Green leafy vegetables are the source of the highest amount of nitrates that humans take in daily, constituting about 17-94%. Though nitrate is necessary for many human processes, part of the nitrate that we ingest becomes nitrite and N-nitroso compounds which have adverse effects on health (Gupta et. al., 2008). There are also many conflicting acceptable daily intake (ADI) amounts of nitrate from various organizations. The Joint Expert Committee on Food and Agriculture in the World Health Organization claims that the acceptable daily intake is 0- 3.7 mg/ kg-1 body weight. The US Environmental Protection Agency’s limit for daily ingestion is 7.0 mg/ kg-1 body weight. In 1995 the European Commissions Scientific Committee for Food designated that the limit for nitrate injection per day is 3.65 mg/ kg-1 body weight. The article titled that we read titled “Health Issues Related to N Pollution in Water and Air” (Gupta et. al., 2008) cited an example that if we assume a body weight of at least 60 kg, the ingestion of just 100 g of fresh green leafy vegetables with a nitrate concentration 2500 mg/ kg-1 weight exceeds the acceptable daily intake of nitrates by about 13% and this does not take into account any other nitrate rich sources such as water or meat that one takes in, in one day (Gupta et. al., 2008).
The nitrate content of samples of both spinach and other similar green leafy vegetables being sold in the Indian market have been found with up to 4451 and 4293 mg/ kg-1 weight. There also exists a large intraspecific variation of nitrate concentration based on genotype studies that have been done (Gupta et. al., 2008). Through these studies it was found that the nitrate concentration was very different based on genotype and then that dictates at which stage in growth the plat has the highest level of nitrates. For example one genotype studied exceeded the acceptable daily intake limit at three weeks growth, while six other genotypes did not exceed that limit until six weeks into growth. It was also observed that the leafstalk contains more nitrates than the leaves of the plant (Gupta et. al., 2008). But one constant in all of the genotypes studied was that they were all on a diurnal cycle on nitrate accumulation, with all having the least amount of accumulation at noon. This is because there is a direct relationship between high irradiance and nitrate accumulation which creates the diurnal pattern (Cárdenas-Navarro, Adamowicz, & Robin, 1999) .
Nitrate levels in green leafy vegetables has a inverse relationship with the enzyme nitrate reductace, which is largely responsible for the different nitrate concentrations that exist in different genotypes in one species.
Nitrate reductace
Source: Wikimedia Commons
So by over expressing nitrate reductace genes in genotypes with high nitrate accumulation we would see a greater reduction in nitrate accumulation throughout that genome as a whole (Gupta et. al., 2008). Along with that it is also important to enhance the expression of nitrate reductace so that the nitrate does not accumulate after converting to nitrite. This all would make the green leafy vegetables safer for consumption for both humans and animals but on top of that we also need to carefully select the vegetables genotypes based on their studied nitrate levels and nitrate reductace activity, harvest the plants during the lowest accumulation in their diurnal nitrate cycle (at noon) and remove the leafstalk before putting on the market so that the ingestion level of nitrates could be significantly lowered (Gupta et. al., 2008).
In another article I read titled “Nitrate Accumulation in Plants: A Role for Water” it was discussed different outside sources that can have an effect on nitrate accumulation in plants (Cárdenas-Navarro, Adamowicz, & Robin, 1999) . It is noted that there is a correlation between plant water and nitrate contents per g-1 of dry weight in that “plant nitrate (mol g-1 dry wt.) mimics water content (m3 g-1 dry wt.) changes and therefore varies highly with the… endogenous and exogenous parameters: genotype, distribution in plant, short and long term dynamics, light irradiance, environmental conditions and N nutrition (Cárdenas-Navarro, Adamowicz, & Robin, 1999) .
This graph shows the whole plant nitrate-water per g-1 of dry weight relationship in lettuce. The top graph shows data for both day and night showing that the water-nitrate relationship exists in both plant cycles for a fully-grown lettuce plant. The graph on the bottom shows the overall relationship between nitrate level and water level per g-1 of dry weight over the lettuce’s life span (Cárdenas-Navarro, Adamowicz, & Robin, 1999) . So these findings, along with what was mentioned earlier about harvesting the plants by noon and selecting genotypes with the most expressed nitrate reductace and other methods, we can reduce the nitrate accumulation and help to make plants safer to eat, especially in developing regions where more nitrate has been present in their water and vegetables.
Bibliography
Cárdenas-Navarro, R., Adamowicz, S., & Robin, P. (1999). Nitrate Accumulation in Plants: A Role for Water. Journal of Experimental Botany , 50 (334), 316-324.
Gupta, S. K., Gupta, R. C., Chhabra, S. K., Eskiocak, S., Gupta, A. B., & Gupta, R. (2008). Health Issues Related to N Pollution in Water and Air. Current Science , 94 (11), 1470.
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