The how the relationship between nitrogen concentration on

The Effect that Nitrogen
Concentration in Fahraeus’ Medium has on the Rhizobium Nodulation of Alfalfa
Plants

Abstract

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Nitrogen fixation of atmospheric nitrogen is essential for
all living organisms as it produces ammonia which is used to make amino acids,
proteins and nucleic acids which are the basic building blocks for life.
Leguminous plants have adapted a way of using the nitrogen-fixing bacteria,
rhizobia, which proliferates in root nodules when nitrogen concentrations are
low. The experiment carried out used Alfalfa (Medicago sativa) seeds sown on 5 different Fahraeus’ media containing different concentrations of nitrogen with
rhizobium solution surrounding them. There was 34-36 replicates and the data
recorded and analysed. This paper elaborates of the importance of root
nodulation in legumes and how the relationship between nitrogen concentration
on the growth of the Alfalfa plants.

Introduction

Rhizobia bacteria are free-living within the surrounding
soil of the legumes and invade the plants roots (New Mexico State University,
2015). The plants supply the bacteria with necessary nutrients as they multiply
within the cortex of the plant and within a week nodules begin to form (New
Mexico State University, 2015). An age gradient is formed where a persistent
meristem develops in the distal end of the root nodule making differentiating
zones giving the nodules their irregular oval shape (Bianco et al. 2014). This involves the use of
phytohormones in particular the auxin indole-3-acetic acid (IAA) which is
produced by the rhizobium bacteria (Bianco et
al. 2014). The bacterially produced auxin can alter that auxin
balance within the plant and can interfere with plant auxin transport so
indirectly influencing auxin homeostasis, resulting in plant growth promotion
or inhibition (Spaepen and Vanderleyden, 2011; Defez et al. 2017).

Nitrogen cannot be directly used by organisms but
it is essential for growth therefor they need to have mechanisms for fixing
nitrogen into ammonia (Haag et al. 2013).
Only a few bacteria and archea are able to fix nitrogen and some plant have
been able to produce symbiotic interactions with these bacteria and us the
nitrogen for growth (Haag et al. 2013).
Within the nodules the rhizobium bacteria are used as endocellular
symbionts which convert atmospheric nitrogen into ammonia which is essential
for making amino acids, proteins etc. (Bianco et al. 2014). The nodules take a while to fully mature and the
rhizobia bacteria don’t carry out much nitrogen fixation in this time, when
they are fully mature they become pink inside indicating nitrogen fixation has
started as leghaemoglobin is controlling the oxygen flow to the bacteria (New
Mexico State University, 2015).

Legumes are one of the most abundant crops and as they have produced
this highly efficient symbiosis with the rhizobium bacteria it therefor reduces
costs and reduces soil/water pollution (ÍñIguez et al. 2016).

The aims of the paper are to describe the effects
of growth conditions, namely inorganic nitrogen concentrations, on nodulation
of Alfalfa roots. It is important to elaborate on this as is key for cutting
costs of agricultural crops and can help with poverty.

Method

Firstly 40 alfalfa seeds are washed for 10 minutes in 80%
ethanol and stirring them occasionally; this sterilise their surfaces but makes
sure not to damage them. The seeds are then rinsed in sterile distilled water 3
times to remove excess ethanol.

Rhizobium bacteria in growth medium is centrifuged at 1000xg
for 10 minutes to pellet the bacteria and the supernatant is poured off and the
pellet re-suspended in the same volume of sterile water. This is then
re-centrifuged and the supernatant removed again and the pellet re-suspended in
5ml of sterile water.

30 surface-sterilised seeds are transferred aseptically into
a small volume of rhizobium suspension in sterile water and incubated for 20
minutes.

5 separate
flasks are set up with Fahraeus’ medium and
different concentrations of nitrogen added to each; 0mM, 0.1mM, 1mM, 5mM, 10mM.
Then using aseptic techniques 6 incubated alfalfa seeds are sown on top of each
growth medium along with a small amount of bacterial suspension. The flasks are
the plugged with non-absorbent cotton wool and aluminium foil then kept in a
growth room in the light so the seeds can germinate; this should take about 2
weeks.

1 mm

 

After 6 weeks the
alfalfa plants were then observed for successful inoculation, if they hadn’t
data was still collected but it was treated as a blind trial, non-inoculated
control set for comparison.

Before opening the
flask the number of plants that grew was recorded and then carefully removed
from the growth medium making sure not to damage the roots and any residual
agar removed from the roots by gently washing and blotting them. For each flask
the roots were cut off all the plants and weighed together getting a total
weight in grams (g); the same was then done for the shoots of each flask.

1 mm

 

Using a binocular microscope the roots of each flask were
examined for the presence of root nodules and then they were counted and
recorded.  

Figure B: Image of a root nodule at 0.1 mM NO-3

 

Once the nodules of each plant had been counted further examination
of the structure was carried out. This was achieved by cutting open the nodules
and looking at them under a binocular microscope as shown in figures A, B and C.
As seen in figure 1 the inside of the root nodule is pink and this is due to
the leghaemoglobin and shows that nitrogen fixation is taking place (New Mexico
State University, 2015).

 

Figure C: Image of a root nodule at 1.0 mM NO-3

 

 

 

 

Results

Once the data had been collected and rearranged a
Shapiro-Wilk test is carried out to determine if the data is normally
distributed at p

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