A, Effect of unilateral UV-B on 2-d-old etiolated seedlings. Seedlings were grown on Jiffy pellets for 2 d in darkness and subsequently exposed to unilateral monochromatic UV-B (0.12 µmol m–2 s–1 of 302 nm). Photographs were taken after 24 h of exposure. B, Kinetic analysis of directional bending in Arabidopsis mutants. Seedlings were grown as in A, with a background of infrared (930-nm) light. Every 15 min, photographs were taken with the focal plane parallel to the direction of UV-B radiation. Quantification of the bending angle was performed on these photographs using ImageJ software (Vandenbussche et al., 2014). Error bars indicate sd (n ≥ 10). wild type, Columbia; hy5, hy5-215; phot1phot2, phot1-5 phot2-1.