Growth Distribution during Phototropism of Arabidopsis thaliana Seedlings.

The elongation rates of two opposite sides of hypocotyls of Arabidopsis thaliana seedlings were measured during phototropism by using an infrared imaging system. In first positive phototropism, second positive phototropism, and red light-enhanced first positive phototropism, curvature toward the light source was the result of an increase in the rate of elongation of the shaded side and a decrease in the rate of elongation of the lighted side of the seedlings. The phase of straightening that followed maximum curvature resulted from a decrease in the elongation rate of the shaded side and an increase in the elongation rate of the lighted side. These data for the three types of blue light-induced phototropism tested in this study and for the phase of straightening are all clearly consistent with the growth rate changes predicted by the Cholodny-Went theory.