| The experiment was carried out with cucumber varieties h02,h082 and Sichuan green no.l as materials. The seedling in bicuspid period was shaded and then sprayed with ALA, using 0 mg/L (CK),0.01 mg/L,0.1 mg/L,1 mg/L,10 mg/L and 50mg/L as treatments. After spraying the ALA Od, 2d.6d,14d 10d. the antioxidant enzymes activity. MDA. and the growth rate of cucumber seedlings were studied. The result shows:under ordinary sunlight, application of low concentration ALA could promote plant height, leaf area, and stem diameter of cucumber seedlings. The effects increased with the increasing concentration, and they reached highest in a certain concentration. They decreased with the concentrations increasing afterwise. The low concentrations application of ALA under weak light conditions had the similar effects and could obviously increase stem diameter and seedling states. The experiment also showed:the application of low ALA concentrations under sunlight or weak light can also improve the activity of plant antioxidant enzymes (superoxide dismutase SOD. catalase CAT, peroxidase POD) and reduce the accumulation of MDA, The effects went up and then down with the concentrations increasing. It shows that ALA can improve the stress resistance of cucumber seedlings under weak light condition, and 1 mg/L ALA had best affect.Cucumber seedlings of varieties h02,h082, Sichuan green no.1 were treated with ALA 1mg/L and water (CK) in illuminating incubator in two leaves stage. The test was seperated into two parts, with the first using weak light conditions of 25℃/18℃,80μmol/m2·s;and the second using normal conditions of 25℃/18℃,200μmol/m2·s. After treatment with ALA the cucumber seedlings in two leaves stage were transferred into artificially climate cabinet and sampled in 2 days interval lasted after 5 times of sample. The last time fresh and dry weight was determined. The experimental was showed by seedling photosynthetic parameters as the net photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration that photosynthesis in the leaves was inhibited significantly under weak light condition. And spraying the exogenous ALA. the leaf photosynthesis has been obviously ebhance. SOD. CAT enzyme activities in three cucumbers varieties of h02, h082. Sichuan green no.1 ubder weak light were significantly lower than under normal sunlight. Contrasted with the declining of CAT as weak light treatment continued, the reduction of SOD and CAT activity in leaves could be significantly relieved by ALA application. POD activity in cucumber seedlings under weak light went up and then down. MDA content had a significant increase under the weak light than under under normal sunlight. As the extension of time, the amount of MDA accumulation also constantly increases. Application of ALA can ease such damage in a certain degree.After weak light treatment, chlorophyll a and b contents of no-weak-light resistance h02 had a decline and foliar spraying of ALA would prevent this tendency, the relatively-weak-light-resistant h082 and weak-light-resistant Sichuan green no.1 it has chlorophyll a and b went up. It indicates that ALA application in low light conditions could conduct accumulation of plant chlorophyll-a and chlorophyll-b as well as carotenoids. Under weak light conditions not weak light resistance h02 chlorophyll a/b ratio got bigger first and then lower compared with commom sunlight, and it kept a higher value throughout experiment. The chlorophyll a/b in seedlings applied with ALA was higher than CK. Under the condition of weak light, the chlorophyll a/b ratio in varieties of cucumber was lower than in normal sunlight and continuously reduce. The ratio in the plant applied with ALA was higher than that without application under weak light condition.
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