As a major food crop in China, maize (Zea mays L.) is relatively sensitive to salt stress, which significantly inhibits its photosynthesis and causes metabolic disorders. Therefore, this study utilized maize varieties with different levels of salt tolerance to systematically investigate their photosynthetic physiology and metabolic responses under salt stress, with the aim of elucidating the underlying causes of inter-varietal differences in salt tolerance at the physiological and metabolic levels.

A pot experiment was conducted using salt-tolerant maize varieties Yinyu 238 (YY238) and Heyu 157 (HY157), and the salt-sensitive variety Xianyu 335 (XY335). Three salt stress concentration gradients were established: 0 mmol NaCl·L^-1 (CK), 120 mmol NaCl·L^-1 (S1), and 240 mmol NaCl·L^-1 (S2). Stress treatment was applied at the six-leaf stage of the maize seedlings. Samples were taken at 24 hours after stress treatment to measure the net photosynthetic rate (P_n), photosynthetic performance index (PI), ion content (K⁺, and Na⁺), and the content of malondialdehyde (MDA), proline (Pro), abscisic acid (ABA), and jasmonic acid (JA). Non-targeted metabolomics technology was used to analyze leaf metabolites. Differential metabolites were screened based on a variable importance in projection (VIP) > 1 and P＜0.05, and pathway enrichment analysis was performed using the KEGG database.

Salt stress significantly reduced both P_n and PI. Compared with CK, the salt-sensitive variety XY335 exhibited a maximum P_n reduction of 39.8% under S2, while the salt-tolerant variety YY238 showed a relatively smaller reduction of 31.9%. In terms of ion homeostasis, salt stress significantly decreased the shoot K⁺/Na⁺ ratio, with XY335 experiencing a reduction of 76.4%, significantly higher than the 74.6% reduction in YY238. Salt-tolerant varieties demonstrated stronger osmotic adjustment capacity, with YY238 showing a 199.2% increase in Pro accumulation. In contrast, the salt-sensitive variety XY335 suffered more severe oxidative damage, with a 172.1% increase in MDA content. Hormonal responses indicated that XY335 had a higher increase in ABA (59.3%), while YY238 showed the largest increase in JA under S2, reaching 56.3%. Metabolomic analysis revealed that salt stress induced variety-specific metabolic reprogramming. The metabolic disorder in the salt-sensitive variety XY335 worsened with increasing stress intensity, with the number of differential metabolites surging to 127. In contrast, salt-tolerant varieties exhibited a stronger ability to maintain metabolic homeostasis. Salt-tolerant varieties specifically accumulated stress resistance-related metabolites: YY238 significantly upregulated amino acid derivatives and organic acids, including Nα-methylhistidine and trans-aconitate; HY157 specifically accumulated amino acids and their derivatives, such as γ-aminobutyric acid and L-glutamine, as well as phenolic acids like salicylic acid. In comparison, the salt-sensitive variety XY335 primarily accumulated nucleic acid metabolites related to energy metabolism, such as β-guanidinopropionic acid, and organic acids like isonicotinic acid. KEGG pathway enrichment analysis revealed that salt-tolerant varieties were commonly enriched in pathways, such as phenylpropanoid biosynthesis and ABC transporters. Additionally, YY238 specifically activated the biosynthesis of flavonoids and flavonols, while HY157 was significantly enriched in alanine, aspartate, and glutamate metabolism pathways.

In summary, salt stress enhanced the adaptability of salt-tolerant maize varieties by maintaining photosynthetic performance and ion homeostasis, mitigating oxidative damage, and activating specific metabolic pathways. Salt-tolerant varieties maintained higher photosynthetic efficiency through precise ion compartmentalization and alleviate oxidative damage by rapidly accumulating osmotic adjustment substances and effectively scavenging reactive oxygen species. At the metabolic level, salt-tolerant varieties specifically activated pathways, such as phenylpropanoid biosynthesis and amino acid metabolism, providing the key material and signaling foundations for their salt tolerance.