Although electron microbeam analysis is relatively new, when applicable to soil research, it may provide direct pictorial evidence of plant nutrient movement and accumulation in undisturbed samples, thereby eliminating hazardous work with radioisotopes. Elemental concentrations and gradients in soil-root interfaces were studied by electron microbeam analysis, used together with a scanning electron microscope. The objective was to observe movement and distribution zones of elements in soil rhizo-sphere and plant cells. Pine (Pinus taeda), cotton (Gossypium hirsutum L.), and peanut (Arachis hypogeae L.) were grown in the greenhouse, and roots with adhering soil were sampled and prepared by impregnation and fixation techniques, according to Humpreys (1975).
Scanning electron microscopy indicated that the preparation procedures used did not distort the plant tissue and, at the same time, it reduced leaching of elements to a minimum. Cellular details were well preserved, and granular structures of adhering soil were displayed clearly. X-ray distribution images revealed higher concentrations of K, P, Ca, and S in pine-root tissue and the soil rhizosphere than in the soil at a distance 0.2-0.3 mm away from the root surface. On the other hand, the X-ray distribution images of Si appeared stronger in the bulk of the soil than in the plant tissue. Magnesium, Mn, and Zn appeared to be evenly distributed over soil and root zones. The highest concentration of K was detected in the cortex, not in the xylem. Calcium was also present, in highest concentrations, in peripheral cells of the root nodule of peanut plants. Phosphorus, Mg, Mn, and Zn were not observed to concentrate in specific cells of the root section investigated. The presence or absence of specific distribution regions of elements in the root tissue-was explained to be related to the functions of the elements in plant growth.
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