Iron toxicity
THE PROBLEM
Poor soils due to iron toxicity limit rice productivity in most rainfed rice areas of South Asia and sub-Saharan Africa. Several million ha of land suited to rice production in Asia are currently unexploited because of iron toxicity, salinity and other related soil problems. Rice is suitable for rehabilitating these soils because of its ability to grow under flooding, and its high potential for genetic improvement. Rice productivity in these affected areas is very low, but can reasonably be raised if planted with varieties tolerant of such stresses. Iron toxicity is particularly common in rice-producing acid soils of Africa, hence more focus is given in that region.
THE STRASA SOLUTION
Multilocation testing
Forty-one (41) lines and four checks were evaluated in the 2018 WS through the ARBTF network in 21 rainfed lowland MET sites (17 NARS sites and 4 AfricaRice sites). These lines included Fe toxicity-tolerant lines. Trial data from 14 sites showed that one site had low heritability (0.08) and was, thus, not considered. Combined analysis of data from the remaining 13 sites gave an overall heritability of 0.32, with ARICA 2 having the highest average yield compared to the test entries and other checks. Yields at the Samogossoni site in Mali and the Kilissi site in Guinea Conakry were significantly correlated with a combined heritability of 0.34. Similarly, yields at the AfricaRice Ndiaye, Senegal site and the Ibadan, Nigeria sites were also significantly correlated with a combined heritability of 0.37.
Line development
During the 2018 WS, iron toxicity tolerance screening was conducted on 272 F4 lines, 135 F6 lines, and 15 F7 lines in Nigeria by the AfricaRice Rainfed Lowland Breeding Unit. From preliminary results, five iron toxicity-tolerant lines with more than 30% yield advantage over FARO 44 and another five lines with over 36% yield advantage over the best check FARO 67 were identified.
Gene discovery
Three mapping populations have been developed for Fe toxicity tolerance. One QTL from NERICA L-19 on Chromosome 1 with LOD of 15 (Figure at right) was identified and SNP markers are being developed for it in collaboration with Cornell University in the USA. No progress has been made on research to gain a better understanding of Fe toxicity tolerance mechanisms, with the aim of identifying reliable traits for effective varietal selection, due to lack of expertise. The scientist responsible for this activity during the earlier phases of the project has been reassigned to the AfricaRice Capacity Development Unit.
Inventory of existing African iron-toxicity-tolerant germplasm completed
A list of 181 accessions was compiled, including two sensitive checks. The material comprises accessions from different institutes (AfricaRice, IRRI, CIAT, and NARES). Six advanced bulk populations for evaluating iron-toxicity tolerance were dispatched from IRRI to AfricaRice and many segregating populations were sent to Nigeria (IRRI-Africa).
Participatory varietal selection conducted for at least 10 iron-toxicity-tolerant lines at key sites in four SSA countries (Nigeria, Guinea, Burkina Faso, Benin) by AfricaRice
Two PVS trials were established at Banfora (Burkina Faso) and Edozighi (Nigeria) with a set of 181 entries. Farmers’ field days were organized at both sites, involving 60 farmers, with women in the majority, at Banfora and 43 farmers, almost exclusively men, attending the field day at Edozighi. The rating sheets collected at both sites have been compiled on farmers’ choices and perceptions.
Phenotyping for iron-toxicity screening at AfricaRice refined
Three approaches are being used to screen for iron-toxicity tolerance—field trials, pot trials, and hydroponics trials. The pot-trial protocol uses plastic pots filled with sand supplemented with Fe2+ as FeSO4. Optimal fertilizer (100 kg ha–1 N, 50–100 kg ha–1 P, and 60–100 kg ha–1 K) were applied to allow normal growth of rice plants in order to avoid nutrient disorders. Forty genotypes were screened with this protocol using two concentrations of Fe2+ (300 and 900 ppm). The first symptoms observed were a reduction in plant height and tillering, whereas leaf bronzing appeared later. In parallel, 180 genotypes were screened in the field at hotspots in Edozighi (Nigeria) and in Banfora (Burkina Faso).
Line development
During the 2018 WS, Fe toxicity tolerance screening was conducted on 272 F4 lines, 135 F6 lines, and 15 F7 lines in Nigeria by the AfricaRice Rainfed Lowland Breeding Unit. From preliminary results, five Fe toxicity-tolerant lines with more than 30% yield advantage over FARO 44 and another five lines with over 36% yield advantage over the best check FARO 67 were identified.
