Abstract: Guar seedlings were grown in greenhouse, and effects of leaf spray (NH₄)₂SO₄ on changes of superoxide dismutase (SOD), peroxidase (POD) activities and malondialdehyde (MDA) content in leaves and death rate of seedlings were studied in different photon flux densities (PDF) after chilling 12 hours at 2±0.5℃. The results showed, SOD and POD activities were lower, MDA content was higher, and death rate of seedlings was higher in the treatment recovered under normal sunlight in greenhouse than those of the control. When the seedlings were recovered under 90 μmol·m^-2·s^-1 weak light for 2~4 days, SOD and POD activities were higher than those of the control, MDA content kept at a low level, and seedlings were survived normally. Application of (NH₄)₂SO₄ increased protective enzyme activities, reduced MDA content in leaves during recovery time after chilling 12 hours, improved seedling resistance to low temperature, and significantly decreased the death rate of guar seedlings. The effect is the best for seedlings sprayed with 0.2% (NH₄)₂SO₄, followed by 0.4% (NH₄)₂SO₄. If weak light is given to seedlings after a short-term chilling, active oxygen species would be reduced and chilling injury could be alleviated or even eliminated.

Key words: guar seedling, low temperature, SOD, POD, MDA