Received 14 Dec. 2004 Accepted 21 Mar. 2005 Supported by the Overseas Scholarship Foundation of the Chinese Education Department (200214) and the National Natural Science Foundation of China (30471181). *Author for correspondence. Tel: +86 (0)25 8439 5917; Fax: +86 (0)25 8439 5266; E-mail:<hxl@njau.edu.cn>. Journal of Integrative Plant Biology Formerly Acta Botanica Sinica 2005, 47 (9): 1084-1091 http://www.blackwell-synergy.com http://www.chineseplantscience.com Promotion by 5-Aminolevulinic Acid of Germination of Pakchoi (Brassica campestris ssp. chinensis var. communis Tsen et Lee) Seeds Under Salt Stress Liang-Ju WANG, Wei-Bing JIANG, Hui LIU, Wei-Qin LIU, Lang KANG and Xi-Lin HOU * ( College of Horticulture, Nanjing Agricultural University, Nanjing 210095, China) Abstract: The seed germination and seedling growth of pakchoi (Brassica campestris ssp. chinensis var. communis Tsen et Lee cv. Hanxiao) were not significantly inhibited until the concentration of NaCl was increased to150 mmol/L. Treatment of pakchoi seeds with exogenous 5-aminolevulinic acid (ALA), at con- centrations ranging from 0.01 to 10.00 mg/L, promoted seed germination when seeds were stressed by salinity, whereas levulinic acid (LA), an inhibitor of ALA dehydrase, significantly inhibited seed germina- tion and seedling growth, suggesting that metabolism of ALA into porphyrin compounds was necessary for seed germination and seedling growth. Determination of respiratory rate during seed germination showed that ALA increased seed respiration under both normal conditions and salt stress. Furthermore, salt stress decreased levels of endogenous ALA, as well as heme, in etiolated seedlings. More salt-tolerant cultivars of pakchoi contained higher relative levels of endogenous ALA and heme under conditions of salt stress. These results indicate that salt stress may inhibit the biosynthesis of endogenous ALA and then heme, which is necessary for seed germination, and treatment of seeds with exogenous ALA prior to germination may be associated with the biosynthesis of heme. Key words: 5-aminolevulinic acid (ALA); germination; heme; pakchoi; respiration; salt stress. The salt tolerance of crops has been of concern worldwide for nearly one century (Liu and Wang 1998). The area of saline soils waiting for agricultural devel- opment is almost 1 billion hm 2 ; furthermore, large ar- eas of fertile land have become sub-saline as a result of unsuitable agricultural practices. For example, the tech- nique of protected horticulture, developed recently, has resulted in the accumulation of subterranean salt on the soil surface because of a shortage of rainfall in the greenhouses or plastic tunnels. Most crops are glycophytes with relatively low salt tolerance. In order to plant crops in saline soil, some cultivars displaying a greater degree of salt tolerance have been bred using either traditional methods or transgenic techniques (Winicov 1998, 2001; Hasegawa et al. 2000; Huang et al. 2000). However, culture tech- niques to improve the salt tolerance of plants have sel- dom been successful. Many agriculturalists expect to regulate salt tolerance by the exogenous application of plant growth substances, such as abscisic acid (ABA), GAs or CTK, but the effects of these substances are too negligible for them to be used in agricultural practice. Recently, Watanabe et al . (2000) proposed that 5- aminolevulinic acid (ALA) could significantly promote the cotton growth of plants treated with 1.5% NaCl after the effects of 12 types of plant growth substances had been compared. They found that the dry weight of plants sprayed with 300 mg/L ALA under conditions of salt stress was almost the same as that of control plants, implying that ALA had the potential to be used to