Estimated reading time: 7 minutes
- Seed-applied technologies represent a revolution in farming.
- Rhizobium inoculants are used on legumes such as groundnuts and soya beans for biological nitrogen fixation (BNF).
- Depending on the variety of legumes and the germination circumstances, tiny nodules may appear two to three weeks after planting.
- There are seven primary steps that are crucial for the infection and nodulation process to develop successfully.
- Micronutrients are essential for plants to grow.
Seed-applied technologies represent a revolution in farming. Today’s sophisticated seed-applied products offer producers multiple benefits, including protection of high-value seed right from the start, flexibility in application, prevention of plant damage or loss, increased plant health and vigour, and fewer environmental and stewardship worries.
To get the best from seed-applied technologies, a combination of different biologicals and chemicals can be used for different functions.
Rhizobium inoculants are used on legumes such as groundnuts and soya beans for biological nitrogen fixation (BNF). BNF is the process where bacteria such as Bradyrhizobium spp. use freely available nitrogen (N2) and convert it enzymatically to a plant-available form (NH3).
In this process, Bradyrhizobium can fixate between 100 to 120kg of nitrogen (N) for groundnuts and between 180 to 250kg for soya beans. The Rhizobium bacteria, or more specifically Bradyrhizobium, which develop in nodules on the groundnut plant’s roots, live off the plant itself. They also give back to the plant by producing enough N to meet all its needs.
The development of a nodule marks the beginning of legume nitrogen-fixing (Figure 1a). Rhizobia bacteria from the soil infiltrate the root and grow inside its cortex cells. The plant gives the bacteria all the nutrition and energy required. Small nodules become evident to the naked eye a week after infection (Figure 1d).
Depending on the variety of legumes and the germination circumstances, tiny nodules may appear two to three weeks after planting. Nodules typically have a white or grey interior when they are young and not yet nitrogen-fixing. They gradually change colour from grey to pink or reddish, signifying that nitrogen fixation has begun. Leghaemoglobin, which is analogous to the haemoglobin in the blood and regulates colour, is what gives the pink or red hue.
There are seven primary steps that are crucial for the infection and nodulation process to develop successfully. First, the plant roots emit flavonoids that bind to receptors in the plasma membranes of compatible rhizobia (Figure 1a). In response to these flavonoids, the rhizobia secrete nodulation (NOD) factors that bind to receptors on the membranes of the host plant root hairs. This completes communication between the bacteria and the host roots.
The binding of the NOD factors to the root hairs increases the movement of calcium ions (Ca2+) into the root hair, causing the root hair to swell up and curl around the rhizobia (Figure 2a to b). Once this occurs, the rhizobia inject infection proteins into the root hair, causing the development of an infection thread (Figure 2c to f).
After the induction of the infection thread, the rhizobia secrete proteins called nodulins that cause the cortex cells of the root to start cell division and form the root nodules. At this stage the rhizobia divide and transform into Bacteroides (Figure 1d). The nodules develop vascular tissue that transports N compounds to the plant shoots and organic carbon from the shoots to the nodule Bacteroides.
For effective BNF, a good quality inoculant needs to be used with a high concentration of viable bacterial cells. Once integrated into the soil, factors such as soil pH, temperature and salinity have a significant impact on rhizobia survival. The most effective Rhizobium strain, being free of unwanted microbial contamination, having a long shelf life, a high concentration of bacteria, being properly packaged and stored, and being able to survive in harsh environments, are all requirements of high-quality inoculants.
Stem-end rot, collar rot and damping off are a few examples of diseases that might threaten groundnut production. Although resistant plant varieties and changes in cultural practices can help manage some plant diseases, other diseases can only be properly treated by using the right chemical and biological fungicides.
For chemical and biological control products to be effective on the seed for disease prevention, the products need to be effective in controlling the desired pathogen, as well as being compatible with other seed-applied technologies such as the rhizobia inoculant that is applied to the seed coat. The use of multi-active products plus a mixture of systemic (carbendazim) and contact (thiram) action is used in controlling seed-borne diseases such as Rhizopus spp., Rhizoctonia solani, Aspergillus niger and dry-stem rot caused by Fusarium monoliforme.
This dual action guarantees early season protection for groundnuts to ensure the desired plant population is reached at harvest time. Carbendazim is a broad-spectrum systemic fungicide that stops and inhibits hyphal growth, affects conidia formation (reproduction) and inhibits fungal spore germination.
Thiram is a contact fungicide that interferes with the normal respiration of fungi and some bacteria. This interference with the metabolism causes mortality of the fungal pathogens, attacking the emerging roots of the germinating treated seed. A critical consideration in choosing a fungicide treatment to be used in combination with rhizobia inoculants on legumes, should be the compatibility of the fungicide with the inoculant.
Trace element nutrients
Micronutrients are essential for plants to grow. The most common ways to provide micronutrients are through soil and foliar applications; however, finding high-quality micronutrient fertilisers is a challenge.
To guarantee optimal nutrient usage, micronutrient seed treatments such as seed priming and seed coating are a simple alternative. Germinating groundnut seeds need rhizobia for effective BNF (inoculant application), as well as micronutrients for effective germination and growth.
In the initial stages of groundnut nodule development, calcium (Ca) plays a vital role in the growth and curling of the root hair around the rhizobia (Figure 2a to b). The function of Ca in the growth and curling is shown in Figure 3. Firstly, the growth of a root hair is directed towards the highest Ca concentration that is usually accumulated in the tip of the root hair. If Ca inflow stops, so does the growth of the root hair.
Calcium influences cell wall strength and elasticity, controls actin filament dynamics, and the incorporation of Golgi- derived vesicles to increase the plasma membrane and cell wall (Figure 3a). When the Rhizobium is attached to the root hair (Figure 3b), it releases NOD factors that induce the inflow of Ca.
This inflow concentrates around the site of attachment and changes the Ca gradient dynamics of the root hair (Figure 3c). This change in gradient leads to the root hair growth shifting towards the higher Ca gradient near the attachment site, and the new growth leads to the curling of the root hair around the attached Rhizobium (Figure 3d).
Understanding this influence of Ca on plant growth and the effect that it can have on the nodulation process in legumes can increase the nutrient-use efficacy of micronutrients as a seed treatment. Using a liquid fertiliser formulated with plant growth regulators, in combination with plant-available amino acid chelated Ca, can stimulate vigorous root growth and improved nodulation.
A holistic approach
Seed-applied technologies have improved greatly over the past few years on different crops, but understanding the interactions between the different types and combinations of products can greatly influence the efficacy of these technologies. – SB Coetzee, regulations and technical specialist, Microbial Biological Fertilizers International