Rice production in these rainfed ecosystems is limited primarily by drought, flooding (submergence), adverse soils (salinity and iron toxicity), and cold.
- Drought regularly affects 13 million hectares of rainfed rice in SA. In the eastern Indian states of Jharkhand, Orissa, and Chhattisgarh alone, rice production losses during severe droughts (about one year in five) average about 40% of total production, with an estimated value of $800 million (Pandey et al 2007). Drought is considered a major constraint to both upland and rainfed lowland rice production in SSA, and the presence of drought tolerance in African rice Oryza glaberrima was an important motivation for developing NERICA rice (Balasubramanian et al 2007).
- Flooding regularly afflicts some 12 million hectares in SA and as much as one-third of the rainfed lowland areas of SSA are thought to be affected by submergence. In SA, annual losses in the region are estimated at more than $1.5 billon. Africa is prone to flooding and this problem is becoming progressively more serious with climate change (Futakuchi 2005). Modern rice varieties are not adapted to these conditions and this is probably the reason these varieties are not widely adopted in these areas and farmers still grow their local landraces with low yield.
- Salinity limits rice productivity in many irrigated and rainfed rice areas of SA and SSA. Salinity is particularly a major problem in tropical coastal regions with predominant rice-based farming systems, because of the intrusion of brackish water during the dry season through tidal movements and capillary rise from shallow saline groundwater. Salinity continues to be high at the start of the wet season during and after rice transplanting, until sufficient rain washes it off the soil.
- Iron toxicity is recognized as one of the most widely distributed nutritional disorders in rice, with considerable areas of lowland rice showing some nutritional disorders associated with iron toxicity. It is widespread in rice fields in Africa when soils derived from iron-rich rocks are inundated with water. Under these anaerobic conditions, insoluble Fe III is reduced to the more soluble Fe II, which is readily taken up by the rice roots. At soil Fe II concentrations above 300 ppm, severe yield losses are incurred in sensitive rice varieties. A study in Côte d’Ivoire, Ghana, and Guinea Conakry found that more than 50% of the lowland rice area was affected by iron toxicity (Chérif et al 2009).
- Cold stress is experienced under two different conditions in rice fields in Africa. One is due to seasonal weather changes experienced in the Sahel zone of West Africa where daily minimum temperatures can descend to less than 12°C between November and March. Likewise, during the wet season in the Sahel zone, delayed planting causes flowering to coincide with a period when night temperatures can fall below 18°C and this leads to sterility of up to 80% in the presently cultivated varieties. Hence, the whole Sahel region of West Africa from Senegal to Chad needs rice varieties tolerant of cold at the vegetative and reproductive stages so as to reduce yield losses due to cold-induced sterility and postharvest losses.
STRASA Phase 2 hopes to bring the major scientific advances in plant genomics to bear on abiotic stress tolerance. This will create a technology platform for developing complementary crop management practices and improved institutional setups for rapid delivery of technology options to give the poorest farmers a realistic chance to improve their livelihoods in rainfed environments.