| Vegetable is indispensable food in human daily life, but the traditional way of soil cultivation is more and more difficult to meet the growth of population demands for the number and quality of vegetables. Aeroponics is a new soilless culture technique for growing vegetables in which the roots are suspended in the air and are intermittently sprayed with nutrient solution to supply the plants with mineral and water, and can effectively solve the problem of root rot in conventional hydroponics systems. It provides a possible solution to the existing problems of soil-borne disease, soil salinity, nutrient imbalance in soil-based systems and oxygen deficiency in general hydroponics systems, at the same time can improve the nutrient and water use efficiency, and give full play to the vegetable production potential, improve the nutritional quality of the vegetable. An optimized nutrient solution is the key determining factor of high yield and product quality of vegetable grown in aeroponic systems. Previous studies have investigated the effects of nutrient solution concentrations, salt concentrations on the yield and nutritional quality of vegetables in hydroponics systems. However, it is rarely reported that the effects of nutrient solution concentrations, salt concentrations and salt stress on the yield and quality of aeroponically grown vegetables. Therefore, it is critical to solve the problems of cost-yield-quality in aeroponics systems by elucidating the mechanism of the effect of concentrations and salt concentrations in nutritional solution. The objective of this study is to investigate the effects of nutrient solution concentrations, salt concentrations and short-term NaCl stress and recovery prior to harvest on the yield, antioxidant contents and antioxidant activity, and to determine the optimal nutrient solution management for aeroponically grown lettuce, which provide scientific basis for the production of vegetables with high yield and nutritional quality. The main results obtained from the experiments are summarized as follows:1. As the South China Agricultural University formula B for leafy vegetables as control (1S), an aeroponics was carried out to study the effects of two nutrient solution supplies, a constant concentration in the whole stage (1/2S、2/3S、/4S、1S、3/2S、2S) and early lower concentration and later normal concentration (1/2S:1S、2/3S:1S、3/4S:1S) on the biomass, antioxidant contents and antioxidant activity in lettuce. Results showed that when at a constant concentration of nutrient supply, with the increased nutrient solution concentrations, the growth rate, biomass, SPAD value increased, but when the nutrient solution concentration exceeds1S, there was no significant difference was observed between the above indicators and1S, while the the content of nitrate increased with the improvement of nutrient solution concentrations. When at a stepwise concentration of nutrient supply, compared to1S, the growth rate, biomass, SPAD value of1/2S:1S treatment significantly reduced, while there was no significant difference of2/3S:1S, and the nitrate accumulation reduced significantly. The contents of soluble protein, soluble sugar, ascorbic acid, glutathione, total phenols, flavonoids, FRAP value and DPPH radical scavenging rate decreased as nutrient solution concentrations were elevated. Although there was no significant difference was observed in the treatments of2/3S:1S and1S. Therefore, aeroponics is beneficial for faster grate rates, higher biomass, antioxidant nutritional quality and reduce nitrate accumulation significantly, its especially apparent under the nutrient solution concentrations of2/3S:1S.2. As the South China Agricultural University formula B for leafy vegetables as control (1.8dS/m), an aeroponics was carried out to study the effects of different electrical conductivity (2.4dS/m、3.0dS/m、3.6dS/m、4.2dS/m) on the biomass, antioxidant contents and antioxidant activity in lettuce. Results showed that because of increased electrical conductivity from1.8dS/m to3.0dS/m, biomass, root growth, SPAD value, net photo synthetic rate, intercellular CO2concentration, soluble protein and soluble sugar were significantly increased. When superfluous salt was supplied decreased the contents obviously. With the increased nutrient solution electrical conductivity, leaf stomatal conductance, transpiration rate and the content of nitrate decreased, while the contents of ascorbic acid, glutathione, total phenols, flavonoids, FRAP value and DPPH radical scavenging rate increased markedly and later decreased gradually, at the salt concentration of3.0dS/m hit their highest point. As biomass and nutritional qualities of lettuce were taken into account, the salt concentration of3.0dS/m might be the optimal electrical conductivity for aeroponically grown lettuce.3. As the South China Agricultural University formula B for leafy vegetables as control (2dS/m), with the method of NaCl regulating nutrient solution conductivity, an aeroponics was carried out to study the effects of different salt stress prior to harvest (4dS/m、6dS/m、8dS/m) on the biomass, antioxidant contents and antioxidant activity in lettuce. Results showed that there was no significant difference of the biomass and the MDA contents was observed in the treatments at12h prior to harvest. Compared with the controls, biomass decreased significantly and the content of MDA increased significantly, while there was no significant difference was observed in the treatments of4dS/m and6dS/m after24h of stress. It is obvious that Salt stress can significantly reduce the nitrate content and improve its antioxidant content and antioxidant activity. With the extension of the treatment time, the contents of ascorbic acid, glutathione, total phenols, flavonoids, FRAP value and DPPH radical scavenging rate increased markedly and later decreased gradually. Along with the increased of the salt stress, the antioxidant contents and antioxidant capacity increased, but there was no significant difference was observed in the treatments of6dS/m and8dS/m. So it is added the6dS/m of salt stress in the nutrient solution24h prior to harvest that can yet regard as one of the effective ways to achieve safe and nutritious aeroponically grown lettuce.4. As the South China Agricultural University formula B for leafy vegetables as control (2dS/m), with the method of NaCl regulating nutrient solution conductivity, an aeroponics was carried out to study the effects of different short-term salt stress and recovery prior to harvest (72h of6dS/m salt stress,24h of6dS/m salt stress and24-48h of recovery) on the biomass, antioxidant contents and antioxidant activity in lettuce. Results showed that although salt stress inhibits the plant growth but there was no significant difference of the biomass compared with the control when the stress finished and recovery24h. Nitrate content decreased significantly under salt stress, the content of nitrate increased slightly at24h of recovery, but there were no significant differences compared with salt stress treatment. But at this time, the MDA content was significantly higher than the control. The highest value of ascorbic acid, glutathione, total phenols, flavonoids, FRAP value and DPPH radical scavenging rate appeared at recovery24h of post-stress. Hence, it is suggested that use24h of mild salt stress salt stress and24h of recovery in the nutrient solution prior to harvest may be suitable to produce aeroponically grown lettuce with low nitrate, high yield, antioxidant content and antioxidant activity. |
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