Humic substances (HS) play important roles in the biotic-abiotic interactions of the root plant and soil contributing to plant adaptation to external environments. However, their mode of action on plants remains largely unknown. In this study the HS distribution in tissues of wheat seedlings was examined using tritium-labeled humic acid (HA) derived from leonardite (a variety of lignites) and microautoradiography (MAR). Preferential accumulation of labeled products from tritiated HA was found in the roots as compared to the shoots, and endodermis was shown to be the major control point for radial transport of label into vascular system of plant. Tritium was also found in the stele and xylem tissues indicating that labeled products from tritiated HA could be transported to shoot tissues via the transpiration stream. Treatment with HA lead to an increase in the content of polar lipids of photosynthetic membranes. The observed accumulation of labeled HA products in root endodermis and positive impact on lipid synthesis are consistent with prior reported observations on physiological effects of HS on plants such as enhanced growth and development of lateral roots and improvement/repairs of the photosynthetic status of plants under stress conditions.
Humic acids (HA) are natural organic compounds that are important components of organic matter. The accumulation, distribution, and fate of tritium-labeled HA prepared from coal were analyzed using wheat (Triticum aestivum L.) seedlings. There was a period of rapid accumulation of HA followed by a slower one in the period from 1 to 24 h. There was a significant decrease in HA accumulation at low temperature, indicating that the slower rate of HA accumulation represented a membrane-mediated process. HA distribution in plant tissues was analyzed using autoradiography. In all cases, HA concentration was considerably higher in the roots than in the shoots. Detailed examination of autoradiograms showed that there was preferential accumulation of HA in the apices of roots and shoots of wheat seedlings. Lipid fractions were extracted from seedlings and analyzed with thin layer chromatography and gas chromatography-mass spectrometry. These analyses revealed that labeled HA were present in the neutral lipid fraction consisting mainly of alkanes and alkenes, which are usually found in plant waxes, associated with the cuticle and suberized tissues. Based on these data, it is suggested that HA-derived hydrocarbons may be used in wax biosynthesis. This role could explain the mitigating activity of humic substances under stress conditions.
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