DIFFERENTIAL MANGANESE TOLERANCES OF COTTON GENOTYPES IN NUTRIENT SOLUTION

Cotton genotypes [Gossypium hirsutum (L.)] C-310-73,-307 (307) and C-Sgl, 70-517 (517), shown previously to differ in tolerance to an acid (pH 5.1), high manganese (Mn) Grenada soil from Arkansas, were grown in nutrient solutions containing variable concentrations of excess Mn to confirm and characterize their postulated differences in Mn tolerance. Based on crinkle leaf symptoms and leaf dry weights, the 307 genotype was significantly more tolerant than 517 to 4, 8, or 16 mg Mn/L at a maintained pH of 4.6 (Experiment 1) and also to 4 or 8 mg Mn/L at an initial pH of 5.0, not subsequently adjusted (Experiment 2). In Experiment 1, the relative leaf dry weight (wt. with no Mn/wt. with 8 mg Mn/L x 100) was 94% for genotype 307 and only 27% for 517. In Experiment 2, the corresponding relative leaf weights were 75% and 26% for 307 and 517, respectively. Plant analytical results indicated that the 307 genotype tolerates a higher concentration of Mn in its leaves than does 517. This failure to correlate Mn tolerance with Mn concentrations in plant shoots agrees with previous findings when these two genotypes were grown in acid Grenada soil. Iron (Fe) concentrations, Fe/Mn ratios, and magnesium (Mg) concentrations were higher in the Mn-tolerant 307 than in the Mn-sensitive 517, but concentrations of phosphorus (P), potassium (K), calcium (Ca), copper (Cu), and zinc (Zn) were not related to Mn tolerance. Because differential Mn tolerance in these two genotypes is associated with differential internal tolerance to excess Mn, rather than differential Mn uptake, studies are needed to determine the chemical forms of Mn in tolerant and sensitive plants whose leaves contain comparable concentrations of total Mn. Because both Mn and Fe (closely related elements in the Mn toxicity Syndrome) have spin resonances, electron paramagnetic resonance (EPR) offers promise in attacking the problem of differential Mn tolerance in plants.