Regulation of redox homeostasis and proline metabolism by tea polyphenols ameliorates salinity tolerance during wheat seed germination and early seedling growth

Salt stress constitutes a significant abiotic constraint impairing seed germination and plant growth. In this study, we employed wheat cultivar 'Longchun 30' to examine tea polyphenol (TP)-mediated regulation of redox homeostasis and proline metabolism in germinated seeds and seedlings under NaCl stress. Relative to ascorbic acid, TP showed a strong scavenging effect on reactive oxygen species (ROS), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), and 1,1-diphenyl-2-picrylhydrazyl. Salt stress severely suppressed seed germination and seedling growth, concurrently elevating the amount of sodium (Na⁺), nicotinamide adenine dinucleotide (phosphate) (NAD(P)⁺), NADPH, ROS, malondialdehyde (MDA) and osmotic regulators while reducing potassium (K⁺), NADH content, K⁺/Na⁺ and NAD(P)H/NAD(P)⁺ ratios. Crucially, TP application mitigated salinity-induced inhibition of seed germination and seedling growth, concomitant with the increases of K⁺ content and K⁺/Na⁺ and NAD(P)H/NAD(P)⁺ ratios, as well as decreases of Na⁺, NAD(P)⁺, ROS and MDA levels. TP supplementation further enhanced proline content in seedling leaves under salinity stress, along with the further up-regulation of ornithine δ-aminotransferase, glutamate kinase, Δ¹-pyrroline-5-carboxylate synthetase, pyrroline-5-carboxylate reductase and proline dehydrogenase activities and their gene expression, particularly during the early stage of Na + TP treatment. Collectively, TP application enhances wheat salinity tolerance by reducing Na⁺ accumulation, reinforcing the redox homeostasis in germinated seeds and seedlings, while tissue-specifically boosting foliar proline accumulation via coordinated activation of anabolic enzymes and associated genes.