65%) when treated with the 0.001 mg L−1 GR24 level. Meanwhile, hydrogen peroxide (H2O2) and Na+ concentrations in roots and shoots increased by 62.53%, 74.66% and 38.55% under saline conditions with a GR24 level of 0 mg L−1. Regarding the plant biomass, a GR24 level of 0.01 mg L−1 with salt stress greatly decreased the root (−47.27% and −50.45%) and shoot (−44.79% and −59.42%) fresh and dry weights, respectively, compared to control conditions. These results reveal that exogenously applied GR24 might be an effective way to mitigate the perilous impacts of salt stress in ornamental sunflower production. It is suggested that the use of molecular techniques to study different processes in which GR24 could play a vital part in various commercial floricultural crops is extremely imperative and can open novel horizons for future investigations in this exhilarating field of plant hormones.