Structural forms of hormone auxins in Madeira vines

Abstract

X-ray diffraction and small-angle neutron scattering were used to identify the naturally occurring hormone auxin in Madeira vine. Two kinds of auxins belonging to indole-3-acetic acid (IAA, C 10 H 9 NO 2 ) and 4-chloroindole-3-acetic acid (4-Cl-IAA, C 10 H 8 ClNO 2 ) have been found. The appearance of well-defined x-ray diffraction peaks from the IAA as well as from the 4-Cl-IAA structures shows that the IAA and 4-Cl-IAA molecules that are produced at the shoot apex crystallize into periodic arrangements, rather than stay in molecular forms, upon transport downward through in stem. Small angle neutron scattering spectra reveal a progressive increase in the length of crystallized IAA and 4-Cl-IAA on transported downward from the shoot apex to the stem, reaching 59 nm in length in the area 8-9 cm below the shoot apex. A portion of IAA loses their carboxyl group COOH becoming inactive in stimulating cell growth, which slows down the growth in an aged stem.