IMPACT OF ALUMINUM TOXICITY ON GROWTH AND EFFICIENCY OF NUTRIENT METABOLISM IN SOME TROPICAL RICE CULTIVARS

Ten-day-old seedlings of six cultivars of rice (Oryza sativa L.), IR 60, IR 20, Co 31, Co 37, IET 1444 and ADT 36, were exposed to Al stress in hydroculture at pH 4.0 for three weeks. The cultivars exhibited a wide range of responses in growth and uptake and efficiency ratios (ER) of utilization of nutrients, both in the presence (222-mu-mol l-1) and absence of Al. Under Al stress, the cultivars Co 37 and Co 31 had maximum uptake of Ca, K, Mg, Mn, P and Fe; ADT 36 showed poor uptake of nutrients. The efficiency ratios (ER) also agreed with the above conclusion on Co 37 and Co 31. It was not possible to identify any relationship between the accumulation of Al in the foliage and the efficiency ratios. Assay of nitrate reductase (NR) activity in vivo suggested that the cultivars Co 37 and Co 31 registered less inhibition in enzyme activity in the presence of Al compared with the other cultivars. These evaluations indicate that cultivars Co 37 and Co 31 emerge as the most tolerant to Al and ADT 36 the most susceptible, the other cultivars being intermediate. The above findings have implications in agriculture on acid soils and also for breeding programmes.