IRON-MANGANESE INTERACTIONS AMONG CLONES OF NILEGRASS

Tetraploid clones of Nilegrass (Acroceras macrum, Stapf.) develop a ch lorosis resembling iron (Fe) deficiency on acid (pH 5.0) soils in the Midlands of KwaZulu, Natal, South Africa. Hexaploid and pentaploid clo nes appear more resistant to the disorder. Iron deficiency would not b e expected in such acid soils, but foliar sprays of Fe sulfate reduce the symptoms within 24 hours. Aluminum (Al) toxicity has been ruled ou t as a cause of this chlorosis on the basis of soil tests. Manganese ( Mn)-induced Fe deficiency has been postulated. Six Nilegrass clones, d iffering in ploidy levels, were grown under low Fe or high Mn levels i n nutrient solutions, in Mn-toxic soil, in calcareous soil and in a st andard potting soil at pH 7.0. Differential chlorosis symptoms, simila r to those observed in the field, were reproduced in plants grown in l ow Fe or high Mn solutions, in neutral potting soil and in calcareous soil at pH 7.8. Based on plant symptoms and dry weights, the tetraploi ds were generally more sensitive to these conditions than hexaploid or pentaploid clones. However, in Mn-toxic soil, plants had leaf tip nec rosis rather than the chlorosis typical of Fe deficiency. When grown i n a standard potting soil at pH 7.0, plants showing chlorosis accumula ted higher concentrations of phosphorus (P), Al, copper (Cu), Mn, Fe, and zinc (Zn) than non-chlorotic plants. Differential susceptibility t o chlorosis is apparently associated with interference of such element s in Fe metabolism, and not with differential Fe concentrations in pla nt shoots. Additional studies are needed to determine the chemical sta tes of Fe and Mn in root zones and within plant shoots of these clones . Resolution of the differential chlorosis phenomenon would contribute to fundamental knowledge in mineral nutrition and could be helpful in tailoring plant genotypes to fit problem soils.