This was the conclusion of Ruben Ladrera Fernández in his PhD thesis, “Models of regulation of nitrogen fixation in response to drought: Soya and Medicago”, in which the different ways of distinct species of legumes respond to drought conditions are explained. The PhD work was directed by Professor César Arrese-Igor Sánchez and senior lecturer Ms Esther González García from the Department of Environmental Sciences at the Public University of Navarre
Biological fixation of nitrogen
In his thesis Mr Ladrera explains that nitrogen is the most abundant element in the terrestrial atmosphere but that it is a very poor source of nutrition for plants. This apparent paradox is due to the fact that atmospheric nitrogen is inert and cannot be used by living things and thus has to be reduced to other chemical forms such as nitrate (NO3-) or ammonium (NH4+) in order to be used by plants. This situation causes a disproportionate amount of nitrogenous fertilisers to be used for agriculture, giving rise to various environmental problems such as contamination of soil and water or the emission of oxides of nitrogen into the atmosphere.
However, some organisms are able to reduce atmospheric nitrogen to ammonium for its subsequent metabolic use, which is known as the biological fixation of nitrogen (BFN). These nitrogen-fixing organisms (also called diazotrophs), can fix nitrogen either as free living or in symbiosis with plants. Amongst the various nitrogen-fixing symbiotic associations, the agriculturally most important is that carried out by plants belonging to the legume and bacteria families, generically known as rhizobes.
In this symbiosis — according to the research undertaken by Mr Ladrera — bacteria dwelling in specialised organs of plant roots known as nodules are capable of using atmospheric nitrogen and reducing it to ammonium, which is exported to the plant, this providing the carbon from photosynthesis to the bacteria and which is necessary to carry out bacteroidal respiration.
What happens in drought or hydric stress
Rubén Ladrera states in his thesis that BFN is a process highly sensitive to drought, to such an extent that it is rapidly inhibited in hydric stress conditions and thus causes significant losses of leguminous crops at a worldwide level. However, it is still not known what the exact mechanism responsible for this inhibition is. Various mechanisms have been put forward, amongst which is a limiting of oxygen in the nodules, a process of retroinhibition using nitrogen and a limiting of the carbon flow to the bacteria.
In this context, the effect of drought on the nodular metabolism and on the plant in different species of legumes (Soya, alfalfa and Medicago truncatula) was studied. To this end, Mr Ladrera used plants from different varieties of each species and that demonstrated different tolerances to hydric stress, with the aim of identifying factors involved in the regulation of BFN.
The results of the research show a limiting of the carbon flow to the bacteria is produced as well as an accumulation of nitrogenated compounds in the nodule (but not in the leaves) of the Soya plants subject to drought, at the same time as the inhibition of the BFN. These results show that the regulation of the BFN in Soya, in hydric stress conditions, is produced at a localised level, in the nodule itself, and that the metabolism of carbon and nitrogen is involved in this.
Nevertheless, in the case of other species analysed - alfalfa and Medicago truncatula -, drought caused an accumulation of carbonated compounds in the nodules, which indicates the regulation of BFN in these species is produced independently of nodular carbon metabolism.
These differences — concludes the author of the PhD thesis – appear to be due to the greater tolerance shown by the species of the Medicago genus to drought conditions.
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