Evaluation of iron toxicity on lowland irrigated rice in West Africa

En zone tropicale, la culture du riz (Oryza sativa L. ) en condition de bas-fond (avec ou sans contrôle de l'eau) est souvent confrontée au problème de toxicité ferreuse. Cette contrainte de type édaphique est couramment observée, notamment dans les bas-fonds de savane et de forêt ouest africains. C'est un trouble nutritionnel associé à de fortes concentrations de fer dans la solution du sol. Les conditions réductrices que l'on rencontre couramment dans les sols engorgés de bas-fond sont précurseurs de la toxicité ferreuse par la solubilisation dans la solution du sol de la quasi-totalité du fer sous forme ferreux (Fe2+). Ces conditions édaphiques des sols de bas-fond, nécessaires à l'apparition de la toxicité ferreuse sont dépendantes des conditions pédoclimatiques expliquant ainsi la forte variabilité spatiotemporelle de cette contrainte. Cette grande quantité d'ions ferreux en solution entraîne chez le riz un déséquilibre en éléments minéraux. Ce fer ferreux (Fe2+) est absorbé en abondance, se concentre dans les feuilles, entraînant une décoloration des limbes, une réduction du tallage, de la taille des plants, et provoque une baisse importante des rendements. Cette contrainte a fait l'objet d'une étude dans trois pays de la sous région ouest africaine (Guinée, Côte d'Ivoire et Ghana) afin de quantifier ses effets sur la riziculture. L'étude confirme que la toxicité ferreuse est une des contraintes édaphiques majeurs des bas-fonds cultivés puisqu'en moyenne plus de 50% des bas-fonds et environ 60% des parcelles cultivées en riz sont affectés par cette contrainte. Dix pourcent de ces bas-fonds ayant même été abandonnés en raison d'une pression de toxicité ferreuse trop importante. Les études montrent également que plus de 55% des surfaces rizicoles sont affectées par l'excès de fer. L'impact sur le rendement est également significatif puisque les parcelles infectées peuvent perdre en moyenne jusqu'à 54% par rapport aux parcelles saines. (Résumé d'auteur

Physiological field evaluation for drought tolerance. Field phenotyping methods for the development of the new synthetic population

The Cirad/CIAT collaborative project has developed a breeding program based on the recurrent selection (RS) for an eco-efficiency use of water. Selection was primarily based on yield and, in the course of the selection process, screening methods were refined with additional secondary relevant traits (morpho-pheno-physiological traits). The infrared (IR) thermography measures the plant canopy temperature, which gives an indication of the plant water status through leaf surface cooling capacity by transpiration along environmental conditions. We developed large scale phenotyping methods, adapting the IR technology to field conditions, to evaluate response to drought among synthetic populations. An experiment was conducted during the 2008/2009 dry season in the Santa Rosa CIAT research station in Villavicencio (Colombia) to adjust the phenotyping method based on IR thermography and to evaluate 400 S1 lines and 1000 S0 plants derived from synthetic populations. This evaluation method allowed us to identify interesting progenitors to be integrated in the population improvement scheme with the goal to develop and diffuse improved material, populations or lines, with increased resilience to drought for various rainfed ecosystems in Latin America and the Caribbean (LAC). (Résumé d'auteur

High throughput phenotyping method for water use efficiency in rice under field conditions : [Abstract, P 6.04]

A collaborative CIAT/Cirad project aims to create new improved upland rice germplasm for drought tolerance based on population improvement through recurrent selection (RS). In the framework of a multidisciplinary team (ecophysiology, molecular genetics and breeding), we seek to enhance this breeding strategy through the integration of marker-assisted breeding tools. This requires improving methods for high throughput phenotyping in the field. Four hundreds lines selected from diverse recurrent populations were screened under drought conditions in Villavicencio CIAT experimental station (Colombia) during the dry season 2008/2009. Drought stress was applied for two weeks at panicle initiation stage. Drought response under water deficit at flowering stage was evaluated based on soil moisture and canopy temperature at flowering with an infra-red thermographic camera and final grain yield. Leaf temperature at/around flowering stage exhibited strongly significant varietal differences that were negatively correlated with soil moisture content and yield. This phenotyping approach permitted identifying genotypes that display good maintenance of transpiration (and thus sustained growth) under drought stress, related mainly to limited water extraction and/or greater root depth; it also enabled identifying genotypes that have high transpirationrates (and thus higher potential growth rates) under irrigated control conditions. Selecting suitable combinations of both characteristics is expected to help identifying drought tolerant material with high yield potential. The methodology will be further improved by (1) including on each thermographic photo a wet/dry object to normalize canopy temperatures against weather fluctuations for situations where micro-meteorological backup is not available, and (2) sampling leaf material for complementary delta 13C measurements indicative of transpiration efficiency (TE). The integrated methodology is being implemented in RS breeding schemes and in parallel in a genetic association study on a japonica rice variety panel, in order to identify promising genomic regions and polymorphisms for marker development. This will eventually permit developing efficient marker-assisted recurrent selection (MARS) approaches. (Texte intégral

High throughput phenotyping methods for screening rice germplasm under water limited field conditions

A Cirad project (Orytage) involving NARS and IRC aims at developing an international phenotyping network for rice adaptations to drought and thermal stresses in the context of gene discovery and genetic mapping. Relevant association mapping depend on improved methods for high quality and high throughput phenotyping in the field. In case of drought the Infrared thermography was used to phenotype the plant transpiration capacity in relation with carbon isotopic discrimination, soil moisture and atmosphere demand. Two hundreds varieties representative of the Oryza sativa japonica group were phenotyped under drought condition at the Villavicencio CIAT experimental station (Colombia) during the dry season 2009/2010. Irrigation was suspended for two weeks at vegetative stage (45-60 DAS). Drought response was evaluated based on canopy temperature with an infra-red thermographic camera. The methodology was improved by CWSI (crop water stress index) calculated to normalize canopy temperatures against micro-meteorological weather fluctuations, and complemented with delta 13C measurements indicative of water use efficiency (WUE). Leaf temperature at vegetative stage exhibited strong and significant varietal differences that were negatively correlated with soil moisture content. This phenotyping approach permitted identifying genotypes with good maintenance of transpiration capacity (and thus sustained growth) under drought stress, related mainly to limited water extraction and/or greater root depth. The integration of high throughput phenotyping with genetic association study on a japonica rice variety panel, present a significant advance in order to identify genomic regions of interest and polymorphisms for marker development (ongoing). This will eventually permit developing efficient marker-assisted selection (MAS) approaches. (Résumé d'auteur

Development of a high-throughput system for phenotyping rice roots traits

A CIRAD project (Orytage) involving NARES and IRC partners aims at developing an international phenotyping network for rice adaptations to drought and thermal stresses in the context of gene discovery and genetic mapping. Root architecture is a key character for improving responses to these stresses. Identifying genes or QTLs that control root development and incorporating these loci in marker-aided selection programs is a promising strategy for the genetic improvement of root traits. However, high-quality phenotyping of a large numbers of individuals, which is necessary for precise gene mapping, remains the main bottleneck of this approach. We set up a high-throughput, low-cost, and precise methodology that allows analysis of root traits of hundreds of plants with limited measurement effort and without soil constraints. The "rhizoscope" system is based on the use of 2-D "nail board rhizoboxes" filled up with glass beads and bathed with aerated nutrient solution. The substrate permits one to study roots under uniform mechanical impedance, which can be manipulated by changing bead size. Growth is visualized in two dimensions and the measurement of multiple parameters such as root angle, number, length, and diameter is based on computer-aided analysis of digital images. The combination of 2-D boxes with glass beads simulates the soil conditions well enough to obtain relevant information, as demonstrated by our comparative studies in soil-filled tubes. Our disposition also maximizes differences among genotypes, making it an ideal system for QTL identification. Although initially developed for japonica rice, our rhizoscope system is generic and can be employed for phenotyping other plant species.(Texte intégral

Genetic variability and correlation analysis of rice (Oryza sativa L.) inbred lines based on agro-morphological traits

In order to evaluate genetic variability of agro-morphological traits and also determine the correlation between grain yield with its components in rice lines, 17 recombinants inbred lines, their parents and a check variety were grown in research station of Africa rice center in Benin republic during two consecutive years 2013 and 2014. The experiments were laid out in a randomized complete block design with four replications. Phenotypic coefficients of variance were higher than genotypic coefficients of variance in all the characters across the two years. High heritability in broad sense (H2) estimates were obtained for biomass (68.77%), date of 50% flowering (98.11%), plant height (81.94%), leaf area (82.90%), number of panicles (64.40%), leaf dry weight (72.91%), root weight (67.43%) and yield/plant (62.23%) suggesting that the traits were primarily under genetic control. A joint consideration of broad sense heritability (H2) and genetic advance as per cent mean expected (GAM) revealed that leaves dries weight and roots weight combined high heritability and high GAM. Furthermore, high (H2) and high GAM recorded in these characters could be explained by additive gene action. However, high estimates (H2) combined with moderate GAM recorded for biomass, day to 50% flowering, leaf area, number of panicle and yield/plant could be due to non-additive gene effect. Grain yield/plant recorded positive and significant correlation with stem weight (r=0.5262) and biomass (r=0.9291). This result indicates that selection based on these two characters will be highly effective for yield improvement in rice. (Résumé d'auteur