Codonopsis lanceolata Benth. et Hook. is a flowering plant native to East Asia. It has small seeds that die easily, and therefore, good germination is hard to achieve. Seed maturation and germination are the two major stages in the alternation of generations in higher plants. Gibberellins participate in numerous developmental processes. Codonopsis lanceolata seeds were pretreated for 7 days with imbibition in one of the following 100 mL solutions and temperatures: in sterile (autoclaved, double distilled) water at 4℃ (TA) or 24℃ (TB), and in a 300 mg·L-1 GA3 solution at 4℃ (TC) or 24℃ (TD). The results of this study showed that three days after cultures were initiated, the seeds in TC and TD showed signs of germination, and the TC had more pronounced effects on the seedlings as compared to the other treatments. The TC increased the percent germination (PG) by 12.8 and 34.2%, respectively, as compared to TA and TB. Furthermore, the maximum germination potential and germination index were also obtained from seedlings pretreated with TC. The contents of soluble sugar significantly increased when seeds were pretreated with cold solutions, whereas the maximum starch content was recorded in seedlings pretreated with TB. The activity of antioxidant enzymes in seedlings varied with the treatment, but only the peroxidase and superoxide dismutase activities were significantly different. Overall, the results suggest that imbibition in a GA3 solution at 4℃ could be the optimal pretreatment for the germination of C. lanceolata seeds.Codonopsis lanceolata is a vine plant with high medicinal values. The cultivation of this plant still needs further research, especially on producing compact seedlings. It has been reported that difference between day and night temperatures (DIF) could highly influence stem extension. The objectives of this experiment were to find an optimum culture condition to produce high quality plug seedlings, and to try to clear the mechanisms of DIF effect on stem elongation. Hence, the effect of DIF on the extension growth of C. lanceolata seedlings was studied in three precisely-controlled units of plant growth chamber. The seedlings were grown under a constant photoperiod of 12 h per day at 250 μmol•m-2•s-1 photosynthetic photon flux density supplied by white light emitting diodes with the same average daily temperature of 22℃, but different DIFs, such as a positive (26/18℃), negative (18/26℃), and zero (22/22℃) DIFs. Growth characteristics were measured after 20 days of treatments. Results showed that length of shoot, average length of node, number of leaves, leaf length, leaf width, and leaf thickness were positively correlated with treatments. As a consequence, the positive DIF regime exhibited superior results for all these traits. However, seedlings grown in the negative DIF had significantly shorter shoot length and average length of node than the positive or zero DIFs. Among all the treatments, the positive DIF portrays a better performance for shoot fresh and dry weights as well as leaf fresh and dry weights. Furthermore, the contents of chlorophyll a, chlorophyll b, and carotenoid in the leaf decreased when DIF was changed from positive to negative. The activities of peroxidase, catalase, ascorbate peroxidase, and superoxide dismutase enzymes were the greatest in the negative DIF treatment. The results revealed that negative DIF did suppress stem elongation and photosynthesis, affecting the synthesis and accumulation of soluble sugar and starch. High night temperatures as a kind of stress can enhance the activities of antioxidant enzymes, and under this stressful environment, seedlings cannot grow and development well.Iron deficiency chlorosis is a plant nutrient disorder symptom that’s widely observed around the world. The aim of this experiment was to identify the causes of leaf chlorosis, and the optimal medium pH and iron concentration to prevent leaf chlorosis and enhance growth in C. lanceolata. For this purpose, three treatment groups were set up, to investigate how the interaction between the iron supply and medium pH affects the plant growth. Seedlings were grown for 60 days in mediums with an iron supply of 0, 15, 30 mg•L-1, and the medium pH was adjusted to 4.7, 5.7, and 6.7. The results revealed that the chlorosis degree increased with decreasing iron concentration or with increasing medium pH. Regardless of the medium pH, at an iron concentration of 0 mg•L-1, seedlings were chlorotic, and at a medium pH of 6.7, seedlings could not survive. At medium pHs of 4.7 and 5.7, and iron concentrations of 15 and 30 mg•L-1, seedlings maintained green leaves, and no visual symptoms of chlorosis were observed. At a medium pH of 5.7, and an iron concentration of 15 mg•L-1, seedlings had the longest shoot length, the highest fresh weight, and high levels of chlorophyll a, chlorophyll b, and carotenoid. Therefore, a medium pH of 5.7 and an iron concentration of 15 mg•L-1 could be the optimal condition for preventing leaf chlorosis and enhancing growth in C. lanceolata.