The photoperceptive sites and the function of tissue light-piping in photomorphogenesis of etiolated oat seedlings*

Abstract. Responses to red light irradiation of discrete areas along the intact, etiolated oat seedling indicate that illumination of the region around the coleoptilar node results in maximal coleoptile growth stimulation and mesocotyl growth suppression. Quantitation of the fibre optic properties of these etiolated tissues shows that the amount of axially transmitted light is log linear as a function of distance for both the mesocotyl and coleoptile (plus primary leaf). Using the fibre optic properties of the tissues to predict the response of the etiolated seedling to defined illumination fields allows one to localize two sites of photoperception: although the mesocotyl response pattern can be explained by the action of a single site found near the top of the mesocotyl itself, the coleoptile response depends on irradiation of both the mesocotyl site and an additional site located just above the node. The very low- and the low-fluence responses of etiolated oats independently predict similar regions of the seedling as sites of photo-perception. The fibre optic properties of the seedling could allow the seedling to increase the effective light signal received by the photosensitive area significantly.