The lettuce cultured using the 2-fold ozonetreated sludge dilution solution showed significantly higher contents of chlorophyll, soluble sugar and Vc. The 2-fold ozone-treated sludge dilution solution improved the quality of hydroponic lettuce compared to the modified Hoagland nutrient solution used traditionally. It was also observed that unsuitable concentrations of ozone-treated domestic sludge dilution solution could inhibit lettuce growth. High sludge concentrations inhibited lettuce growth and at low sludge concentrations the nutrition provided was insufficient to satisfy the lettuce growth demand . The leaf number, plant height, fresh weight and dry weight in the lettuce cultured by the modified Hoagland nutrient solution were significantly higher than those cultured by any of the ozone-treated sludge dilution solutions. While the nitrate content in the lettuces cultured using the 1-, 2-, 3-, and 4-fold ozone-treated sludge dilution solutions were 34.3, 53.93, 55.34, and 68.60% lower than that in the modified Hoagland nutrient solution treatment, respectively, although the chlorophyll, soluble sugar and Vc contents increased. The various physiological indexes of lettuce were not independent and there was a certain relationship between them . As shown in Table 3, nitrate content had a significant or extremely significant correlation with leaf number, plant height and dry weight, and the correlation coefficient were 0.99, 0.86, 0.87, respectively. The result indicated that the nitrate content would increase with the leaf number, plant height and dry weight increasing. Thus, the leaf number, plant height and dry weight could be appraisal indexes of nitrate content. So, the nitrate content could be controlled within the scope of the security by the leaf number, plant height and dry weight.
And dry weight had a significant or extremely significant correlation with leaf number, plant height and root length, with the correlation coefficient of 0.89, 1.00, and 0.90, respectively. This was consistent with others reports . But the correlation for chlorophyll or soluble sugar content with other physiological indexes was not significant. Additinally, Vc had a significant correlation with leaf number. Some researches showed that the significant correlation among physiological indexes and quantitative trait loci of corresponding physiological indexes may exist chain effect . Therefore, vertical farming racks the correlation among physiological indexes could be understood further by the correlation analysis among physiological indexes. The results could provide basic theory for genetic analysis of lettuce stress resistance in the future. The ozone-treated domestic sludge dilution solutions investigated in this study were thought to be an efficient liquid fertilizer substitute. Here, improvements in terms of physiological, biochemical and nutritional characteristics and safety in the lettuce growth process were observed. Before the sludge was treated by ozone, the number of fecal E. coli bacteria was measured. The results showed that the number of the feces in the sludge before ozone treating ismore than 1×105 L–1, which was measured with the most probable number method, while the number of the feces in the modified Hoagland nutrient solution is zero. And the fecal E. coli was not detected in the 1-, 2-, 3-, and 4-fold ozone-treated sludge dilution solutions after ozone treated for 120 min, so ozone treatment showed strong bactericidal ability, which was beneficial to the effective control of pathogenic microorganisms. But heavy metal distribution in the sludge solution and lettuce cultivated with sludge must be studied in subsequent research. And further studies will also be performed on food safety.
Using ozone treated sludge dilution solution as fertilizer in hydroponic systems has the potential to outperform traditional fertilizers and to cut costs for growers, however, further research is necessary.It is evident from this study that increased accumulation of bio-active compounds, antioxidant properties and mineral composition can be achieved without compromising the yield by selecting less N sensitive varieties that can use N effectively. N application generally improved leaf area and attributed increased chlorophyll content due to the effective use of photosynthetically active radiation . However, the morphological response of the green lettuce varieties to different N supply rates differed from the reports of Mahlangu et al. where, Lollo Bionda Lobi leafy lettuce grown in non-circulating hydroponic system failed to show any increase in fresh weight although the form of N source was NH4 NO3 . In this study, fresh weight in the green varieties showed strong correlations with increasing N supply . However, discrepancies in results could be due to the type of growing system used or it can also be due to the genotype. In red lettuce varieties, photosynthetically produced carbon molecules are used more for the production of phenolic compounds than for the use of plant growth and fresh weight . This relates to the C/N shift theory that shows a shift to carbon based metabolites under N deficiency for the biosynthesis of phenolic acids and flavonoids . The reports of Becker et al. further confirmed that the activity of phenylalanine ammonia-lyase that is responsible for the biosynthesis of polyphenol propanoids are higher at lower N concentrations, which further explains the observed increased phenolic acids and flavonoids and lower fresh leaf weight in var. Multired 4 in this study. Slight divergence in the trends were observed with the biosynthesis of phenolic acids in green varieties with respect to N application rates in this study and the reports of Becker et al. . The lettuce genotypic differences could have attributed to these observed differences.
Increasing N application rates were shown to reduce phenolic acids and flavonoids in butter head lettuce cv. Egery . However in this study the response to increasing N supply with regards to the accumulation of different phenolic acids and flavonoids composition differed . N supply affects leaf chlorophyll content and in this study N application rates between 90 to 120 mg L–1 were sufficient to maintain the chlorophyll content to carry out photosynthesis effectively in both green cultivars . This observation coincides with the findings of Mahlangu et al. on the green lettuce cultivar Lollo Bionda Lobi grown in a non-circulating hydroponics system where 100 to 120 mg N L–1 was reported to be sufficient to maintain photosynthesis. However in cultivar Lollo Bionda Lobi, the relationship model between the applied N rates and the chlorophyll content fitted linearly whereas in Mulitigreen 1 and Multigreen 3, they showed a polynomial model . The observed discrepancies in chlorophyll content could be due to the lettuce genotypic differences. Increased rates of N applications have been reported to reduce the ascorbic acid content in different fruit and vegetable crops . On the contrary, Chiesa et al. reported that high N increased ascorbic acid content in lettuce. Variation in accumulation of ascorbic acid content with respect to different rates of N applications varied with the findings of Mahlangu et al. where the ascorbic acid concentration was higher at 100 to 120 mg N L–1. In this study, the highest ascorbic acid content was obtained at 60 mg L–1 in both green and red varieties and as shown by Mahlangu et al. it declined with increasing rates of N application. The observed divergences with regards to the N application rates and ascorbic acid content in lettuce can be explained due to the type of N source and application methods. Type of fertilizers such as ammonium nitrate or urea was shown to increase the ascorbic acid content compared with ammonium sulphate . Abd El-Rehem and Abd El-Baky demonstrated that the use of ammonium nitrate reduced the ascorbic acid content with higher application rates. IM and FNB recommended daily intake for an adult male and female is 90 and 75 mg of ascorbic acid. It is evident in this study that 100 g of Multired 4 provides 17 mg of ascorbic acid at 60 mg N L–1 and similarly 100 g of Multigreen 1 and Multgreen 3 contained 14 and 15.5 mg of ascorbic acid, respectively.
Phenolic acids, vertical rack system flavonoids and ascorbic acid are known as antioxidants and play a major role in human nutrition. Stefanelli et al. reported that the changes in total phenol content and the antioxidant power revealed similar trend with increased rates of N applications and that the highest FRAP activity and total phenolic content was attained at 400 mg N L–1. Heimler et al. established a positive correlation between total phenolic content and antioxidant capacity in 5 out of 10 lettuce cultivars. The trend in antioxidant activity in this study differed from the findings of Mahlangu et al. , which showed higher activities at 120 mg L–1. In this study in all three cultivars the antioxidant power and capacity were higher at lower N concentrations and decreased with higher rates of N application.Differences in nitrate accumulation between the green lettuce and red lettuce varieties were noted in this study and a similar observation was reported by Becker et al. . There is discrepancy between the findings of this study and the reports of Becker et al. with regard to increasing nitrate content in the leaves with increasing rate of N application. Becker et al. stated that the nitrate residue did not increase steadily with increasing N concentration in the nutrient solution. Permitted nitrate content for greenhouse lettuce is set at 4 500 mg kg−1 of fresh weight when grown from 1 October to 31 March and at 3 000 mg kg−1 from 1 April to 30 September by the European Community . Although the nitrate levels in the leaves were calculated on a dry weight basis in this study, the contents were much lower than the levels set by the European Community. It can be recommended to use less N sensitive varieties to promote the production of phytochemicals and to reduce accumulation of nitrates in a controlled production system.
Reports of Stefanelli et al. stated that Ca increased while B, Mg, Mn, and Zn significantly decreased with increasing N application rates. In this study N application rates helped to improve the mineral composition in the leaves and different accumulation trends were noted with respect to the different cultivars with the red cultivar responding well. Petropoulos et al. showed the influence of N application on Mn and Zn in lettuce leaves . However, in this study, N application influenced the accumulation of Mn in Multigreen 3 . An increase in minerals especially Fe, and Mn that are an important source of micro elements in lettuce will add value by enhancing the nutritional value in the food chain. Rice is the foremost staple food in Asia, feeds more than 60% of the population in China and constitutes up to 40% of the total national grain production . At present, there is an urgent need to shift rice production toward mechanized farming . Mechanical transplantation is an emerging rice planting method in China and represents an important opportunity for scaled, simplified, commercialized, and modernized development . Mechanical transplantation has several advantages, such as high and stable yields and high efficiency , and has developed rapidly in China in recent years; the use of mechanical transplantation will guide the future direction of national agricultural production to solve the problems concerning severe shortages in the rural labor force and to achieve full mechanization of rice cultivation. However, traditional rice seedling cultivation methods for mechanical transplantation are time consuming and laborious, and collecting topsoil for the boxes can disrupt soil structure and subsequently reduce crop yields. All of these factors limit the development of mechanical transplantation of rice in China. The use of hydroponically grown long-mat rice seedlings represents a novel seedling cultivation method that reduces labor costs and improves efficiency . This method presents two main features: first, non-woven cloth is used as the medium instead of soil, and the seedlings are cultivated hydroponically; second, nursery seedling boxes that are 3 to 6 m in length are used rather than traditional small rectangular boxes . The grain yield and economic benefit of HLMS were equivalent or superior to those of traditional nutritive soil methods because of the robust seedling quality, better transplantation quality and higher dry matter accumulation during late growth of the former ; in addition, the root entwining force of HLMS is enhanced, resulting in significantly increased flexibility of seedling age and enabling high-yielding and -efficiency rice production .