Comparative Analysis of Physiological Parameters, Antioxidant Defense, Ion Regulation, and Gene Expression in Two Distinct Maize Hybrids Under Salt Stress at Seedling Stage

Salinity significantly impacts maize production globally, requiring a deeper understanding of maize response mechanisms to salt stress. This study assessed the response of two Egyptian maize hybrids, SC-10 and TWC-321, under salt stress (200 mM NaCl) and non-stressed conditions to identify traits and mechanisms linked to enhanced salinity tolerance. Both hybrids accumulated similar Na⁺ levels in leaves, but TWC-321 exhibited better ion regulation, with lower Na⁺ concentrations and Na⁺ to K⁺ ratio in roots. While SC-10 showed a reduction in leaf K⁺ levels, TWC-321 maintained stable K⁺ levels, highlighting its superior salinity tolerance. TWC-321 also demonstrated better oxidative stress management, as evidenced by lower malondialdehyde levels and significantly higher total chlorophyll content, relative water content, and stomatal conductance. Proline accumulation was more pronounced in TWC-321, and it showed higher antioxidant enzyme activities (SOD, CAT, and POD) compared to SC-10, which exhibited lower SOD and POD activities. Gene expression analysis demonstrated distinct responses to salt stress between the hybrids. Although <i>zmHKT1;5</i> was similarly induced in both hybrids, TWC-321 exhibited higher expression levels of <i>zmHKT2</i> (1.96-fold compared to 1.42-fold in SC-10) and upregulated <i>zmNHX1</i> (1.92-fold), whereas <i>zmNHX1</i> expression was slightly reduced in SC-10 (0.8-fold). Additionally, TWC-321 achieved a greater total dry weight than SC-10 under salinity stress, highlighting its superior performance and resilience. These findings indicate that enhanced Na⁺ exclusion and sequestration mechanisms mediate the salinity tolerance of TWC-321. Correlation analysis under salinity stress identified key indicators of salinity tolerance, including increased activity of CAT and SOD, elevated proline accumulation, and higher K⁺ content. Consequently, the salinity tolerance of TWC-321 can be attributed to its effective ion regulation, stable photosynthetic pigment levels, improved osmotic adjustment, enhanced water retention, and potent antioxidant defense system. These insights are highly valuable for breeding programs focused on developing salt-tolerant maize hybrids.