7 ǀ Conclusions
  1. There is ample variation in tolerance of Fe toxicity in both O. sativa and O. glaberrima gene pools for identifying donors and markers for breeding. However to date, no markers have been identified with large enough effects to be utilized in applied breeding programs.
  2. Germplasm screening is complicated by large genotype by environment effects and weak correlation between visible symptoms and beneficial stress response strategies, including below-ground plant-soil interactions.
  3. Severe nutrient deficiencies are the norm in most Fe toxic soils, and applications of N, P and K fertilizers are essential for reasonable rice yields. Toxicity symptoms become more apparent as nutrient deficiencies are alleviated.
  4. Genotypes whose tolerance depends on Fe exclusion by oxidation in the rhizosphere make deficiencies of cationic nutrients (such as K and Mg) worse because of the accompanying acidification of the rhizosphere and resulting impairment of cation mobility. Genotypes with enhanced Fe storage in roots and stems may be better suited to such conditions.
  5. There has been recent progress in understanding the molecular physiology of tolerance mechanisms, including below-ground processes controlling Fe retention in roots and root-shoot transport, as well as above-ground partitioning and tissue tolerance. Each of these interacts in specific ways with nutrient deficiencies.
  6. To successfully identify markers for use in breeding, the complex tolerance response should be broken down into component traits based on the tolerance mechanisms outlined above, and tailored screening methods for individual tolerance mechanisms developed.