The relationship between phototropism and axis extension was examined in light‐grown mustard (Sinapis alba L.) seedlings using the low pressure sodium lamp (SOX)† technique to eliminate growth responses due to phytochrome. Addition of blue light caused no net inhibition of hypocotyl elongation, but plants showed a phototropic response. Curvature was caused by a simultaneous inhibition of growth on the illuminated side of the hypocotyl and an acceleration on the shaded side. Phototropism thus occurs independently of axis elongation and suggests that they are two separate processes. The results are inconsistent with the Blaauw theory of phototropism.
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Rich et al. (1985) studied this question.
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