Proline (Pro) is not only an important osmotic adjustment substance, but it also plays an important role in regulating plant abiotic stress resistance. The maize varieties, Xinxin 2 (low temperature insensitive) and Damin 3307 (low temperature sensitive), were chosen as materials, setting a normal temperature for germination (22 °C/10 °C, 9d), low temperature germination (4 °C/4 °C, 5d) and normal temperature recovery (22 °C/10 °C, 4d), combined with a proline (15 mmol·L⁻¹) soaking treatment, to reveal the seed germination and regulation mechanism in maize embryos. The results showed that proline significantly improved the germination potential, germination rate, germination index and vigor index of seeds under low temperature stress, increasing the length of the coleoptile and radicle, increasing the dry and fresh weight of young buds (coleoptile + radicle), and increasing the activity of α-amylase. Proline enhanced the activities of Δ1-pyrroline-5-carboxylic acid synthetase (P5CS) and ornithine aminotransferase (OAT) in maize embryos under low temperature stress, enhanced the proline synthesis pathways, and further enhanced proline accumulation. Proline induced the activity of proline dehydrogenase (ProDH) in the early stage of low temperature stress and stress relief. Under low temperature stress, the activities of hexokinase (HXK), phosphofructokinase (PFK), pyruvate kinase (PK), isocitrate dehydrogenase (IDH), succinate dehydrogenase (SDH), malate dehydrogenase (MDH) and glucose-6-phosphate dehydrogenase (G-6-PDH) and glucose-6-phosphate dehydrogenase (G-6-PDH and 6-P-GDH) in maize embryos were decreased, resulting in a decrease in the glycolysis (EMP) pathway, tricarboxylic acid (TCA) cycle and pentose phosphate pathway (PPP). Proline alleviated the inhibition of key enzyme activities of the EMP pathway, TCA cycle and PPP of maize embryos under low temperature stress, increased the activities of cytochrome oxidase (COX) and alternative oxidase (AOX), increased the ATP content, alleviated the inhibition of low temperature stress on main cytochrome pathway activity (ρV_cyt), while further increasing the total respiratory activity (V_t) and the actual operational activity of the alternative pathway (ρV_alt) during seed germination at the initial stage of low temperature stress, as well as improving the inhibition of the V_t and ρV_alt in the middle and late stages of low temperature stress. Under low temperature stress, the content of abscisic acid (ABA) increased significantly, while gibberellin (GA), auxin (IAA) and zeatin nucleoside (ZR) decreased significantly. Proline alleviated the decrease in IAA, ZR and GA contents in maize embryos under low temperature stress, reduced the increase in the ABA content, and reduced the inhibition of low temperature on seed germination.

This study was to explore the nitrogen metabolism and transcriptome mechanism of spermidine (Spd) under drought stress conditions. Firstly, maize variety Xianyu 335 (drought insensitive type) and Fenghe 1 (drought sensitive type) were chosen as experimental materials under hydroponic conditions. The effects of PEG-6000 combined with Spd application on nitrogen metabolism were studied. Secondly, we chose maize variety Xianyu 335 for the field experiment. At the flowering stage, normal water treatment and moderate drought stress were carried out, respectively. The results showed that: (1) Hydroponics experiment showed that the content of NH₄⁺ in the leaves of maize seedlings under drought stress increased significantly, while the content of NO₃⁻ and nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamine dehydrogenase (GDH), glutamic oxaloacetic transaminase (GOT) and glutamic pyruvic transaminase (GPT) increased significantly. Spd can promote the assimilation of excess ammonia by enhancing the activities of ammonia assimilating enzymes GS/GOGAT and GDH, and transaminase (GOT and GPT), effectively alleviate the ammonia toxicity and nitrogen metabolism disorder induced by drought stress. (2) Pot experiment showed that Spd significantly promoted the root growth of maize under drought stress, so as to improve the absorption and utilization of water and nutrients. In addition, Spd can improve the chlorophyll content and photosynthetic rate of maize leaves under drought stress. After the application of exogenous Spd, the photosynthetic green leaf area increased, the leaf senescence rate slowed down, and the dry matter accumulation increased after anthesis, resulting in the increase of grain weight and grain number per ear, and finally improve the maize yield.