Usurbil, Basque Country
January 23, 2008
The regulation of the biological
fixation of nitrogen in hydric stress conditions varies with the
different species of legume plants studied. 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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