Endogenous IAA Levels in Boron-Deficient and Control Root Tips of Sunflower

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Abstract
Bo deficiency in sunflower resulted in abnormal root morphology. Six hours after transfer to Bo-deficient nutrient medium, root elongation was inhibited; numerous ultrastructural symptoms, such as altered cell wall deposition and perturbations in the plasma and mitochondrial membranes, were evident. These early responses to Bo deficiency are similar to those caused by exogenously supplied optimal indoleacetic acid (IAA). Using a sensitive radioimmunoassay for IAA, no significant difference in free IAA was detected between Bo-deficient and control root tips after treatment for 6, 24 and 72 h. The growth rate of Bo-starved root tips was less than control root tips, but both showed similar levels of endogenous free IAA. In sunflower, early Bo deficiency symptoms cannot be correlated with elevated levels of endogenous IAA.