The mechanism of excessive iron‐uptake (iron toxicity) of wetland rice

Abstract

The hypothesis that iron toxicity is the result of a multiple nutritional stress (deficiencies in P, K, Ca and Mg) was studied in the greenhouse (pot experiments) using a typical iron toxic soil and the rice varieties IR 22 (susceptible to iron toxicity) and IR 42 (relatively tolerant). During the growth period the effect of N, P, K, Ca + Mg‐fertillzation on Fe(II)‐formation, the total rhizosflora (reflected by the dehydrogenase activity), iron reducing microorganisms (total number and N₂‐fixing), Fe‐reducing bacteria) as well as on the sulfate reducing bacteria was investigated by regular sampling. Plant and root samples were analysed for total Fe and N, P, K, Ca and Mg.

Though the total number of iron reducing bacteria increased with fertilization, Fe(II)‐formation and dehydrogenase activity was much more intensive in the rhizosphere of the unfertilized treatment transplanted to IR 22. Population densities of the strictly anaerobic N₂‐fixing Fe‐ or sulfate reducers in the unfertilized pots were as high as those applied with N, P, K, Ca and Mg. Fertilization improved the Fe excluding mechanism efficiently (less Fe but more K, Ca and Mg in the roots), but increased the amount of Fe in leaves and culms. Iron toxicity is discussed in relation to an sufficient supply of nutrients.

Key words:

• Iron toxicity ‐ N
• P
• K
• Ca
• Mg‐fertilization ‐ Dehydrogenase activity
• Iron reducing bacteria ‐ Iron‐formation ‐ Iron‐ and nutrient‐contents of rice plants