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Aluminium toxicity - breeding for resistance

<em>Brachiaria</em> grass cultivars are being trialled that combine tolerance of high aluminium levels with other desirable traits such as fodder quality (© Neil Palmer (CIAT))
Brachiaria grass cultivars are being trialled that combine tolerance of high aluminium levels with other desirable traits such as fodder quality
© Neil Palmer (CIAT)

Soil acidity, a common feature of highly weathered soils, affects over 40 per cent of potential arable land in the tropics, significantly limiting plant growth. In particular, high acidity accentuates the twin problems of aluminium toxicity and phosphorus deficiency, leading to poor root development and low drought-tolerance. But in Rwanda, Nicaragua and Malawi, participatory plant breeding of bean and fodder crops offers new hope to crop and livestock farmers struggling with acidic soils and low rainfall.

Exactly how aluminium impairs root growth at a genetic level is not entirely clear, but according to plant nutritionist, Idupulapati Rao, from the International Center for Tropical Agriculture (CIAT), the result is easy to see. "Soil acidity causes aluminium in the soil to become more soluble and more toxic to plants," he explains. "It makes the roots of many important food and pasture crops grow shorter and thicker. This means they are unable to access residual water deep in the soil." They are also less able to absorb phosphorus, an important element in plant growth, which tends to be fairly static in soils and therefore not easily found by the stunted plant roots.

Beans and Brachiaria - high priorities in Rwanda

In Rwanda, soil acidity impairs root growth leading to low drought-tolerance (© Neil Palmer (CIAT))
In Rwanda, soil acidity impairs root growth leading to low drought-tolerance
© Neil Palmer (CIAT)

Under the Fighting drought and aluminium toxicity project, funded by the German government (BMZ-GTZ), a partnership between CIAT and national research institutes*, conducted participatory varietal selection with farmers growing beans and forage grasses in semi-arid parts of eastern Rwanda. Beans are a major staple, with per capita consumption of up to 60 kg per year. In addition, a national 'one cow per family' policy has created a huge demand for livestock feed, increasing the need for improved forage plants that can thrive in the dry conditions found in livestock-keeping areas.

The farmer-led trials have involved several 'best bet' local bean varieties, plus some brought from CIAT headquarters in Cali, Colombia. Three commercial Brachiaria forage grass cultivars, and five hybrids have also been tested by farmers, to assess their drought and aluminium resistance, nutritional quality, and resistance to spittlebug and foliar blight, two common threats to hybrid Brachiaria plants.

Bringing in farmers from the start

Early involvement of farmers in this process has been key, according to Idupulapati Rao. Conventional breeding tends to develop plant lines and then expose them to farmers. With this project, the farmers were exposed to parent plants before the lines were developed, and were also able to assess the first crosses, to test their suitability. Such farmer involvement was already part of the long term PABRA (Pan-African Bean Research Alliance) bean breeding programme in Rwanda, but it has been a new development for forage breeding.

Farmer-led trials have involved several 'best bet' local bean varieties (© Neil Palmer (CIAT))
Farmer-led trials have involved several 'best bet' local bean varieties
© Neil Palmer (CIAT)

The selection trials have been supported by innovative work carried out at CIAT and the Leibniz University of Hannover, Germany, in collaboration with Professor Walter Horst. Phenotypic screening has been used to assess root and shoot traits. Use of clear soil cylinders has allowed root elongation to be measured under aluminium or drought stress, or both. At shoot level, high yielding bean varieties have been observed to have superior grain filling and thinner pod walls.

Genetic analysis

Adopting a holistic approach, the project has also studied the genetic processes involved in aluminium and drought tolerance. Though work is ongoing, several genes and gene families, called candidate genes, are expressed at root tips under stress conditions, which may be linked to resistance. Further isolation and identification of these genes is now needed, and work also continues in crossing lines derived from runner bean and common bean types, to further boost the level of resistance to aluminium and drought.

Crop breeders are able to monitor root development in conditions of acidity and drought (© CIAT)
Crop breeders are able to monitor root development in conditions of acidity and drought
© CIAT

Brachiaria has a naturally high level of resistance to aluminium toxicity, so breeding work is focussed on maintaining this, while adding other desirable traits. Producing Brachiaria seed, to allow for expanded planting of the selected varieties, is expensive, although necessary for large-scale pasture improvement. But at smallholder level, vegetative propagation by farmer groups is the preferred approach, and is now being implemented by the participating farmers.

Given the challenges faced by smallholders in Africa and beyond, Rao is in no doubt of the importance of these developments. "These are exactly the scenarios we need to be prepared for, as climate change brings more unpredictable rainfall and increasing incidence of drought," he says. He is also highly satisfied with the shared learning that has contributed to the success thus far. "It's an excellent example of the global reach of CIAT. We're combining our expertise in Latin America with that of our partners in Africa, to develop crops that can benefit small farmers across the Tropics."

*Rwandan Agricultural Research Institute (ISAR) and the National Department of the Ministry of Agriculture (DARS)

Date published: March 2011

 

Have your say

Breeding resistance to Al toxicity is one thing. But for man... (posted by: Dr.R.Palaniappan)

 

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