Sorgoleone exudated from the root hairs of sorghum (Sorghum bicolor (L.) Moench) is a potent bioherbicide. The living root hairs of young developing sorghum plants exude higher content of sorgoleone. The concentration of sorgoleone is proportional to the formation of root hairs. A set of factors that enhance root hair formation and subsequently sorgoleone production were studied under in vitro conditions. The effect of auxins on root growth and root hair formation were studied by incorporating IAA, IBA, NAA and 2,4-D in the concentration range of 0.5- 2.0 mg/L respectively, as a supplement in half-strength Murashige and Skoog (MS) medium. In addition, the effect of gibberellic acid (GA3) was also studied by supplementing it in half-strength MS medium at a concentration range of 0.5-2.0 mg/L. Among the auxins tested for their in vitro rhizogenic potential, the optimal response was observed in IBA at 1.0 mg/L. Further, GA3 at 1.0 mg/L also showed a significant optimal effect on root hair formation. The effect of strength of MS medium, addition of copper, phloroglucinol, polyamines such as spermidine and putrescine on optimal root hair induction were also investigated in separate experiments. The results indicated that root hair formation was beneficially promoted in half- strength MS medium containing 1.0 mg/L of IBA and GA3 respectively. Also inclusion of copper sulfate at 0.5 mg/L in half-strength MS medium in the presence of 1.0 mg/L of IBA and GA3 respectively, resulted in enhanced root growth and root hair formation. The individual addition of phloroglucinol, spermidine and putrescine to root induction media did not provide any additive effects on promoting root hair formation but however, it was observed that a combination of optimal phytohormones together with copper sulphate and phloroglucinol improved the root hair formation to a significant extent. Further, the optimal temperature for root hair formation was identified to be 30ºC and the roots of 7-days old seedlings showed an increased synthesis of root hairs. An aqueous root extract containing sorgoleone showed bioherbicidal activity against Acalypha indica during in vitro phytotoxic assay.
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