| Pak-choi/Bok choi (Brassica campestris ssp. chinensis) is one of the most important vegetable crops in China. Particularly, in South China, huge numbers of pak choi and their relatives are harvested throughout the year. It is often said that "A large bowl of rice, some bean curd, and a plate of cabbage are daily components of meals for the people".It is a cool season crop preferring moist and uniform conditions in full sunlight. High temperatures with long days will induce bolting especially in the white-stemmed varieties. The ideal temperature during growth is between 8℃and 20℃and while best grown in spring and autumn, pak choi can be grown all year round.Generally, pak choi is cultivated as annual, although most cultivars are biennial. It can be direct seeded or transplanted. They are harvested when about 15 cm tall and commonly used in stir-frys. As their bite indicates, they help move stuck energy and so are beneficial for people with colds, arthristis, or depression. It’s relatively easy to grow and is high in vitamin C, beta carotene, folic acid and calcium, as well as many essential minerals. Just as well, since it’s a good source of betacarotenes, vitamins B2 and B6, folic acid, iron and magnesium, as well as calcium. Each 100 g of leaves provides 46 calories (190 kilojoules) of food energy and contains 4 g of protein,0.5 grams of fat,7 g of carbohydrates.Most of pak choi is cultivated in non-saline soil. However, there are about 100 million hm2 saline soils in the world. Whether pak choi can be planted in saline soils is seldom reported. Recently, Wang et al. (2005) suggested that pak choi is rather salt tolerant, and exogenous 5-aminolevulinic acid (ALA) could promote seed germination of the vegetable. Here, we present more evidence to show its salt tolerance and tempt to elucidate the mechanism of ALA promotion on plant salt tolerance. 1. Study on salt tolerance of pak choi(Brassica campestris ssp. chinensis) at germination and seedling growthSalt had considerable effect on the germination, root length, cotyledons, hypocotyls and root fresh weight and dry weight, as well as respiration (CO2). The seed germination, root length, fresh and dry weight of roots, cotyledons, hypocotyls and respiration (CO2) were decreased when NaCl concentration were increased (50,100,150,200,250 mmol I-1 NaCl). "Za-1", "Su-1", "Ak-1" and "Ha-1" responded high germination in low salinity level (50 mmol I-1 NaCl) compared to control, while "Ai-1" responded longest root length (6.39cm/P) and highest seed respiration (20μmol/g FW min) in also low concetration. "Ak-1" responded highest (16.8 mg/p) hypocotyls fresh weight and "Ba-1" responded highest cotyledon fresh weight (25 mg/P) at 50 mmol I-1 NaCl solution as compared to control.2. Study on promotive effects of ALA on seed germination of pak choi (Brassica campestris ssp. chinensis) under salt stressSalt had considerable effect on the germination, root length, cotyledons, hypocotyls and root fresh weight and dry weight, and respiration (CO2). Seed germination percentage and seedling growth were significantly increased when increased the ALA concentration. "Lb-1" and "Ai-1" responded highest germination (98%) in T6 (200 mmol I-1 NaCl+50 mg I-1 ALA), meanwhile the longest root of "Lb-1", "Za-1" and "Su-1" were recorded in T3 (200 mmol I-1 NaCl+20 mg I-1 ALA). "Qi-1" strongest response of fresh and dry weight (25mg/p,1.6mg/P) of hypocotyls, and fresh and dry weights of cotyledons (30.4mg/P, 2.9mg/P) were obtained in T6. Levulinic acid (LA), an inhibitor of ALA dehydrase significantly an inhibited seeds and seedling growth. In 20 mmol I-1 LA solution, the seedling growth was inhibited. ALA growth-promoting effect in seed germination and seedling growth might be caused mainly by heme, and heme is essential activity of cytochrome c in the respiration chain of the mitochondrion.3. Study on effect of ALA on sodium, magnesium, potassium, chlorine, calcium content in pak choi under salt stressSalts had affected on ions of pak choi leaves. Magnesium, potassium and calcium content were decreased under salt stress, as well as sodium and chlorine was increased. ALA treatments increased Mg2+, Ca2+and K+content of pak choi under salt stress, while those Na+and Cl" decreased. It was observed that the Na+content was not decreased in "Qd-1", Na+ content indeed decreased in "Li-1" and "Ai-1". Magnesium ion, Ca2+, K+, Na+ and Cl- content were different in different times. The highest concentrations of Mg2+(4.07 mg/g) content and Cl- (7.0 mg/g) content in "Li-1" were recorded at day twenty four, and the highest concentration of Ca2+content (36.08 mg/g) in "Ai-1" was at eight day. The highest concentrations of Na+(47.16 mg/g) content in "Qd-1" and K+(43.34 mg/g) content in "Ai-1" were observed at day twenty four.4. Study on the role of ALA on chlorophyll, antioxidative enzymes (POD, CAT, SOD), and photosynthesis of pak choi under salt stressPhotosynthesis rate, activitivties of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and chlorophyll content were determined. It was observed; influnced of ALA increased the salt tolerance of pak choi. "Qd-1" (salt moderate cultivar) responded highest Pn (15.41 pmol m-2 s-1) at PFD of 1000μmol m-2 s-1 in 100 mg l-1 ALA treatment under 150mmol l-1 NaCl stress, with the highest Chl a (1.08 mg g-1 FW), Chl b (0.59 mg/g FW), total Chl (1.67 mg/g FW) and Chl b/a ratio (0.54). The "Ai-1", a salt tolerant cultivar, shows the strongest response of activity of SOD (72 U g-1 min-1) in T 3 and the highest POD enzyme activity (0.08 U g-1 min-1) in T 2. Meanwhile, CAT activity in "Li-1" increased at 50mg l-1 and 150mg l-1 NaCl. The effect of ALA, the CAT activity decreased 155% at 150mg l-1 NaCl+100mg l-1 ALA and decreased 180% at 50mg l-1L NaCl+100mg l-1 ALA. It was determined that 100mg l-1 ALA increased the activities of antioxidative enzymes chlorophyll content and photosynthesis rate under salt stress.5. Study on the changes of chlorophyll fluorescence parameters in pak choi by ALA under salt stressThree cultivars (salt sensitive-"Lichuandasuomian", salt moderate-"Qingdi", salt tolerant-"Aijiaohuang") of pak choi were examined under salt stress. The result shows that "Qingdi" (salt moderate cultivar) responded highest efficiency of light absorbed by chlorophyll in the presence of ALA under salt stress. The electron acceptor (Qa) (pastoquinones) and the electron transfer rate increased in "Qingdi". Salt stress was quite harmful to the stability of PSII in pak choi. ALA significantly improved tolerance of photosynthetic function of leaves. 6. Study on Expression of S44 gene induced by ALA in pak choiS44 is an important component of salt tolerance in halophyte species, which may play an important role in plant tolerance to salt stress. "Ai-1" and "Qd-1" cultivars of pak choi were determined the gene expression. It was investigated that ALA effect on the S44 gene expression was high in root as compared to leaf. The identification and sequencing of the S44 gene in pak choi, including fragment of sequence (294 bp).7. Study on the promotive effect of ALA on chlorophyll, antioxidative enzyme and photosynthesis rate on pak choi under hot stress conditionPak choi has especially large area of production in winter season. Under high temperature conditions, the plant growth very slow but it in enough soil moisture can tolerate temperatures. In this investigation, effects of ALA on the chlorophyll content antioxidative enzymes and photosynthesis under hot stress were determined. "Qd-1" shows strongest response Chl a (1.96 mg/g FW) in T3 (250 mg l-1 ALA), and "Za-1" highest response of Chl b (1.74 mg/g FW) and highest total chlorophyll (2.32 mg/g FW) in T1 (50mg l-1 ALA), "Ai-1" responded highest Chl b/a ratio (3.63 mg/g FW) in T2 (150 mg l-1) to control. The enzymes activity was reduced under hot stress. "Zh-1" strongest response of POD activity (1.23 Ug-1.min-1), "Zy-1" highest response of CAT activity (0.93 Ug-1.min-1) and strongest response in SOD activity (287 Ug-1.min-1) in "Qi-1" in T3 (250 mg l-1 ALA) as compared to control were recorded. It was observed that "Ba-1" responded highest Pn (14.78 (μmol m-2 s-1) in T3 (250 mg l-1 ALA) and "Zh-1" stomatol conductance (Gs) (0.599μmol m-2 s-1) was highest in T1 (50 mg l-1) as compare to control. Meanwhile "Qi-1" highest (405μl l-1) intercellular CO2 concentration (Ci) in T3 (250 mg l-1 ALA) was observed as compare to control. Highest transpiration rate (E) (11.2μmol m-2 s-1) was recorded in "Za-1" at 250mg l-1 ALA treatment as compare to control. ALA is a key precusor in porphyrin (heme and chlorophyll) biosynthesis. ALA increased the activities of enzymes, chlorophyll content and photosynthesis rates under hot stress. |
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