Tuesday, August 16, 2011
Rice that thrives on iron-rich soils
Iron is a ‘trace element’ that must be available to rice plants is very small quantities for normal growth and development. However, at high concentrations, iron is toxic to rice plants (i.e. it poisons them). Affected plants develop small brown spots that spread and merge, and finally result in reddish-colored leaves. Moreover, iron toxicity alters the plant’s root structure and development, and leads to spikelet sterility (i.e. reduced yield). Iron toxicity is a major problem in lowland rice, including irrigated systems. Rice yield losses attributable to iron toxicity range from 10 to 100%, with an estimated average of 50%. Several management and cultural practices can be used to reduce the occurrence of iron toxicity in rice fields, but most of these are not affordable to African resource-poor farmers. Consequently, AfricaRice focuses on improving varieties’ tolerance to iron toxicity.
For many years, AfricaRice has evaluated rice varieties, selected promising lines and developed appropriate agronomic practices that can help farmers cope with iron toxicity. In recent years, AfricaRice molecular biologist Khady Nani Dramé and her colleagues have added new dimensions to the iron-toxicity research. They used GIS and remote-sensing tools to map the potentially toxic areas. These tools will help reveal the extent and severity of the constraint and also guide breeders in their evaluation and dissemination strategies.
A key issue is how different varieties grow and develop in different environments (in this case, under different levels of iron toxicity). What a plant (or any organism for that matter) looks like is known as its ‘phenotype’, and evaluating how varieties grow in different environments is known as ‘phenotyping’. To make such screening for iron-toxicity tolerance efficient, it would be invaluable to have a standardized, controlled and reliable method. AfricaRice continues to review screening methods, including fields in hot spots, pots on station and a hydroponic (soil-free) environment. Field screening is difficult, requiring many repetitions, because iron concentration levels, and consequently toxicity, vary widely within and among fields even in hot-spot areas. However, the farmer’s field is the environment where rice is grown for food, so any new variety considered tolerant has been verified in field trials.
The experimental pot screening that AfricaRice conducted turned out to be useless, as the performance of varieties in pots on station was completely different from that of the same varieties tested in the field in a hot spot. Meanwhile, testing in a hydroponic environment can only be carried out during the vegetative stage (i.e. before the flower-heads start to grow). As yet, there is no proof that tolerance during vegetative stage is directly correlated with tolerance during reproductive stage (from the time flower-heads start to grow to harvest), as reflected in final yield. This aspect is still under investigation.
The battle against iron toxicity is also being fought with molecular breeding. AfricaRice is identifying and validating molecular markers (QTLs) associated with iron-toxicity tolerance. A few QTLs have been validated that will be used in marker-assisted selection to improve popular varieties nominated by national programs.
With tolerant material already available, participatory varietal selection (PVS) trials were started at three sites in four countries (Burkina Faso, Ghana, Guinea and Nigeria) in 2009. The first year saw farmers visiting rice gardens planted with 80 varieties (including a local check), from which they selected the one they preferred. Ten varieties were retained per country for further testing. Farmers are growing these varieties alongside their local varieties and making further selections with a view to keeping the best three. These three best iron-toxicity-tolerant varieties will be nominated for national release. “Even in the first year, we were seeing new varieties performing well in farmers’ fields in comparison with their local variety”, Dramé enthuses.