| Special environment, intensive fertilizer input, and no rainwater leaching and other cause during protected cultivation, often led to soil salinization. Soil salinization not only seriously influenced plant growth, but also decreased yield and quality. Cucumber (Cucumis sativus L.) is widely planted in greenhouse. Cucumber roots are shallowly distributed and show preference for fertilizer, but no tolerance to excess fertilizer, so cucumber can be easily affected by salinization. Soil salinization has been a main barrier of protected cucumber production. Lanthanum can to a certain degree improve the resistance of plants to envirvnmental stresses.Therefore, study on mechanism of LaCl3 nelieving nitrate stress has the important theoretical and practical meaning to guide sustainable production and increase utilization efficiency of protected cultivation.In this study, hydroponic culture experiments were conducted to investigate the effects of LaCl3 on seed germination, carbon and nitrogen metabolism, protective enzyme activity, mineral elements uptake and subcellular distribution and the changes of endogenous hormones. The objective of this paper is to lay theoretical formelation and propse new measures for alleviation of salinity injury. The main results were presented as follows:1. Research on effects of lanthanum alleviating nitrate stress on cucumber seeds germinationSeeds of two cucumber varities were used to study the effects of different LaCl3 concentrations on seeds germinabality and the physiological and biochemical properties during the germination under nitrate stress.The results showed that 0.05 mmol·L-1, 0.1 mmol·L-1 and 0.5 mmol·L-1 LaCl3 could, to different degress alleviate salt stress, low concentration LaCl3 could enhanceα-amylase,β-amylase,dehydrogenase activity and increase the content of soluble protein, thereby promoting the growth of branch roots, radicel and axis; but high concentration(1mmol·L-1) LaCl3 inhibited the germination of seeds.Low concentration LaCl3 (less than 0.5 mmol·L-1) alleviated the nitrate injury and improved resistance of seeds to nitrate stress by reducing the content of MDA and relative electrolytic leakage. But high concentration (1mmol·L-1) LaCl3, which inhibited the germination of seeds, was deleterious to the seeds and thus failed to alleviate the cucumber seeds germination under nitrate stress. Compared all indexes comprehensively, we found that 0.05 mmol·L-1 LaCl3 was the best concentration in alleviating the cucumber seeds germination under nitrate stress.2. Effects LaCl3 on carbon and nitrogen metabolism under nitrate stress in cucumber seedlingsIn this experiment, cucumber seedlings were treated which high concentration nitrate (140mmol·L-1NO3-) and LaCl3. The results showed that NR activity in leaves increased at early stage of stress, but GS and GOGAT activties decreased;NR activty decreased at later stage of stress, compared to the control. Nitrate and NH4+ contents increased in cucumber seedlings under nitrate stress, which inhibited growth of cucumber seedlings. As a result, such as plant height, leaf area and biomass accumulation were reduced. Exogenous LaCl3 could improve NR,GS and GOGAT activtives in cucumber seedlings and reduceud nitrate and NH4+ contents, which could alleviate the reduce of nitrogen metabolism and promote cucumber seedling growth. Excess nitrate significantly reduced SS activity in cucumber seedlings, but had obvious effect on SPS. Souble sugar and total sugar contents of cucumber increased at eafly stage of stage, but reduced at later stage. Low LaCl3 could improve SS activity of ccucumber seedlings and promote carbon metabolism, but high LaCl3 brought combined effects to cucumber seedolings under nitrate stress.3. Effects of LaCl3 on photosynthetical characteristics in cucumber seedlings under nitrate stressThe effects of LaCl3 on photosynthetical characteristics of hydroponically cultivated cucumber seedlings were investigated under nitrate stress, and we discussed the mechanism of LaCl3 alleviating nitrate stress. The results showed that chorophyll content and carotenoids contents of cucumber seedlings were significantly restrained by nitrate stress, and quickly decreased with prolength of treatment ; Decreased photosynthetic rate due to stomatal limitation during the early nitrate stress period, but was non-stomatal limitation at later stage of stress. Exogenous LaCl3 could enhance chorophyll content of cucumber seedlings under nitrate stress and improve the gas exchange. Fv/Fm,ФPSII, AQY, CE and qp in cucumber seedlings under nitrate stress increased by applying LaCl3 , resulting in increased cucumber light utilization efficiency and CO2 assimilation. When dealing with it ,in addition,it could accelerate the effect of photosynthesis , but the effect of 0.02 mmol·L-1 LaCl3 also significanted, 0.2 mmol·L-1 LaCl3 reduced the photosynthetic rate of cucumber at threatening anaphase. Addition of different concentration LaCl3 could improve photosynthesis at primary stage of nitrate stress; but high LaCl3 (0.2 mmol·L-1) decreased photosynthesis, while low LaCl3 (0.02 mmol·L-1) also could still maintain high photosynthesis at later stage of stress.4. Effects of LaCl3 on antioxidant enzymes activities in cucumber leaves under nitrate stressEffects of LaCl3 on plant growth and antioxidant enzymes in cucumber leaves under nitrate stress were studied by hydroponics. The results indicated that the growth of cucumber roots and shoots was significantly inhibited by nitrate stress, especially the shoot. The shoot fresh weight per plant reduced 12.77 g , activities of SOD, POD and CAT increased, but APX,DHAR and GR activities significantly reduced compared to control. Application of LaCl3 markedly increased fresh weight , thermally stable protein content of leaves, activities of SOD, POD, CAT, APX, DHAR and GR, and resulted in decreased MDA content and electrolytic leakage of cucumber leaves under nitrate stress. After 7 days of treatment, addition of 0.05 mmol·L-1LaCl3 alleviated inhibition of cucumber growth by nitrate stress to some extent, shoot fresh weight increased by 35%; Application of high concentration LaCl3 had no alleviated obvious effect. So, we concluded that addition of certain concentration LaCl3 could alleviate nitrate stress on cucumber by increasing activities of antioxidative enzymes and content of thermally stable protein.5. Effects of LaCl3 on absorption and subcelluar distribution of the mineral elements in cucumber leaves under nitrate stressEffects of LaCl3 on absorption and subcelluar distribution of the mineral elements in cucumber leaves under nitrate stress were studied by hydroponics. Ca and K content of cucumber seedlings increased Fe and Mg content reduced compared to control. Application of low LaCl3 treatment considerably reduced the Ca,K content and increased Fe,Mg conten.High nitrate had effectes on subcelluar distribution of Ca,K and Fe,Mg in cucumber seedlings, especially increased Ca,K content of cytoplasm and reduced Fe,Mg content of chloroplast. Low LaCl3 significantly improved cytoplasm distribution of Ca,K and increased Fe,Mg content of chloroplast. LaCl3 treatment could improve activity of H+-ATP, Ca2+-ATPase and H+- ppase in the plasma and vacuole membrane of cucumber seedlings, and promote ion transportation and P absorption, which were causes of the effects of LaCl3 on subcelluar distribution of the mineral elements. LaCl3 of high concentration could reduce absorption of Ca,K, but La3+ could enter cytoplasm, causing injury on cucumber seedlings.6. Effects of LaCl3 on changes of endogenous hormones in cucumber leaves under nitrate stressEndogenous hormones of root were higher than those of leaf in cucumber seedlings under nitrate stress, the content of auxin and cytokinin reduced, but ABA content increased in cucumber, compared to control. Application of LaCl3 could promote cytokinin synthesis and shoot transportation, so increased cytokinin content of shoot, thereby maintaining higher level of endogenous hormones, which was helpful in alleviating nitrate stress, however, LaCl3 of high concentration had no allevating effects. |
PDF Full Text Request