Increasing the shelf life of cassava

Crop scientists have identified several genetic mechanisms to improving the shelf life of cassava roots. Long an unsolvable problem, the research has the potential to benefit the poorest of the poor, widening and strengthening the markets for cassava, reducing marketing costs and losses along the marketing or value addition process.

The research team, led by Hernán Ceballos at the International Center for Tropical Agriculture, identified four different sources of tolerance to a phenomenon known as post-harvest physiological deterioration. This process renders the cassava roots inedible and unmarketable a mere two to four days after harvesting.

This study started with an accident. A few roots from a cassava clone (belonging to a new high-carotene generation) were left on a shelf for more than two months between. When inspected these roots did not show any symptoms of deterioration. This result prompted the planning and execution of a study whose results are presented in the July-August 2010 edition of Crop Science, published by the Crop Science Society of America.

One source of tolerance was found in the only species of cassava native to the United States. A second source was induced by mutagenic levels of gamma rays which silenced one of the genes that cause the deterioration symptoms.

A third source was a group of high-carotene clones. The authors suggest that the antioxidant properties of carotenoids protect the roots from deterioration, which is basically an oxidative process. Finally, tolerance was also observed in a waxy-starch mutant. The researchers believe that the waxy-starch gene is co-located next to a tolerance gene, and resistance to deterioration is not directly caused by the mutant gene.

“This kind of discovery includes sources from wild relatives of cassava, which supports the need for germplasm collections,” says lead researcher Hernán Ceballos. “It also has new sources of tolerance induced through mutagenesis or exposed through inbreeding, which is a rather new thing for cassava. Finally, some of the tolerance comes from the antioxidant properties of high carotene cassava. But mostly, this is very important for women who sell cassava roots in markets throughout Africa or Latin America, who will not necessarily lose their products if they fail to sell them within a day or two after harvest.”

The economic relevance of these discoveries is expected to be huge. Cassava is one of the most important staple crops worldwide, and is the most important in many arid regions, such as sub-Saharan Africa. It can produce reasonable root yields under adverse climatic and soil conditions, and offers the advantage of a flexible harvesting date, allowing farmers to keep the roots in the ground until needed. In addition to the important role cassava plays in food security, there is a growing demand for cassava roots by the starch, food, animal feed and ethanol processing industries.

The results of this study suggest that tolerance to post-harvest physiological deterioration can be found in different sources, and they seem to be acting through different biochemical/genetic mechanisms. They also highlight the importance of germplasm collections and their screening, the usefulness of inbreeding cassava (in search of recessive traits) and the potential of induced mutations, particularly with the advent of molecular tools.

Future research will focus on finding additional sources of tolerance and identifying molecular markers linked to those traits. This will allow for early identification of tolerant varieties, overcoming the current limitations of cassava research, which involve growing large numbers of roots to obtain sufficient data.

This study will go a long way towards addressing bottlenecks that prevent cassava having a larger impact on the livelihoods of the communities that depend on it, as identified by the Food and Agriculture Organization of the United Nations (FAO) Global Cassava Initiative, which culminated in 2000. It concluded that cassava could become the raw material base for an array of processed products and contribute to agricultural transformation and economic growth in developing countries.