Lateral redistribution of auxin is not the means for gravitropic differential growth of coleoptiles: A new model

Gravicurvature in water- and auxin (IAA)-incubated coleoptiles of rye (Secale cereale L.) is similar, despite a general strongly enhancing effect of exogenous IAA on the overall (cell) elongation of these organs. Longitudinally split coleoptiles or isolated longitudinally halved coleoptiles (horizontally positioned as upper or lower halves) respond gravitropically in the same way as water-incubated intact coleoptiles, irrespective of whether the halves are incubated in distilled water or IAA. A new model for the principal mechanism of regulation of gravitropic growth is proposed which depends on, yet does not involve, the redistribution of IAA as the means for gravistimulated differential growth, as postulated by the Cholodny-Went hypothesis (CWH). It is based on a gravimediated temporarily restrained infiltration of IAA-induced wall-loosening factors into the growth-limiting outer epidermal walls of the concave organ flank.