Effects of Sorghum (Sorghum bicolor L.) root exudates on the cell cycle ofthe bean plant (Phaseolus vulgaris L.) root

Two experiments were conducted to test the allelopathic effect of sorghum ( Sorghum bicolor L.) root exudates on bean (Phaseolus vulgaris L.) cell divi sion. Research was conducted in the greenhouse of the Wistock Agricultural Research Institute of Minas Gerais State (EPAMIG) and in a laboratory of th e Federal University of Lavras (UFLA). Sorghum variety BR-601 and bean vari ety Carioca MG were used. The exudate, called sorgoleone (SGL), was obtaine d by methylene chloride and acetic acid extraction from sorghum roots seven days after sowing on Petri dishes, and refrigerated until use. Solutions o f 0.01, 0.05, 0.10, and 0.15 mM were prepared using Johanson solution as th e SGL solvent. Seven-day-old bean seedlings grown in vermiculite in a green house were transplanted to the solution. Seven days after transplantation, the beans did not show any sign of phytotoxicity; however, cytogenetic obse rvations showed that SGL reduced the number of cells in prophase, metaphase , and anaphase stages. Colchicine effects were observed among cells in meta phase on the third and fifth days after treatments and varied with SGL conc entrations. By the seventh day, the colchicine effects were inversely propo rtional to concentration, which varied from 34.3% for 0.01 mM to 6.6% for 0 .15 mM. SGL acts as a mitotic inhibitor. It probably depolymerizes the micr otubular proteins and induces the formation of colchicine metaphases causin g polyploid nuclei. A largest period of SGL treatment also induced chromoso me breaks and bridge formation in anaphase and telophase. Although SGL cann ot be used as a herbicide for bean cultures, its allelochemical effects on other cultures are the factors that will define the use of sorghum as a nat ural herbicide.