Physiological response in the leaf and stolon of white clover under acid precipitation and freeze-thaw stress.

Freeze-thaw (FT) in northern China is a common event in spring and autumn, and the release of sulfur dioxide from coal-burning in winter is apt to trigger acid precipitation. Both these stresses can aggravate the wintering ability of white clover (Trifolium repens L.). Acid precipitation and FT simulation experiments were carried out in the field and an indoor alternation refrigerator, respectively. The contents of soluble protein, soluble sugar, malondialdehyde (MDA), proline and antioxidant activity were tested under acid precipitation and FT stress. The results showed that under acid precipitation stress, the content of MDA, peroxidase and superoxide dismutase increased in both leaves and stolons, whereas soluble protein and soluble sugar content declined compared with the control groups. During the freezing period, the content of antioxidant enzyme activity, soluble protein and proline increased at first and then dropped, whereas MDA and soluble sugar content both increased. As a conclusion, the stolon of white clover is more sensitive than the leaf to short-term stress, either as the single FT stress or the combined stress of FT and acid precipitation, suggesting that maintaining more leaves can contribute to the resistance of white clover to these stresses.