Browning is one of the key factors that influenced the callus subculture of tree peony (Paeonia suffruticosa Andr.). Effects of medium composition and exogenous hormones: macro elements of Murashige and Skoog (MS salts) and iron salt (Fe2+), pH, agar and 6-benzylaminopurine (6-BA), 1-naphthaleneacetic acid (NAA) and kinetin (KT) on the callus browning of P. suffruticosa ‘Shan Hu Tai’ in vitro were studied in this paper. Results showed that the browning of P. suffruticosa callus were more sensitive to KT than 6-BA in different concentrations of 6-BA and KT separately with different concentrations of NAA, and reduced to the lowest (13.3%) under 0.5 mg·L-1 NAA plus 0.3 mg·L-1 KT. 1/4 × MS plus 1/4 × Fe2+ was the best basic medium in which the browning rate was only 18.2%. The browning rate of the callus was the lowest of 4.0% under pH 6.5 and the callus grew better in 7.0 g·L-1 agar than others. This study indicated that the best medium preventing P. suffruticosa callus in vitro from browning was: 1/4 × MS medium supplemented with 6.95 mg·L-1Fe2+, 0.3 mg·L-1 KT, 0.5 mg·L-1 NAA, 6.0 g·L-1 agar and 30 g·L-1 sucrose in pH 6.5.

Introduction
 Materials and Methods
  Plant material and culture conditions
  Exogenous hormones treatment
  MS salts and Fe2+ treatment
  pH treatment
  Agar treatment
  Growth measurements
  Data analysis
 Results and Discussion
  Effects of different combinations of exogenoushormones on the browning of P. suffruticosa callus
  Effects of different combinations of MS saltsand Fe2+ s alt medium o n the b rowning ofP. suffruticosa callus
  Effects of different pH values on the browningof P. suffruticosa callus
  Effects of different concentrations of agar onthe browning of P. suffruticosa callus
 References

• Fang Fang Zhou(College of Forestry, Henan Agricultural University, Wenhua Road, Jinshui District, Zhengzhou, Henan, P. R. China)
• Zheng Wang(College of Forestry, Henan Agricultural University, Wenhua Road, Jinshui District, Zhengzhou, Henan, P. R. China)
• Li Yun Shi(College of Forestry, Henan Agricultural University, Wenhua Road, Jinshui District, Zhengzhou, Henan, P. R. China)
• Jia Jia Niu(Institute of Horticulture, Henan Academy of Agricultural Science, Huayuan Road, Jinshui District, Zhengzhou, Henan, P. R. China)
• Wen Qian Shang(College of Forestry, Henan Agricultural University, Wenhua Road, Jinshui District, Zhengzhou, Henan, P. R. China)
• Dan He(College of Forestry, Henan Agricultural University, Wenhua Road, Jinshui District, Zhengzhou, Henan, P. R. China)
• Song Lin He(College of Forestry, Henan Agricultural University, Wenhua Road, Jinshui District, Zhengzhou, Henan, P. R. China) Corresponding author