Climatic and soil factors are limiting rice growth in many countries. In Vietnam, a steep gradient of temperature is observed from the North to the South, and acid sulphate soils are frequently devoted to rice production. We have therefore attempted to understand how temperature affects rice growth in these problem soils, by comparison with rice grown in nutrient solution. Two varieties of rice, IR64 and X2, were cultivated in phytotrons at 19/21°C and 28/32°C (day/night) for 56 days, after 3 weeks preculture in optimal conditions. Two soils from the Mekong Delta were tested. Parallel with the growing experiments, these two soils were incubated in order to monitor redox potential (Eh), pH, soluble AI and Fe, soluble, and available P. Tillering retardation at 20°C compared to 30°C was similar in nutrient solutions and in soils. The effect of temperature on increasing plant biomass was more marked in solutions than in soils. The P concentrations in roots and shoots were higher at 20°C than at 30°C, to such an extent that detrimental effect was suspected in plants grown in solution at the lowest temperature. The translocation of Fe from roots to shoots was stimulated upon rising temperature, both in solutions and in soils.This led to plant death on the most acid soil at 30°C. Indeed, the accumulation of Fe in plants grown on soils was enhanced by the release of Fe 2+ due to reduction of Fe(III)-oxihydroxides. Severe reducing conditions were created at 30°C: redox potential (Eh) dropped rapidly down to about 0 V. At 20°C, Eh did not drop below about 0.2 V, which is a value well in the range of Fe(III)/Fe(II) buffering. Parallel to Eh drop, pH increased up to about 6-6.5 at 30°C, which prevented plants from AI toxicity, even in the most acid soil. Phosphate behavior was obviously related to Fe-dynamics: more reducing conditions at 30°C have resulted in enhancement of available P, especially in the most acid soil.
L'absorption du technétium, radionucléide artificiel essentiellement issu du cycle du combustible nucléaire, a été étudiée chez le riz (Oryza sativa L.) en aquiculture et en sol irrigué par une solution contaminée. Les cinétiques d'absorption en flux continu pour différentes concentra tions en 99TC (0,017 à 17 *Ci. I-1) révèlent l'existence de deux phases : la première à dominance passive (diffusion dans les espaces libres) et la seconde, linéaire, résultant de la pénétration du technétium dans les cellules et de ses conséquences. L'évolution de la désorption met en évidence la présence de deux compar timents dont le premier correspondrait aux "espaces libres apparents" tandis que le second, comptant plus des 95 % de l'activité de la plante, représenterait la fraction absorbée. La demi-vie biologique dans ce second compartiment est telle (40 à 50 jours) qu'en cas de pollution accidentelle, une décontamination totale des plantes ne peut être attendue. Comme les essais en aquiculture, les essais en sol montrent l'apparition de symptômes de toxicité pour une concentration dans la solution d'irri gation de 17 / µCi.l-1. Les facteurs de transfert calculés sont très élevés, de l'ordre de 1000 pour les parties aériennes végétatives. La répartition du technétium entre les différentes parties de la plante varie selon la concentration en 99TC mais plus de 90 % sont localisés dans les parties aériennes; les caryopses (parties comestibles) n'en renferment que 1 % .
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