Does indoleacetic acid promote growth via cell wall acidification?

Growth of Triticum aestivum L. cv. Cappelle Desprez coleoptiles is promoted by 5.7×10(-5) M indole acetic acid (IAA) as effectively in pH 3.4 buffer as in water, but IAA is not effective in the presence of buffer at pH 3.0 or 3.2 A combination of 5.7×10(-5) M IAA and pH 3.4 buffer promotes growth to a greater extent than pH 3.2 buffer alone, which is optimal for acid-induced growth. IAA employed at 10(-7) M is still effective at promoting growth in the presence of pH 3.4 buffer, moreover, IAA at 10(-7) M interacts synergistically with the acidic buffer to promote growth. It is concluded that IAA and acid promote growth via separate mechanisms, and that IAA does not promote cell wall loosening by rendering the cell wall more acid.