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 (Seca le cereale L.) is similar, despite a general strongly enhancing effect of e xogenous IAA on the overall (cell) elongation of these organs. Longitudinal ly split coleoptiles or isolated longitudinally halved coleoptiles (horizon tally positioned as upper or lower halves) respond gravitropically in the s ame way as water-incubated intact coleoptiles, irrespective of whether the halves are incubated in distilled water or IAA. A new model for the princip al mechanism of regulation of gravitropic growth is proposed which depends on, yet does not involve, the redistribution of IAA as the means for gravis timulated differential growth, as postulated by the Cholodny-Went hypothesi s (CWH). It is based on a gravimediated temporarily restrained infiltration of IAA-induced wall-loosening factors into the growth-limiting outer epide rmal walls of the concave organ flank.