The Role of Boron in Plant Growth: IV. INTERRELATIONSHIPS BETWEEN BORON AND INDOL-3YL-ACETIC ACID IN THE METABOLISM OF BEAN RADICLES

The hypothesis that boron deficiency is equivalent to a stateof IAA toxicity was explored. Bioassays showed that extractsof substances similar to IAA taken from boron-deficient rootswere significantly more inhibitory to the growth of bean-rootsegments than those from normal roots. Supplied IAA and borondeficiency together restricted root growth to a greater extentthan either deficiency or IAA treatment separately. Roots werefound to recover more quickly from the inhibitory effects ofsupplied IAA if boron was present at high (0.5 ppm) rather thanlow (0.01 ppm) concentrations. Experiments with ¹⁴C-labelled IAA showed that deficient rootsabsorbed ¹⁴C more slowly than boron-fed roots and there wasalso a lower rate of decarboxylation in the deficient tissue.Consideration of the published evidence showed that many ofthe effects of boron deficiency could follow from an upset inIAA metabolism. It is suggested that boron-deficient tissuesuffers from excess auxin either because the element is necessaryfor some growth process, such as cell wall formation or nucleicacid synthesis, which, when impaired, results in the accumulationof auxin, or because the IAA-oxidation system is affected byphenolic inhibitors which boron normally inactivates by complexformation.