INFLUENCE OF SOIL-MOISTURE REGIMES ON SUBSEQUENT SOIL MANGANESE AVAILABILITY AND TOXICITY IN 2 COTTON GENOTYPES

Differential plant tolerance to high levels of Mn and effects of chang ing soil aeration on Mn availability and are major factors in the prob lem of Mn toxicity in acid soils. A pot study was conducted to evaluat e the effects of temporary flooding or drying on Mn forms and toxicity during subsequent cotton (Gossypium hirsutum L.) growth. Two acid, hi gh-Mn Hapludalfs were incubated for 16 d in flooded, air-dry, or moist conditions, then restored to field moisture content and planted with two cotton genotypes: C-310,73-307 (307, Mn-tolerant) and C-Sg1,70-517 (517, Mn-sensitive). After the incubation period, levels of exchangea ble Mn in the Jackland soil (pH 4.8) were 9.0, 2.6, and 0.3 mmol kg(-1 ), respectively, for hooded, air-dry, and moist pretreatments. Corresp onding values for the Myersville soil (pH 5.2) were only 4.4, 0.6, and 0.1 mmol kg(-1). Three days after seedling emergence in the flooded p retreatment, exchangeable Mn declined to 4.4 mmol Mn kg(-1) in the Jac kland soil and 0.2 mmol Mn kg(-1) in the Myersville soil. Manganese to xicity symptoms (crinkle leaf) for both cotton genotypes developed onl y on the Jackland soil, with severity of symptoms following the order: flood > air dry > moist. With both soils, flooding pretreatment decre ased dry matter production; generally more for 517 than for 307. Tissu e Mn concentrations were 20 to 50 times higher in leaves from Jackland soil compared to those from Myersville soil. Where flooding caused to xicity symptoms in both genotypes, lower exchangeable Mn was measured in pots growing Mn-tolerant cotton. Differential Mn tolerance of cotto n genotypes was not associated with differential Mn uptake or tissue c oncentration.