Objective   Effect of exposure to light of varied spectra at night on the cold tolerance of tomato (Lycopersicon esculentum) seedlings was investigated.

  Method  Tomato seedlings grown under supplemental lighting at night by using white (W), red (R) or blue (B) light or combined 2R/8B, 5R/5B or 8R/2B were taken as treatments, and seedlings grown under natural conditions were taken as control. After 20d of cultivation, the plants were subjected to a low-temperature stress of 10/5 ℃ (day/night temperatures) to determine the tolerance of the seedlings toward mild hypothermia.

  Result   The ranges of increased electrolyte permeability and MDA content in the leaves of the treatment seedlings under low-temp stress were significantly lower than those of control. After being exposed at night to the combination of 2R/8B, 5R/5B or 8R/2B, the seedlings could maintain high activities of SOD, CAT, and POD under the low-temp stress. These activities peaked when the 8R/2B was applied to reach significantly 399.49% on SOD, 129.01% on CAT, and 135.74% on POD higher than those of CK, 6d after the seedlings were moved to the low-temp environment.

  Conclusion  Exposing tomato seedlings to light of different spectra at night improved the ability of the plants to tolerate mild hypothermia. The effect was maximized when the seedlings were grown for 20d under the nighttime lighting with 8 red and 2 blue lamps.