TOXICITY OF NITRATES AND NITRITES IN PLANTS
C Kameswara Rao
Foundation for Biotechnology Awareness and Education
Nitrogen plays a very crucial role in the life of organisms.†† An innumerable number of nitrogen compounds are formed and utilized endlessly in metabolic processes.†
Nitrates are an integral part of the nitrogen cycle in our environment. Nitrates form from fertilizers, decaying plants, farm yard manure and other organic residues and are a common component of water and vegetarian food.† Excessive use of natural or synthetic nitrogenous fertilizers and/or use of sewage water for crop irrigation lead to an accumulation of nitrates, particularly in the leaves.† Nitrates are also byproducts of animal or human waste.† Rain or irrigation water removes nitrates into water sources.†
Nitrate concentration is an indicator of food and water quality. One part of nitrate per a million parts of water is considered as the safe limit.Elevated nitrate levels in food and water are matters for concern, because plasma nitrate can interfere with blood-oxygen levels, leading to methemoglobinemia and gastric cancer.
Methaemoglobinaemia arises out of the excessive conversion of haemoglobin (containing ferrous iron) to methaemoglobin (containing ferric iron), which is incapable of binding and carrying oxygen.†
Methaemoglobinaemia was reported in all age groups consequent upon high nitrate ingestion.† However, infants and those above 45 years of age are most susceptible to nitrate poisoning.†
The causes, symptoms and treatment of methaemoglobinaemia are well studied.† While methaemoglobinaemia may be due to anaemia, and cardiac or pulmonary disease, it is more often due to high concentrations of chemical contaminants in food or water.†† Nitrates and nitrites are an important cause for methaemoglobinaemia, aggravated by congenital deficiencies of the enzyme nitrate reductase that effectively lowers nitrate concentrations.††† Methaemoglobinaemia on account of chemical poisoning cannot be reversed by oxygen administration, unlike in cardiac and pulmonary disorders.
The blood sample of a patient with over 15 per cent of methaemoglobin has a chocolate brown colour which does not change when exposed to air. A visible symptom of methaemoglobinaemia is cyanosis, a grayish-blue colour in people, even when about 10 per cent of total haemoglobin was affected.††† Other symptoms are nausea, headache, weakness, abnormally rapid heart beat (tachycardia), breathlessness (dyspnoea), chest pain, and mental changes, which progressively worsen as concentrations of methaemoglobin increase.† Methaemoglobin concentrations of about 50 per cent result in depression of the central nervous system, while at more than 70 per cent life is threatened.†
Identification of offending agent and prevention of further exposure alone are usually adequate for mild cases methaemoglobinaemia.† Methylene blue (1 to 2 mg/kg body weight) is the specific antidote for methaemoglobinaemia, but is not effective in ruminants and does not help in respiratory distress.† There is no specific antidote for animals.† Proper grazing management is the only means.
Excess quantities of nitrates are converted in the mammalian digestive system to nitrite.† Nitrites are transformed into Nitro-compounds, many of which, such as the nitrosamines, adversely affect metabolic processes and so are health hazard.† Two other nitro-compounds derived from propionic acid, NPA and NPOH, affect respiratory enzymes such as succinate dehydrogenase.† Miserotoxin and nitrotoxin are derivatives of NPA, which enhance methane production in ruminants.† The quantity of toxic nitro-compounds can be estimated indirectly through an estimate of nitrites.††
The European Commissionís Scientific Committee on Food (ECSCF) fixed the Acceptable Daily Intake (ADI) of nitrate ion by people at 3.65 mg/ kg body wt/day.† Sheep and cattle may tolerate four or five times this level.†
Toxicity of nitrate related compounds causes two syndromes.† The Acute State (syndrome) is when a single very high dose was consumed leading to rapid onset of symptoms such as loss in co-ordination, distress, laboured breathing, excitability, cyanosis, muscular weakness, and collapse leading to death, in a few hours to one day after ingestion.†† Sheep are very susceptible.† The Chronic State (syndrome) results from the consumption of several sub-lethal doses over a long period of time, cumulatively reaching hazardous concentrations leading to weight loss, nasal discharge, respiratory distress, poor hair coat, hind limb weakness, and roaring sound.† Cattle, horses, sheep, goats, pigs, chicks, rabbits, mice and pigeons are affected, but lactating cattle and sheep are far more susceptible.†
Nitrate toxicity is concentration related.† Lighter body types are more susceptible than heavy body types and smaller animals such as sheep and goats are more at risk than cattle.
Good agricultural practices reduce nitrates in agricultural produce to prevent toxic effects.† Certain substances such as vitamins C and E inhibit the transformation of nitrate to nitrite to nitrosamines, reducing nitrate related toxicity to some extent.†
A research article in the peer reviewed journal Current Science (February 2007) reported that the nitrate content of several samples of the leafy vegetables spinach (up to 430 mg/100g) and chenopodium (up to 445 mg/100 g) from local markets in different parts of Delhi was far higher than the limits set by the ECSCF, when taken at 100 g/day, which is† about 109.5 mg/day for a 30 kg person and 219 mg/day for a 60 kg person.†
Nitrate concentration in fenugreek, coriander and sowa samples, all far less fibrous than spinach and chenopodium, was well within the safe limits.† Leaf stalks of spinach contained several times higher nitrate (up to 6269 mg/kg) than the leaves (up to 4293 mg/kg).† Apparently, fibrous/woody parts accumulate more nitrate than the herbaceous parts.
Six week old spinach and chenopodium contained higher levels of nitrate than three week old plants as nitrate seems to accumulate with age of the crop and increased fibrous tissue.†
Nitrate content was lowest at about noon time, corresponding with higher levels of activity of the enzyme nitrate reductase, which utilizes nitrate.† There were genetic differences in the crops studied in the ability to produce nitrate reductase, which in turn influences the quantity of residual nitrate.†