Objective  Physiological mechanism and exogenous melatonin (MT) application to improve cold tolerance of green pepper seedlings were investigated.

  Method  Seedlings of green pepper cultivar, Britney, were planted in matrix pots and sprayed with a MT solution of 0 (CK), 25, 50, 75, or 100 µmol·L⁻¹. Effects on the growth, antioxidant enzyme activity, osmotic regulatory substances, and photosynthetic characteristics of the seedlings under chilling at 4 ℃ were determined.

  Result   Comparing to CK, spraying MT increased the contents of catalase (CAT), superoxide dismutase (SOD), and proline (Pro), as well as the net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s), maximum quantum yield (F_v/F_m), maximum photosynthetic potential (F_v/F_o), electron transport activity (F_m/F_o), leaf SPAD, while decreased malondialdehyde (MDA), relative electrical conductivity, and intercellular CO₂ concentration in leaf. At 50 µmol·L⁻¹ level, the MT spray showed the greatest effect in alleviating cold injury on the seedlings. The treatment also allowed the underground dry weight, CAT, SOD, MDA, relative conductivity, P_n, C_i, leaf SPAD, F_v/F_m, F_v/F_o, and F_m/F_o to reach significant heights and Pro, T_r, and G_s at extremely significant levels.

  Conclusion   The exogenous MT spray mitigated injury by chilling temperature and improved cold tolerance of the green pepper seedlings through mechanisms such as a heightened antioxidant enzyme activity and photosynthetic rate as well as suppressed membrane lipid peroxidation and regulated osmotic substances. The optimum MT application on the green peeper plants was determined to be at 50 µmol·L⁻¹.