The Biological Activity of Organoboron Compounds in the Promotion of Root Growth

The growth of the bean (Vicia faba var. minor) radicle in nutrientsolution requires the presence of borate. Optimum extensiongrowth, over 41 h, was obtained in the presence of 0.5 µMboric acid. This requirement of borate for root growth couldalso be satisfied by PhB(OH)₂, 2-OCH₃PhB(OH)₂, and 4-OCH₃PhB(OH)₂.These three compounds also complex in vitro with catechol-3,5-disulphonic acid. NaPh₄B and 2, 6-OCH₃)₂PhB(OH)₂ did not formsuch complexes in vitro, but were biologically active as sourcesof boron for radicle growth. This activity of NaPh₄B was absentif roots were grown in solutions which were changed every 8h. The activity of both NaPh₄B and 2,6-(OCH₃)₂PhB(OH)₂ was increasedby using stock aqueous solutions which were not freshly prepared.Theresults are considered to provide support for the hypothesisthat the activity of borate, as an essential plant nutrient,depends upon its ability to form a biologically active complexwith an in vivo cis-diol compound.