GROWTH DISTRIBUTION DURING PHOTOTROPISM OF ARABIDOPSIS-THALIANA SEEDLINGS

The elongation rates of two opposite sides of hypocotyls of Arabidopsi s thaliana seedlings were measured during phototropism by using an inf rared imaging system. In first positive phototropism, second positive phototropism, and red light-enhanced first positive phototropism, curv ature toward the light source was the result of an increase in the rat e of elongation of the shaded side and a decrease in the rate of elong ation of the lighted side of the seedlings. The phase of straightening that followed maximum curvature resulted from a decrease in the elong ation rate of the shaded side and an increase in the elongation rate o f the lighted side. These data for the three types of blue light-induc ed phototropism tested in this study and for the phase of straightenin g are all clearly consistent with the growth rate changes predicted by the Cholodny-Went theory.