Description

[Synopsis]

Rice farmers in Asia are adopting direct seeding because this practice requires less labor than transplanting. The main constraint of the adoption is the inconsistent seedling establishment, because O₂ is deficient in flooded soil. It has been found recently that ferrous iron (Fe^＋＋), whichis commonly present in flooded soil, stimulates coleoptile elongation. This study was conducted to analyse the mechanism of Fe^＋＋- stimulation of coleoptile elongation. Germinated rice seeds were placed and sealed in Erlenmeyer flasks or vials and then 2 mM Fe^＋＋ or water (as a control) was administered. We found that Fe^＋＋ application increased ethylene production as well as the elongation of coleoptile. When ethylene was applied exogenously to the germinating seeds, the effect of Fe^＋＋on coleoptile elongation was eliminated. When Fe^＋＋ was applied to the germinated intact seeds, germinated caryopsis, or husks, ethylene production was stimulated by Fe^＋＋ in intact seeds and husks and not in caryopsis, suggesting that ethylene was produced in the reaction between husks and Fe^＋＋. Trichloroacetic acid did not inhibit the reaction. Ethylene production was as low as at 0.02 mM Fe^＋＋ and maximum at between 2 and 200 mM. Ethylene production increased linearly up to 3hr after the application of Fe^＋＋, even in the absence of O₂．The results suggest that Fe^＋＋ in flooded soil reacts with husks of germinating seeds, produces ethylene, and elongates the coleoptile to the soil surface where O₂ is present.