Influence of gravity on the photomorphism of secondary moss protonemata

In dark-grown plantlets of the moss, Pottia intermedia, negatively gravitropic secondary protonemata differentiate from the superficial cells of leafy shoots. When transferred to the light, distal parts of the protonemata nearest to the apical cells begin to ramify and the apical cells of the side branches as well as of the main protonemal filaments often differentiate as buds. Dark-grown protonemata were oriented horizontally and illuminated from below with white light of different intensities. Only light with an intensity of 4.5 μmol·m−2·s−1 was sufficient to induce: (a) phototropism in the apical cells, (b) light-directed initiation of branch primordia, and (c) directed growth of side branches and bud differentiation. Apical cells illuminated with light of lower (0.03–0.37 μmol·m−2·s−1) intensity grew upwards (i.e., away from the light). It was shown that this upward growth was determined by the action of gravity. Although initiation of branch primordia was only slightly affected, their growth was strongly stimulated on the upper side of the protonemata.