Seed germination, seedling growth performance, genetic stability and biochemical responses of papaya (Carica papaya L.) upon pre-sowing seed treatments

It is necessary to move towards using innovative, safe and environmentally friendly materials to break seed dormancy, increase germination percentage (GP) and obtain strong seedlings. This investigation was carried out during 2022/2023 and 2023/2024 seasons at the Faculty of Agriculture’s nursery, Damietta University, Egypt, to study the influence of presowing of papaya seeds (Carica papaya L.) cv. Red Lady for 24 hr in magnetic water (M.W) at 2000 Gauss, chitosan (CH) at 1.5% and gibberellic acid (GA₃) at 1000 ppm, as separate treatments or in combination, on seed germination, seedling growth parameters and genomic-DNA template stability. Among all the investigated treatments, the individual application of GA₃ ranked first followed by the M.W treatment resulting in the highest GP (100% and 86.36%), germination rate index (15.53 and 14.06) and seedling vigour index (883.33 and 490.30), respectively. As for seeds’ biochemical content, the highest values of 2,2-diphenyl-1-picrylhydrazyl (DPPH) (47.04% and 46.11%), total phenolic content (5.41 and 4.81 mg · g⁻¹ DW), α-amylase enzyme (16.45 μmol · min⁻¹ · mL and 12.37 μmol · min⁻¹ · mL), proline content (3.87 mg · g⁻¹ and 3.67 mg · g⁻¹ DW) and total phosphorus content (0.18% and 0.13%) were highest in the GA₃- and M.W-treated seeds, respectively. Genome template stability percentage (GTS%) derived from papaya seed protein profiles using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) technique showed higher percentages if CH is added to any treatment. Nevertheless, the most significant adverse effect on GTS% was generally noticed in response to mixed treatments. Mixed treatments may up-regulate gene expression towards the formation of antioxidants and secondary metabolites rather than the formation of primary metabolites responsible for increasing plant growth parameters, which might be the reason for reduced growth parameters in these treatments. Similarity index obtained by unweighted pair-group method with arithmetic averages (UPGMA) showed the highest values with M.W and GA₃ treatments both sharing 28 bands, similar to the control by 80% and 75%, respectively, and inducing significant changes in the morphological growth parameters besides seed phytochemicals when compared to the control. By the end of this research, it could be concluded that soaking papaya seeds in a 1000 ppm GA₃ or 2000 Gauss M.W solution separately for 24 hr before sowing could significantly improve seed germination and development; while using mixed forms of the investigated concentrations, they had no significant effect or the germination process and seedling growth were negatively affected. Further studies are needed in this regard.