Photoperceptive site of the photoinduction of root hairs in lettuce (Lactuca sativa L. cv. Grand Rapids) seedlings under low pH conditions

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.