High Yield And Quality Of Aeroponically Grown Lettuce (Lactuca Sativa L.) Produced By Modulating Of Nutrient Solution

Aeroponics is a soilless culture technique for growing vegetables and other crops in which the roots are suspended in the air and are intermittently sprayed with a nutrient solution to supply the plants with mineral and water. It provides a possible solution to the existing problems of soil-borne diseases, soil salinity in soil-based systems and oxygen deficiency in general hydroponics systems. Aeroponics is considered to be an economical and environmentally friendly modern technology for producing natural and healthy crops. It has been widely used in commercial horticultural production, spacecraft life support system and courtyard agriculture. On the other hand an optimized nutrient solution is a key determining factor of high yield and product quality of vegetable grown in aeroponic systems. Unfortunately, the nutrient solutions with high concentration of nitrate nitrogen are commonly used in present aeroponics, which not only results in raising production costs, enhancing difficulties of management, but also in high nitrate accumulation. Therefore, it is critical to elucidate the mechanism of the effect of nitrogen supply on the yield and quality of aeroponically grown vegetables. Modulating the nutrient solution of leafy vegetables in aeroponic production is scarcely reported at present. The objective of this study is selecting appropriate nutrient formulation and comparing the difference between aeroponic and hydroponic systems, and to investigate the effect of nitrogen nutrient supply on the yield, antioxidant contents and antioxidant activity and finally to determine the optimal nutrient solution management for aeroponically grown lettuce (Lactuca sativa L.), which would provide scientific basis for the production of low-nitrate lettuce with high yield and nutritional quality. The main results obtained from the experiments are summarized as follows:1. The effects of five hydroponic nutrient solutions, Hoagland formula, Japanese Yamazaki formula (JY), Japanese Garden formula (JG), South China Agricultural University (SCAU) formula B for leafy vegetables and England Hewitt formula (EH), were compared on the biomass production and nutritional quality of aeroponically grown lettuce. The results showed that the pH levels of five solutions were increased to some extent during the treatment. The highest increase in pH was observed in EH, while the lowest was in SCAU formula B. Plants grown in SCAU formula B solution showed high fresh and dry weight of roots and shoots, compared to the plants grown in other four nutrient solutions. Furthermore, SCAU formula B treated plants showed the highest ascorbic acid content, FRAP (ferric reducing/antioxidant power) value, DPPH (1,1-diphenyl-2-picrydrazyl) radical scavenging rate as well as soluble sugar content in the leaves among all nutrient solution-treated plants. However, the soluble protein content was comparable to that of JG treated plants, while the glutathione content was lower than that of JG treated plants. Furthermore the nitrate content of stems and leaves of SCAU formula B treated plants were significantly lower than those of EH and Hoagland treated plants, but a little higher than those of JG and JY treated plants. These results indicate that aeroponically grown lettuce in SCAU formula B solution produced high biomass yield, high antioxidant content and antioxidant activity, and low nitrate content, suggesting that SCAU formula B is the most ideal nutrient solution for aeroponically grown lettuce, whereas EH and JY formula are unsuitable for aeroponically grown lettuce.2. Hydroponic and aeroponic experiments were carried out to study the effects of two different cultures on the biomass production and nutritional quality of lettuce under different nitrogen levels (2,8 and 20 mmol/L). Results showed that with the increased nitrogen level, the contents of nitrate and soluble protein in the leaves and stems increased, while concentration of soluble sugar, ascorbic acid and glutathione, FRAP value and DPPH radical scavenging rate decreased in both hydroponics and aeroponics. Highest root morphology index, photosynthetic index, fresh weight, accumulation of soluble protein, soluble sugar, ascorbic acid and glutathione were observed at the nitrogen level of 8 mmol/L. Under the same nitrogen level, root growth, biomass, relative growth rate, SPAD value and photosynthesis in aeroponics were significantly higher than those in hydroponics, while glutathione and nitrate content in levels and stems in aeroponics were much lower than in hydroponics, and with similar or higher FRAP value and DPPH radical scavenging rate compared with hydroponics. Although there was no significant difference was observed in soluble sugar, soluble protein, ascorbic acid in two culture systems, but the accumulation of soluble protein, soluble sugar, ascorbic acid and glutathione in aeroponics were markedly higher than those in hydroponics. Therefore, aeroponics is beneficial for higher biomass, antioxidant accumulation and antioxidant activity, its especially apparent under the nitrogen level of 8 mmol/L.3. An aeroponics was carried out to study the effects of different nitrogen levels (4,8,12,16 and 20 mmol/L) on the biomass, antioxidant contents and antioxidant activity in lettuce. Results showed that because of increased nitrogen level from 4 mmol/L to 8 mmol/L, biomass of lettuce increased markedly, but biomass of lettuce was not affected with nitrogen level further increasing up to 12 mmol/L. When superfluous nitrogen was supplied (16 and 20mmol/L) not only decreased biomass obviously, but it also caused the highest nitrate accumulation. Furthermore, soluble sugar content in the leaves and stems decreased and soluble protein content increased firstly and then decreased slightly with nitrogen levels increasing. The contents of ascorbic acid, glutathione, total phenlics and flavonoids in leaves decreased as nitrogen levels were elevated. But when nitrogen level was increased from 4 mmol/L to 8 mmol/L, except for ascorbic acid and DPPH radical scavenging rate which decreased, there was no significant changes in the contents of glutathione, total phenlics, flavonoids and FRAP value. As biomass and nutritional quality of lettuce were taken into account, nitrogen level of 8 mmol/L might be the optimal nitrogen level for aeroponically grown lettuce.4. An aeroponics was conducted to investigate the effects of different NO3-:NH4+ ratio (100:0,75:25,50:50,25:75,0:100) on the biomass, nutritional quality and antioxidant activity in lettuce. Results showed that with the increasing proportion of ammonium in the nutrient solution, the biomasses, SPAD values, photosynthetic index and accumulation of ascorbic acid, glutathione, total phenlics, flavonoids and carotenoid increased markedly and later decreased gradually, nitrate and soluble sugar decreased, while soluble protein, ascorbic acid, glutathione, total phenlics, flavonoids, carotenoid, FRAP value and DPPH radical scavenging rate increased gradually in aeroponically grown lettuce. The highest contents of soluble protein, antioxidant contents and antioxidant activity were observed when nitrate was totally replaced by ammonium, but ammonium inhibited fresh biomass seriously. When NO3-:NH4+ratio of 75:25 was supplied, photosynthesis, fresh biomass and accumulation of antioxidant in leaves were found to be highest. This indicated that appropriate amount of ammonium (25%) could promote the accumulation of biomass and antioxidants, enhance photosynthesis ability and decrease nitrate content. Hence, NO3-:NH4+ratio of 75:25 should be optimal for NO3-:NH4+ratio for the production of both high-yield and high-quality aeroponically grown lettuce.5. An aeroponic culture was carried out to investigate the effects of nitrogen supply in the nutrient solution (100% NO3-,50%NO3-,0%NO3-,50%NO3-+50%NH4+ 50%NH4+) before harvest on biomass, antioxidant content and antioxidant activity in lettuce. Results showed that, after 3 days'treatments, there were no significant effects on fresh biomass, dry weight, nitrate and antioxidant content of aeroponically grown lettuce. After more than 6 days'treatments, nitrate content in leaves and stems were significantly reduced by all the nitrogen supply methods, compared to continuous supplying 100%NO3- continually, and biomass were reduced in treatments of 0% NO3-,50%NH4+ and 50%NO3- in a certain extent, but was significantly increased in 50%NO3-+50%NH4+. Except for the treatment of nitrate withdrawal (0%NO3-) in which ascorbic acid and glutathione content were reduced, the other treatments increased antioxidant contents and improved antioxidant activity in leaves of lettuce, and after more than 6 days' treatment, the influence could be more and more remarkable. Highest contents of ascorbic acid and glutathione were obtained in 50% NH4+, and then in 50%NO3-+50%NH4+, and highest contents of total phenlics and flavonoids were obtain in 0% NO3-, and then in 50%NO3-+50%NH4+. It is suggested that the 100% of nitrogen concentration supplied with mixed nitrogen source (50% NO3-+50%NH4+) in the nutrient solutions 6 days prior to harvest may be suitable to produce aeroponically grown lettuce with low nitrate, high yield, antioxidant content and antioxidant activity.