Light induces root hair formation in etiolated lettuce (Lactuca sativa L. cv. Grand Rapids) seedlings under acidic conditions by a phytochrome-mediated response. Microbeam irradiation of 1 mm root segments with the first (100 Jm(-2)) and second (1,000 Jm(-2)) maxima of the fluence 'response curve for red light induction of root hair initiation indicated that the ca. 5 mm apical portion of 12 mm long roots was the site of photoperception. The root hair-forming portion of the root was situated at a distance of 1.7 mm from the root tip at the time of red light irradiation and extended (at most 1.5 mm) towards the basal end at a later stage of development, irrespective of which portion of the root was irradiated with red light.