RICE GROWTH AS AFFECTED BY COMBINED AMMONIUM AND NITRATE SUPPLY

In rice cultivated under flooded conditions, the anaerobic condition favors the formation of NH4+ in the soil, and is therefore considered the main available N source for this crop. However, the process of O-2 transport through the aerenchyma and its release by roots, create a favorable environment for nitrification in the plant rizosphere. Nitrification intensity determines the proportions between available NH4+ and NO3-. In this case, it is believed that the presence of NO3- can favor rice growth and simultaneously avoid NH4+ toxicity. This experiment was carried out to evaluate the effect of different proportions between NH4+ and NO3- in nutrient solution on rice growth. The experiment was performed in a greenhouse, from September to November 2007, in a nutrient solution with the following NH4+ and NO3 proportions: 100: 0, 75: 25 and 25: 75, in a 10.0 mmol L-1 N concentration. The plots were fulfilled with a inert substrate to create conditions to root growth. The genotypes IRGA 417 and Sasanishiki were compared and the biomass production, root length, root distribution and the N, Ca, Mg and K uptake were evaluated. The presence of NH4+ in the 100: 0, 75: 25 proportions resulted in plant toxicity, however, as the NO3- proportion increased, toxicity was reduced. The combined supply of NH4+ and NO3- increased biomass production compared to solely NH4+ supply. The rice root system of the two cultivars developed mainly in the 0-10 cm layer and the greatest root length was observed in the treatment NH4+ 75: 25 NO3- for IRGA 417. The presence of NH4+ in soil solution affected negatively calcium absorption, whereas no affect was observed on N, Mg or K absorption. The total absorbed N, K, and Ca indicated a higher absorption efficiency with the combined supply for IRGA 417, but for cultivar Sasanishiki no differences were verified. It was concluded that NH4+ is toxic to rice plants, and is not the main available source of nitrogen. Nitrate is indispensable to increase the N uptake, normal plant growth and total cation absorption.