Interactions between light and gravitation in moss protonema tropisms

Growth movements of protonemata of the moss Ceratodon purpureus (Hedw.) Brid. were studied in darkness and during and after unilateral illumination. We found that the direction of protonema growth was determined at least by three stimuli responsible for photo-, gravi-, and autotropism. In darkness, protonemata showed negative gravitropism; under uniform illumination, they grew radially over the substrate surface; unilateral illumination induced phototropic growth. Blue and far-red light blocked gravitropism; red light blocked both gravi- and autotropism. Reversible inhibition of autotropism involved the phytochrome system, as is evident from red and far-red effects. Gravitropism was coupled with starch synthesis and amyloplast formation.