Abstract
In greenhouse experiments, 4 tropical forage crops: Chloris gayana Kunth. cv. Katambora, Sorghum bicolor L. Moench. cv. Sweet Soix IV, Pennisetum typhoides (Burmf.) stapf and Coix lacryma-jobi L. var mayuem (Roman) stapf were grown in low pH nutrient solution (pH 4.2) containing 0, 5, 20ppm aluminum (Al) and in high pH nurient solution without Al (pH 5.0) for 15 days and assesed for Al tolerance in terms of shoot and root lenght, root and shoot dry weight, Al and nutrinet (Ca, Mg, K, P) uptake. The pH of nutrinet solution was adjusted intitially to 5.0 and 4.2 with H_2SO_4 and NaOH and adjusted by the automatic pH adjusted controller through the experiment period. Both of root and shoot growth were depressed in 4 species grown with Al. Relative values of root dry weight (rate of dry weight of roots grown in low pH with 20ppm Al to that of roots in high pH without Al) was very high in C. lacryma-jobi, P. typhoides and low in S. bicolor and C. gayana. As aluminum-affected roots in aluminum sensitive species, the roots of C. gayana and S. bicolor were relatively short and thick and had many stunted laterals. And also, in C. gayana and S. bicolor, leaves showed chlorosis resembling to Al-induced magnesium deficiency, and reduced the growth of tillering. The absorption amount of all nutrients of C. gayana tended to be depressed severly by Al treatments, however, in other 3 species, the absorption of calcium (Ca) and magnesium (Mg) were more severely decreased than that of potassium (K) and phosphorus (P). C. gayana and S. bicolor that are aluminum sensitive species contained excess K and were deficient Ca and Mg, especially Mg. K/Ca+Mg (meq) ratios of C. gayana and S. bicolor in high pH culture medium without Al was 1.31 and 2.33, respectively. However, these ratios increased remarkably up to 5.85 and 5.55 in low pH culture medium contained 20ppm Al. Although the Al concentration of root increased by Al treatment, there were no differences among the species. It is conculuded from results obtained that Al tolerances among the species of tropical forage crops might be related to the ability of plant roots to absorb Ca and Mg, especially Mg, from culture medium containing high Al.
