Résumé
Iron toxicity is a widespread nutrient disorder of lowland rice ecosystems in West Africa. Soluble iron present in the soil solution under waterlogged and reducing conditions is absorbed by roots and accumulates in shoots. It causes leaf discoloration associated with poor growth, reduced tillering and severe yield reduction. Strong correlation (r = 0.84) was observed between leaf iron-toxicity symptom score and grain yield across seasons and nutrient treatments. Field experiments were conducted during 1997 and 1998 at an iron-toxic lowland site at Korhogo (Ivory Coast) to study iron-toxicity avoidance mechanisms for three lowland rice (Oryza sativa) cultivars. Iron concentration in various plant organs was determined by atomic absorption spectrophotometry of digested tissue. Despite the similar morpho-phenological characteristics and iron uptake levels of the three cultivars, the iron distribution among their organs (roots, stems and leaves) was different, indicating the presence of a specific physiological avoidance mechanism. The tolerant cultivar stored more iron in its stem and less in its leaves than did the sensitive cultivar. The very sensitive cultivar did not have any iron barrier mechanism between the different organs. For this cultivar, the iron content was higher in all organs. Iron content in leaves had a negative effect on the photosynthetic rate. Application of P, K and Zn, with N, significantly reduced iron-toxicity symptoms, corrected the photosynthetic rate and increased yield in all three cultivars. The correlation of yield with iron distribution among plant organs will permit the use of the iron partitioning in rice plants as a breeding criterion for the selection of improved materials. (Résumé d'auteur)