7 ǀ Conclusions
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.