Physiological responses of wheat (Triticum aestivum L.) germination to elevated ammonium concentrations: reserve mobilization, sugar utilization, and antioxidant metabolism

To elucidate the physiological mechanisms of wheat (Triticum aestivum L.) germination in response to elevated ammonium (NH₄ ⁺) concentrations (EAC), Yumai49 (NH₄ ⁺-tolerant) and Lumai15 (NH₄ ⁺-sensitive) cultivars were supplied with a 5.0 mM NH₄ ⁺-N (EAC) treatment or a 5.0 mM NO₃ ⁻-N (CON) treatment as the control. There was a reduction in the seed germination index, plant biomass, radicle length, and other deleterious effects following the EAC treatment in both cultivars, but Yumai49 was more tolerant to EAC than Lumai15. When compared to CON, α-amylase activity and ATP content in seeds were significantly lower after the 3rd and 5th day of the EAC treatment in Lumai15 and Yumai49, respectively. Moreover, the ratio of abscisic acid to gibberellins in seeds was higher under EAC treatment as compared to CON, with a smaller ratio increase in Yumai49 than in Lumai15, indicating that the differences in hormone ratio caused by the EAC treatment was associated with lower seed reserve mobilization. The soluble sugar content in radicles increased while it was reduced in coleoptiles for both cultivars under the EAC treatment. However, Yumai49 exhibited a slight increase in seedling soluble sugar content, while Lumai15 exhibited a significant reduction in sugar content, as compared to CON. Moreover, Yumai49 exhibited a lower ratio reduction in coleoptile to radicle soluble sugar content as compared with Lumai15, indicating that Yumai49 was undergoing greater reserve mobilization and transportation to the radicle to support growth. The activities of pyruvate kinase and phosphoenolpyruvate carboxylase were increased under the EAC treatment in the radicles, with a greater increase seen in Yumai49 than in Lumai15, indicating that Yumai49 had a greater capacity to assimilate NH₄ ⁺. In addition, the EAC treatment enhanced the content of malonaldehyde and superoxide anions, and the activities of superoxide dismutase and peroxidase in the coleoptiles and radicles of both cultivars, with the magnitude of these increases greater in Yumai49 than in Lumai15. It is concluded that the NH₄ ⁺-tolerant cultivar, as compared to the sensitive cultivar, has a greater capacity to undertake reserve mobilization and ATP production in its seeds, a greater sugar utilization in its radicles, and a stronger antioxidant protection mechanism.