CHAPTER ONE
1.2 SOYBEAN
1.2.4 Nodulation .1 Nitrogen
About 80% of the earth’s atmosphere is nitrogen gas (N2) present in an inert or stable state. With a strong stable triple bond between the two nitrogen atoms. Plants require nitrogen for healthy growth. Although there is an abundance of nitrogen available in the atmosphere, this nitrogen is not in an accessible form for plants.
Plants receive most of their nutrients from the soil. These nutrients that are present or put into the soil are usually soluble in water and are then easily absorbed by the roots of the plant and used to manufacture proteins, fats and carbohydrates. Dinitrogen present
in the atmosphere is unable to dissolve in water and therefore needs to be fixed or changed to a soluble form (Tikhonovich
1.2.4.2 Nitrogen fixation by bacteria
A variety of prokaryotic organisms are able to fix free atmospheric n
accessible form. However, eukaryotes lack this ability. Nitrogen fixation is an enzymatic reaction that involves the reduction of dinitrogen from the atmosphere
(Sathish Kumar and Bhaskara R
Soybean plants form a symbiotic relationship with the bacterium
1.7) in order to fix dinitrogen into ammonia which is soluble in water and easily taken u by the plant. In South Africa the natural soil population of this particular bacterium is very low and often inoculation of the bacterium is required (Duxburg
japonicum has the enzyme system, nitrogenase, which provides the biochemical machinery for nitrogen fixation and has the ability to induce nodule formation on the roots of specific leguminous plants
Figure 1.7 View of Bradyrhizobium
19
in the atmosphere is unable to dissolve in water and therefore needs to be fixed or changed to a soluble form (Tikhonovich et al., 1995).
bacteria
A variety of prokaryotic organisms are able to fix free atmospheric n
eukaryotes lack this ability. Nitrogen fixation is an enzymatic reaction that involves the reduction of dinitrogen from the atmosphere
Sathish Kumar and Bhaskara Rao, 2012).
a symbiotic relationship with the bacterium B. japonicum
in order to fix dinitrogen into ammonia which is soluble in water and easily taken u h Africa the natural soil population of this particular bacterium is very low and often inoculation of the bacterium is required (Duxburg et al
the enzyme system, nitrogenase, which provides the biochemical fixation and has the ability to induce nodule formation on the leguminous plants (Tikhonovich et al., 1995).
Bradyrhizobium spp. under microscope (Lorquin et al
in the atmosphere is unable to dissolve in water and therefore needs to be fixed or
A variety of prokaryotic organisms are able to fix free atmospheric nitrogen into an eukaryotes lack this ability. Nitrogen fixation is an enzymatic reaction that involves the reduction of dinitrogen from the atmosphere into ammonia,
japonicum (Figure in order to fix dinitrogen into ammonia which is soluble in water and easily taken up h Africa the natural soil population of this particular bacterium is et al., 1990). B.
the enzyme system, nitrogenase, which provides the biochemical fixation and has the ability to induce nodule formation on the
et al., 1997)
1.2.4.3 Formation of root nodules
Nodulation occurs in the roots when the plant secrets a stimulant, usually sugars and amino acids, to attract the nitrogen-fixing bacteria already present in the soil to form nodules on the root hairs. The plant then produces flavonoids and these flavanoids in turn induce the expression of nod genes in the bacterium. There are many genes that are involved in nodulation. The main nod gene that is involved in nodulation in the bacterium is the nodD gene. This gene is subdivided into nodD1 and nodD2. It was found that nodD1 is more important as nodulation cannot take place without it (Figure 1.8). Nod factors are produced from the nod genes and these Nod factors then react with the plant and cause the root hairs to curl (Tikhonovich, 1995).
Figure 1.8 Diagram showing the process of root nodule formation and the secretion of flavonoids in the soil that induces the expression of nod genes in bacterium and the activation of Nod factors (Long, 1996).
During the curling of the root hair the bacterium attaches itself to the root hair and is taken up (Figure 1.9). Primordium formation occurs when the bacteria induces cell division in the root cortex. The bacteria are released into the plant by an endocytotic process as the infected threads grow towards the centers of mitotic activity and enter the primordium cells. The primordium cells mature and differentiate to form an ideal
21
environment for the bacteria to fix nitrogen. Thereafter, nitrogen fixation takes place by the bacterium for the plant and the plant provides nutrition for the bacterium. Thus, this is the manifestation for mutualist relationship between soybean plants and nitrogen- fixing bacteria. There may also be inactive nodules that develop on the plant. These nodules do not facilitate nitrogen fixation. When cut the inside of the nodule is white, green or grey whereas active nodules are red or brown in colour (Tikhonovich, 1995).
Figure 1.9 Induced nodule formation and uptake of bacteria in legume plants (Farabee, 2010).
1.2.4.4 Successful nodulation practices
In order for nodulation to be successful certain factors have to be present. The bacterium required for nodulation, i.e., B. japonicum, does not thrive well in the presence of excessive heat, sunlight, acid soils and particularly dry conditions. These remain some of the reasons for the lack of this bacterium in most South African soils due to the weather conditions. As such it is very important to plant crops in moist soils and have an adequate irrigation system especially during dry hot seasons. Also it is
recommended that for proper nodulation to take place it is vital to inoculate the plant with the specific strain of bacterium for that particular genotype. In some cases the inoculums can be stored in a cool dark place for optimal results (Keyser and Li, 1992).
1.2.5 Production of soybean