Auxin Transport in Geotropic Curvatures of a Branched Plant.

Our present un('erstanduing of growth curvatures produced by negative geotropism is based otl the concept of excess of auxin in the lower side. Dolk's (5) report of this imbalance of IAA in coleoptiles has been confirmed by refined techniques (7, 8). Evidence for a downward movement of auxin through other organs has also been ol)taine(l but it too was (lerive(l from the distribution of free auxin in juvenile structures rather than from measurements of auxin supply to the growth zones of established plants. Dijkmnan (4) obtained data from diffusible auxin in hypocotyls of Lupinufs after a few hours in the horizontal position. Van (ler Laan (14) measured diffusible auxin in the upper and lower halves of Vicia epicotyls for periods up to 90 minutes. Brauner and Bock (2) used ethanol to extract IAA from hypocotyls of Helianthits after 45 minutes of gravity stimulus. Boysen-Jensen (1) extracted auxin fromt horizontal epicotyls of Phaseolus and TIicia. Lateral transport by gravity within the tissues of typical branches and axial internodes is only a reasonable assumption from the evidence for seedling structures. The movement of auxin molecules has not been demonstrated in nonseedling tissues. The probability for such downwar(l transport in a nonvertical stenm is supported, however, b) the appearance of full epinastic curvatures in branches (10) only when growth of the plant on a horizontal clinostat eliminates an apparent lateral transport effect of gravity. This report presents direct evi(lence from dicotvledonous stems for a definite transport effect of gravity on exogenous IAA-2-C14, based on the distribution of the compound and its reaction products within immature internodes after response to a geotropic stimulus had produced growth curvatures in them. The resulting imbalance of radioactive conpounds between upper and lower sides closely resembles that of unaltered auxin in seedling organs.