Study On Effects Of Nitrate Stress On Nitrogen Assimilation Of Cucumber (Cucummis Sativus L.) Seedlings

Soil secondary salinization in protected farmland has seriously limited sustainable development of agricultural production. According to the present studies, accumulation of ions in protected farmland is greatly different from seaside or inland. In the protected farmland, the main cation and anion are Ca2+, K+ and NO3-, respectively, while Na+ and Cl- are the main forms of ions in seaside or inland.Now, most of researches about salt stress to plant are focused on NaCl, however, there are fewer investigations about nitrate stress to horticultural crops. Cucumber is one of the most important horticultural crops, and it has been reported that excessive accumulation of nitrate widely inhibited the growth and development of cucumber in protected farmland of China. Therefore, we studied how excessive nitrate influenced cucumber (Cucummis sativus L., variety Xintaimici) seedlings nitrogen assimilation and the mechanism of nitrate stress to cucumber with hydroponic culture. The results were as follows:1. Effects of nitrate stress on the growth of cucumber seedlingsIn this experiment, the effects of three nitrate concentration levels—14 mmol·L-1(CK), 56 mmol·L-1(T-1) and 140 mmol·L-1(T-2)—on the growth of the cucumber seedlings were studied with hydroponic culture. The results suggested that plant height, stem diameter, leaf area and leaf number of cucumber seedlings increased with increasing of nitrate concentration from 14 mmol·L-1 to 56 mmol·L-1, then decreased when nitrate concentration was beyond 56 mmol·L-1.2. Effects of nitrate stress on the photosnthesis of cucumber seedlingsThe results showed that with increasing of treatment time, the photosyhthetic rate and Hill reaction activity of seedlings grown in nutrient solution containing 140 mmol·L-1 NO3- obviously decreased, while qN increased; photosynthetic pigments content increased at the beginning of trratment , and then decreased to the content level of CK; there was little significant difference in Chla/Chlb, Car/Chl,ΦPSⅡand qP between seedlings under 140 mmol·L-1 NO3- stress and CK during preperiod. However, Chla/Chlb,ΦPSⅡand qP decreased gradually, while Car/Chl increased over the late treatment course.3. Effects of nitrate stress on the respiration of cucumber seedlingsThe results suggested the respiration in roots and leaves of cucumber seedlings under 140 mmol·L-1 NO3- stress increased obviously during two days after treatment, and then decreased damatically from two days to four days. After four days, the respiration decreased slowly.4. Effects of nitrate stress on the content of mineral elements in cucumber seedlings.The results suggested that nitrate, nitrite and ammonia content in the roots, stems and leaves of cucumber seedlings grown in nutrient solution containing 140 mmol·L-1 NO3- increased significantly with increasing of treatment time, while glutamine, asparatine and amide content decreased dramatically over treatment course. Magnesium, manganese and zinc content in roots, stems and leaves of cucumber seedlings under 140 mmol·L-1 NO3- stress decreased, respectively; while iron and copper content increased.5. Effects of nitrate stress on nitrogen assimilation enzymes in cucumber seedlingsThe results showed that in vivo nitrate reductase activity in roots and leaves of cucumber seedlings under 140 mmol·L-1 NO3- stress had little significan difference from CK during preperiod, and then decreased over late treatment course. The nitrate reductase activity in leaves was influenced more seriously than in roots. Compared with in vivo nitrate reductase activity, in vitro nitrate reductase activity in roots and leaves of cucumber seedlings was much higher than CK. Glutamine synthetase activity in roots increased significantly over two days after treatment, and then decreased gradually. On the twelveth day of treatment, the glutamine synthetase activity in roots was much lower than CK. In leaves, glutamine synthetase activity had little significant difference from CK between zero day to two days after treatment, then decreased dramatically after two days. Glutamate synthetase activity in roots of seedlings under 140 mmol·L-1 NO3- stress significantly increased from zero day to two days, and decreased dramatically between two days to four days. After four days, the activity of glutamate synthetase decreased slowly. In leaves, the glutamate synthetase activity decreased gradually from the beginning of treatment. Glutamate dehydrotase activity in roots obviously decreased from the beginning of treatment. In leaves, the activity of glutamate dehydrotase dramatically increased during one day after treatment, and decreased gradually after one day.