Metabolization of Elemental Sulfur in Wheat Leaves Consecutive to Its Foliar Application

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Abstract
The qualitative and quantitative aspects of elemental sulfur metabolization in wheat leaves and its effect upon photosynthetic metabolism were studied through the application of micronized sulfur upon the third leaf. Energy-dispersive x-ray analysis combined with scanning electron microscopy emphasized the existence of a sulfur peak associated with a strong potassium peak in the spectra of different tissue regions for treated leaves only, supplying an original evidence of sulfur uptake. Experiments with 35S-labeled micronized sulfur showed that about 2% of the labeled S was absorbed and metabolized into cystine, methionine, glutathione, and sulfate. The close correlation between the excess of oxygen uptake and oxygen needs for sulfur oxidation in conjunction with the absence of hydrogen sulfide released by treated leaves support direct and fast oxidation of sulfur into sulfate according to a pathway still unclear but independent of photosynthetic CO2 metabolism in treated leaf. The mechanisms involved in the primary metabolism of element sulfur in wheat therefore appear to be different from those in fungi.