The current use of nitrates as a nitrogenated fertiliser in intensive farming has given rise to environmental problems such as the contamination of water or the degradation of the ozone layer. There are also health problems such as deficiencies in the oxygenation of blood in breast-feeding mothers or the accumulation of nitrosamines, substances that are said to be cancerous, in the adult stomach.

This is why it is necessary to find a balance between agricultural productivity and quality crops, while maintaining environmental conditions.

These are the conclusions of M� Dolores Dom�nguez Valdivia of the Department of Environmental Sciences at the Public University of Navarre in her PhD thesis entitled, Mechanisms of tolerance to ammonium in plants of agronomic interest.

The study is within the framework of research being carried out by researchers at the Public University of Navarre in this field and which has enabled new data to be obtained about the response mechanisms of various plant species arising from the stress produced as a result of the use of exclusively ammoniacal fertilisers. Stress in this sense is understood as the processes that produce a decrease in plant growth and thereby the productivity of crops. A thorough knowledge of these questions enables a more rational use of fertilisers in farming.

Research for sustainable farming

The possibility of using animal ammonium or urea as nitrogenated sources from certain plants enables the study of these forms of nitrogen as an alternative to nitrates. Nevertheless, ammonium, despite being the most respectful with the environment, does not produce the same growth in most crop plants as that when nitrate is used as a fertiliser. The studies carried out to date had revealed that the use of nitrate over years favours � in most plants � a sensitivity or even toxicity to ammonium. However, the tolerance shown by some crop species such as the pea is of great interest due to the studies of the physiological, metabolic and molecular mechanisms that the ability to tolerate ammonia confers on these plants.

In this study, M� Dolores Dom�nguez has compared the effect of ammoniacal nutrition on the metabolism of sensitive plants, such as spinach and on tolerant ones, like the pea. The results indicate that the processes associated with tolerance to ammonia in the pea are multiple with respect to spinach. In the pea plant there is greater regulation of the absorption of ammonia at the roots and control of the ammonium levels, above all in the leaf tissue. Moreover, important changes are observed in relation to the carbon and nitrogen of the majority aminoacids (asparragine in the pea and glutamine in spinach).

With respect to the activities of the GS (glutamine synthetase) and GDH (glutamate dehydrogenase) enzymes, responsible for the assimilation of ammonium, this substance is not modified in moderate concentrations in the pea but it is so in spinach. Regarding the main metabolic modifications, these take place in the roots in the case of the pea and in the leaves in the case of spinach. Moreover, in the roots of the pea, morphological changes have been observed which could be indicative of adaptations in order to assimilate the ammonium without modifying its internal processes.

In another part of the study the nature of the stress produced in plants by the use of ammoniacal fertilisers is investigated. The high concentration of ammonium does not carry oxidative stress with it (i.e. the molecules are oxidised due to alteration produced in the enzymes), unlike that which occurs in the majority of these states. The activation of some antioxidant enzymes is reflection of stress that ammonium produces in plants. Moreover, this reaction can favour the growth of plants tolerant to ammonium in another situation of added stress.

The researchers have shown that tolerance to ammonium experienced in one type of pea can be extrapolated to the rest of the varieties of the species. Nevertheless, different ranges of tolerance and strategies are observed for growth under ammoniacal nutrition.

Cloning pea genes

To study the ammonium assimilation enzymes, GS and GDH, in depth, these pea genes have been cloned and sequenced and, by means of a novel technique, two new antibodies have been produced that recognise the enzymes of these plants and of other plant species. The results show that the contents of the GS and GDH enzymes in the root tissues of the different varieties of pea are increased when ammoniacal fertiliser is applied and, this increment is related to the concentration of ammonium applied.

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