Although the experiments were conducted under hydroponic conditions without rice paddy soil which have thesorption capacities to 2,4-DBP and 2,4-DBA to limit their direct volatilization and bioavailability, it still could be concluded that phytovolatilization was an effective process for the exchange of 2,4-DBP and 2,4-DBA between contaminated sites and the air phase. Moreover, phytovolatilization as well as total volatilization of 2,4-DBP and 2,4- DBA in planted treatments were closely related to the exposure time, biomass and growing status of the rice plants. With regarded to the long term exposure of those two contaminants, the biomass of rice seedlings gradually increased with growth, and the phytovolatilization would be significantly enhanced and become more important in the long distance transportation of 2,4-DBP and 2,4-DBA.Methylation and demethylation metabolites of 2,4-DBP and 2,4- DBA were detected in their individual exposure systems. Xenobiotic contaminants are generally metabolized under the biocatalysis of plant enzymes after being taken up by plant. O-methyltransferase and demethyltransferase have been extensively observed and mediate the methylation and demethylation of contaminants in high plants. None of daughter 2,4-DBP and 2,4-DBA were detected in both planted blank controls without PUF and the unplanted treatments with PUF, con- fifirming that methylation of 2,4-DBP and demethylation of 2,4-DBA were all mediated by rice plants, not chemical reactions in exposure systems. Similar interconversion was also reported between OH-PCBs and MeO-PCBs within rice plants . For the 2,4-DBP exposure system without PUF, the methylation product was rapidly formed in rice roots after 6 h exposure , and then constantly decreased. The methylation product could also be detected in both leaf and leaf sheath.

The concentration was firstly increased and then decreased in leaf sheath  but always increased in rice leaf during the exposure . The total amount of methylation metabolite increased during 6–48 h,nft growing system and then decreased from 155.3 ng  to 97.78 ng . Majority of methylation metabolite was distributed in the culture solution, accounting for 68.6–83.8% of the total amount of metabolite. While for exposure of 2,4-DBA, as seen from Fig. 4B, no demethylation metabolite was detected in exposure solution. The demethylation product was extensively detected in various rice tissues. The concentrations of demethylation product in rice leaf, root and leaf sheath and the total concentration in whole rice plant were all constantly increased during the exposure period, and reached to 16.34, 42.66, 61.78 and 39.53 mg kg−1 at the end of exposure. A majority of the total demethylation metabolite was accumulated in rice roots before 72 h, while demethylation product became dominant in the leaf sheath and leaf at 120 h. Likewise, the formed metabolites, 2,4-DBP and 2,4-DBA, were phytovolatilized from rice seedlings into the headspace. And the methylation metabolite  which observed in hydroponic solution could also be directly volatilized into the headspace. Herein, all of metabolites in exposure solution, plant tissues and those volatilized into the headspace were included to evaluate the transformation ratio  between 2,4-DBP and 2,4-DBA in the sealed system. The total volatilized methylation and demethylation metabolites detected in air phases of exposure systems with PUF were 126.8 ng and 218.5 ng . Including these volatilized metabolites, the demethylation ratio of 2,4- DBA was 12.0%, that is 32.0 times higher than the methylation ratio of 2,4-DBP  after exposure for 120 h . The demethylation process was obviously faster than the methylation process. A similar phenomenon was previously reported on the interconversion of OH– and MeO-PCBs in rice seedlings, in which the demethylation process was 7.70–18.2 times greater than methylation . Results illustrate that bromoanisoles are readily demethylated within rice plants and subsequently released as bromophenols into environment. Unfortunately, bromophenols are precursors to form more lipophilic and bioactive products  in rice plants, such as hydroxylated polybrominated diphenyl ethers  and polybrominated dibenzo-p-dioxins/dibenzofurans.

Therefore, the occurrence of bromophenols and bromoanisoles needs to be considered as a potential risk factor in consumption of food plants.The contributions of inter conversion processes to the volatilizationof parent contaminants were further quantitatively evaluated and summarized in Fig. 5C and D. According to the equimolar reaction between parent and daughter compounds, the volatilized methylation and demethylation metabolites were transformed from 0.48 and 0.87 nmol of parent 2,4-DBP and 2,4-DBA accounting for 0.07% and 0.13% of their initial amounts , respectively. These amounts enhanced 4.95% and 2.69% of total volatilization mass , and 12.1% and 36.9% of phytovolatilization mass of parent 2,4-DBP and 2,4-DBA, respectively . Obviously, methylation and demethylation processes served as important strategies to volatilize their corresponding parent chemicals out of intact plants. Furthermore, the volatilization of metabolites was also an important process to reduce bioaccumulation of parent chemical in plants. These observations indicate that interconversion provides another pathway to volatilize phenolic chemicals and methyl-phenols pollutants from contaminated sites into the air phase. After calculation the mass balance of 2,4-DBP and 2,4-DBA in the treatments and exposure systems with PUF , it was found that the total recovered 2,4-DBP and 2,4-DBA in the unplanted treatments with PUF were only 88.5% and 87.0%, respectively. The photodegradation of target chemicals was avoided in incubation system by wrapping aluminum foil outside the brown glass reactors, and microbial transformation  was minimized by pre-autoclaving the solutions and glass bottles before exposure experiment. Therefore, we inferred that those unrecovered target compounds were resulted from the incompletely sampling of the volatilized chemicals in air phase. Although PUFs were placed in the systems, the analytes detected on PUFs only represent a fraction of the volatilized amounts. Namely, the total volatilization and the contribution of the interconversion to the volatilization are all underestimated. In planted exposure systems with PUF, the recoveries of 2,4-DBP  and 2,4- DBA  were significantly lower than those of unplanted treatments with PUF. Though non-sterilized rice plants were used for exposure experiments, our former study has shown that the microbial transformation  of 2,2′,4,4′-Tetrabromodiphenyl Ether  was far lower than pumpkin plant in the similar cultivation and exposure conditions . Thus, the significant differences between unplanted and planted treatments were inferred with the occurrence of other metabolic pathways or bound residues in rice plants .Lettuce  is a rich source of antioxidants such as polyphenols, ascorbic acid and carotenoids .

However, antioxidant content in plants can be manipulated by growing conditions and agronomic practices . At the same time, pre-harvest factors such as fertiliser application play a major role in determining product quality and shelf life of fruits and vegetables . Shortage or excess of nitrogen can positively or negatively affect quality parameters and nutritional components of lettuce . Although information is available on the influence of nitrogen in soil  or hydroponic systems on quality, the information on post harvest quality and bioactive compounds is limited. Nitrogen application of 225 kg ha–1 showed the least postharvest decay in romaine lettuce and defects in iceberg lettuce.Higher N application rates have been reported to negatively affect post harvest quality in romaine lettuce . Furthermore, Luna et al. recommended moderate levels of N to obtain better postharvest quality in lettuce. Growing salad vegetables with a short growing cycle like lettuce in a hydroponic system is a popular practice, which has many advantages such as providing good quality and sanitary products without soil contaminants, while benefiting the environment by reducing water and nutrient usage.Weight loss, colour, texture and appearance greatly affect post harvest quality,consumer acceptance and the saleable price of fresh cut lettuce.The term appearance describes the size, shape, wholeness, presence of defects and consistency for fresh cut vegetables . Texture of vegetables is influenced by cellular turgor pressure, which determines the consistency or weight loss of the product . In current marketing practice, fresh cut lettuce are packed in modified atmosphere packaging  and maintained at low temperature storage to retain quality and nutritional components similar to the whole original product at harvest . Shelf life of fresh cut vegetables is limited due to enzymatic browning that alters the colour of the product due to the production of brown pigments . Browning in fresh cut lettuce is one of the primary causes of quality loss . Furthermore, browning of lettuce affects sensory and biochemical properties which affect consumer acceptance of the product . Phenylalanine ammonialyase activity, polyphenol content , polyphenol oxidase and peroxidase activities are involved in the production of o-quinones browning pigments . Although several chemical treatments have been recommended to control the phenolic metabolism associated with browning, there are concerns about chemical toxicity related to food safety with regards to the recommended anti-browning agents or treatments .

Agronomic practices such as nitrogen  application rates were demonstrated as a tool to manipulate the enhancement of phytochemicals . Furthermore, N application rates can influence the quality and shelf life of fresh cut products . Since N application influences cell size, nft hydroponic system number of leaves and fresh mass, N available during the growth phase of the plant can also influence post harvest quality and shelf life . It is also important to understand the impact of nitrogen application on storage loss and phytochemical properties in red and green loose leafy lettuce cultivars used for fresh cuts. Optimum N application rates to obtain desirable yields with higher bioactive compounds and quality attributes differ between varieties . However, in practice, limited standard protocols are available for lettuce breeding companies in relation to fresh cut processing on cultivar selection or recommended preharvest N application. Therefore, the aim of this study was to determine the influence of different N application rates on green and red lettuce varieties grown for fresh cuts, packed in standard MAP and held at 5°C up to 12 days on the retention of overall quality , browning enzymes PAL, polyphenol oxidase , peroxidase , browning substrates  and ascorbic acid content. Percentage weight loss increased with storage time irrespective of the rates of pre-harvest N application in Multigreen 1  and Multired 4 . However, in Multigreen 3, lower rates of N application at preharvest stage showed higher weight loss with the storage time increasing . When compared the three varieties of fresh cut lettuce, the red variety, Multired 4 revealed higher percentage of weight loss. Percentage weight loss in Multigreen 1 was lower than in Multired 4 with the storage time increasing. In green variety Multigreen 3, the weight loss  increased around 1 to 2% with preharvest application less than 90 mg L–1 N. Weight loss is a vital factor associated with the saleable weight during marketing and weight loss higher than 5 to 10% has been reported to reduce the saleable value of fresh produce due to wilting . Weight loss is associated with water loss due to transpiration  which can occur through damage of the barriers that protect against transpiration during fresh cut processing . Weight loss within the MAP is affected by transpiration and moisture condensation within the packaging . However, in this study, the response to preharvest N application rates during storage differed between the different varieties. Lower rates of N application influenced weight loss with the storage time increasing in fresh cut green lettuce Multigreen 3 .

From our findings, the percentage weight loss was higher in Multired 4 due to lower thickness of the cuticle. According to previous reports, the influence of N application rate was very low on weight loss in crisphead cultivars Marius and Saladin . Unfertilised Butterhead lettuce  planted in soil showed higher weight loss than lettuce fertilised with 100 kg ha–1 N during 12 days of storage .Leaf colour of fresh cuts of the different types of lettuce varieties varied in response to preharvest N application during storage. Colour value L* , was not significantly affected by the different N application rates in fresh cuts of variety Multigreen 1 . When the light intensity decreased, leaves became darker with the storage time increasing . Although Multigreen 3 fertilized with lower preharvest N application retained the lightness or glossiness of the leaf up to 3 days during postharvest storage , the light intensity decreased irrespective of lower or higher preharvest N application rates with the storage time increasing However, in red variety Multired 4, light intensity was not remarkably influenced by storage time or preharvest N application rates .