Ibnou Dieng

Genetic variation and host-parasite specificity of Striga resistance and tolerance in rice: the need for predictive breeding.

Summary The parasitic weeds Striga asiatica and Striga hermonthica cause devastating yield losses to upland rice in Africa. Little is known about genetic variation in host resistance and tolerance across rice genotypes, in relation to virulence differences across Striga species and ecotypes. Diverse rice genotypes were phenotyped for the above traits in S. asiatica‐ (Tanzania) and S. hermonthica‐infested fields (Kenya and Uganda) and under controlled conditions. New rice genotypes with either ecotype‐specific or broad‐spectrum resistance were identified. Resistance identified in the field was confirmed under controlled conditions, providing evidence that resistance was largely genetically determined. Striga‐resistant genotypes contributed to yield security under Striga‐infested conditions, although grain yield was also determined by the genotype‐specific yield potential and tolerance. Tolerance, the physiological mechanism mitigating Striga effects on host growth and physiology, was unrelated to resistance, implying that any combination of high, medium or low levels of these traits can be found across rice genotypes. Striga virulence varies across species and ecotypes. The extent of Striga‐induced host damage results from the interaction between parasite virulence and genetically determined levels of host–plant resistance and tolerance. These novel findings support the need for predictive breeding strategies based on knowledge of host resistance and parasite virulence.

Genotypic Variation in Grain P Loading across Diverse Rice Growing Environments and Implications for Field P Balances

More than 60% of phosphorus (P) taken up by rice (Oryza spp.) is accumulated in the grains at harvest and hence exported from fields, leading to a continuous removal of P. If P removed from fields is not replaced by P inputs then soil P stocks decline, with consequences for subsequent crops. Breeding rice genotypes with a low concentration of P in the grains could be a strategy to reduce maintenance fertilizer needs and slow soil P depletion in low input systems. This study aimed to assess variation in grain P concentrations among rice genotypes across diverse environments and evaluate the implications for field P balances at various grain yield levels. Multi-location screening experiments were conducted at different sites across Africa and Asia and yield components and grain P concentrations were determined at harvest. Genotypic variation in grain P concentration was evaluated while considering differences in P supply and grain yield using cluster analysis to group environments and boundary line analysis to determine minimum grain P concentrations at various yield levels. Average grain P concentrations across genotypes varied almost 3-fold among environments, from 1.4 to 3.9 mg g−1. Minimum grain P concentrations associated with grain yields of 150, 300, and 500 g m−2 varied between 1.2 and 1.7, 1.3 and 1.8, and 1.7 and 2.2 mg g−1 among genotypes respectively. Two genotypes, Santhi Sufaid and DJ123, were identified as potential donors for breeding for low grain P concentration. Improvements in P balances that could be achieved by exploiting this genotypic variation are in the range of less than 0.10 g P m−2 (1 kg P ha−1) in low yielding systems, and 0.15–0.50 g P m−2 (1.5–5.0 kg P ha−1) in higher yielding systems. Improved crop management and alternative breeding approaches may be required to achieve larger reductions in grain P concentrations in rice.

Soil-based screening for iron toxicity tolerance in rice using pots

Abstract The objective of this study was to assess the reliability of pot-based screening method for iron (Fe) toxicity tolerance in rice using soils from hot spots. Five lowland rice varieties with known reaction to Fe toxicity were grown in pots in a screen house for three seasons. Fe-toxic soils from two hot spot fields – Edozighi, Nigeria and Niaouli, Benin were used and soil from Africa Rice Center (AfricaRice) experimental farm, Cotonou, Benin was included as control. Leaf bronzing score (LBS) was determined at different stages, and grain yield was determined at maturity. Heritability was estimated using data across the three seasons. High heritability was recorded for LBS and grain yield. Grain yield reduction in stress treatment relative to control varied from 15 to 56% depending on the variety and soil. Bao Thai, Suakoko 8, and WITA 4 had better performance under Fe toxicity in terms of LBS, yield and relative yield reduction, whereas Bouake 189 and IR64 had poorer performance. Grain yield and LBS were significantly correlated but negatively at 60 days after sowing (DAS). Overall, the results found in this experiment were consistent with previous field studies. Therefore, pot screening using soils from hot spots can be used by rice breeding programs to reliably assess Fe toxicity tolerance ex situ.

Interactions Between Rice yellow mottle virus (RYMV) Isolates and Rice Germplasm from Niger

Rice yellow mottle virus (RYMV) is the most damaging virus in Niger’s rice agrosystems. We analysed host plant-RMYV interactions using 175 accession from the rice germplasm collection of Niger and 52 accessions from Mali. Five different virus isolates from Niger (three), Benin (one) and Burkina Faso (one) were used for inoculation. The assessment was based on visual disease symptom scoring, and secondary disease-related traits such as leaf chlorophyll content and plant height. Most rice accessions were susceptible to RMYV but a few African rice accessions displayed a level of resistance to some virus isolates, which was similar to that of the highly resistant TOG5681. Their host plant resistance was characterised by the absence of symptoms, low chlorosis and limited plant height reduction. Examining alleles using primers derived from the RYMV1 resistance gene revealed that one of these accessions has the rymv1-3 allele and other two accessions bear the rymv1-4 allele. We could not identify any known allele in one highly resistant accession, suggesting the presence of another resistance gene. The RYMV isolate BF1 from Burkina Faso was more aggressive than the three isolates from Niger, which were in turn found to be more aggressive than the isolate from Benin.

Variations in agronomic and grain quality traits of rice grown under irrigated lowland conditions in West Africa

Abstract Rice breeding in West Africa has been largely skewed toward yield enhancement and stress tolerance. This has led to the variable grain quality of locally produced rice in the region. This study sought to assess variations in the agronomic and grain quality traits of some rice varieties grown in this region, with a view to identifying sources of high grain yield and quality that could serve as potential donors in their breeding programs. Forty‐five varieties were grown under irrigated conditions in Benin and Senegal with two trials in each country. There were wide variations in agronomic and grain quality traits among the varieties across the trials. Cluster analysis using paddy yield, head rice yield, and chalkiness revealed that 68% of the total variation could be explained by five varietal groupings. One group comprising seven varieties (Afrihikari, BG90‐2, IR64, Sahel 108, WAT311‐WAS‐B‐B‐23‐7‐1, WAT339‐TGR‐5‐2, and WITA 10) had high head rice yield and low chalkiness. Of the varieties in this group, Sahel 108 had the highest paddy yield in three of the four trials. IR64 and Afrihikari had intermediate and low amylose content, respectively, with the rest being high‐amylose varieties. Another group of varieties consisting of B6144F‐MR‐6‐0‐0, C74, IR31851‐96‐2‐3‐2‐1, ITA222, Jaya, Sahel 305, WITA 1, and WITA 2 had high paddy yield but poor head rice yield and chalkiness. The use of materials from these two groups of varieties could accelerate breeding for high yielding rice varieties with better grain quality for local production in West Africa.

Generation mean analysis to estimate genetic parameters of some traits for riceweed competitiveness

Weeds are the most widespread biotic production constraint of rice in Africa and one of the major factors limiting grain yield. An efficient breeding strategy could be particularly important for improving weed management in sub-Saharan Africa (SSA) because most smallholder rice farmers use few external inputs. To understand rice weed competitiveness, experiments on reciprocal interspecific crosses derived from FKR19 (Oryza sativa) and CG20 (Oryza glaberrima) were carried out to estimate gene effects and heritability of traits: plant height at five leaves, plant height 30 days after transplanting, plant height at maturity, number of tillers at 30 and 60 DAT, number of fertile tillers, width of leaves at 80 DAT and at maturity, and length of leaves at 80 DAT and at maturity for rice–weed competitiveness. Six generations – P1, P2, F1, F2, BC1F1 and BC2F1 – were raised and subjected to generation mean analysis. The lowest heterosis of F1 was obtained in both crosses (CG20/FKR19 and FKR19/CG20), except for plant height at 30 days after transplanting and leaf width at maturity in the CG20/FKR19 cross. The majority of traits displayed higher dominance gene effects (H5_L, H30 and L_80 for CG20/FKR19; W_mat and L_mat for FKR19/CG20) than additive gene effects; the latter were slight and non-significant for the majority of traits. Duplicate epistasis was observed for the number of tillers 30 days after transplanting and leaf length at maturity and plant height at maturity. Additive genetic variance values were higher in CG20/FKR19, revealing that the CG20 variety can be used as a donor parent. Plant height at maturity, length of leaves at 80 DAT and at maturity showed high narrow-sense heritability (hn 2 >0.70), influencing weed competitiveness.