Ferrous iron toxicity : mechanisms of resistance and impact on nutrient elements at the vegetative stage in cultivated rices (Oryza sativa L., Oryza glaberrima Steud and Interspecific Hybrids)

In order to improve our understanding of the resistance mechanisms and impact on nutrient elements of cultivated rices (Oryza sativa L., Oryza glaberrima Steud and interspecific hybrids) in the presence of an excess of ferrous iron, a study was performed in hydroponic conditions at the vegetative stage. Seventeen cultivars were submitted to 0 or 250 mg L-1 Fe2+ during 28 days in the Yoshida solution. Leaf iron concentration and level of bronzing correlated positively and highly significantly. Both parameters correlated positively with quantity of iron coating, but negatively and highly significantly with leaf dry weights and weight of roots organic matter, thus showing that efficient regulation of leaf iron concentration play a primordial role in resistance to ferrous iron toxicity. The time course evolution of Fe revealed that the capacity to insure homeostasis of iron at a relatively low concentration in resistant cultivars would be of primary importance for resistance to ferrous iron toxicity. Iron coating that correlated negatively with all growth parameters may be considered as a symptom of ferrous iron toxicity rather than a mechanism of resistance. The mineral concentrations in the plant tissue decreased with time, except Fe in shoots and roots and P in roots under Fe2+ treatment. The reduction was clearly noticeable in bivalent cations Mg, Ca, Zn, Mn. However all concentrations remained in between the physiologically adequate range. Thus the development of symptoms of ferrous iron toxicity was mainly attributable to excess of Fe concentration rather than imbalances in other nutrients. The proportion of aerenchyma was not influenced by ferrous iron treatment. However, the cultivars that showed the highest rate of aerenchyma were the most resistant. The development of lateral broom-type roots was induced under Fe2+ treatment in resistant cultivars and should also be considered as related to the resistance, since it corresponded to the formation of healthy and active lateral roots. Obvious differences were found between cultivars. The glaberrima's produced high biomass, highest aerenchyma rate, high number of lateral broom-type roots, had low levels of iron in the leaf tissue, produced few iron coating, and generally had low levels of bronzing. They may thus be considered as ferrous-iron resistant mainly because of avoidance mechanisms related to a lower Fe concentration in the leaf tissue.Doctorat en sciences agronomiques et ingénierie biologique (AGRO 3)--UCL, 200