Gravity can induce epinastic bending of isolated leaves

Abstract Isolated leaves of Plectranthus fruticosus were grown in cubic plastic cuvettes, and were supplied via their cut petioles with nutrient solution and indole-3-acetic acid (10^?6m ). Holes bored in the cuvette walls allowed the petioles to be oriented at approximately 60°, 90° or 120° to the vertical. Growth of the leaves initially oriented at angles of 60° and 90°, which simulated the situation in the intact plant, did not result in epinastic bending of the petiole. Inversion of the leaves (adaxial surface of the petiole downwards) and orientation of the adaxial/abaxial surfaces of the horizontal petiole parallel to the gravity vector, however, yielded strong epinastic bending of the petioles. In the latter case, this bending was not in the direction of the gravity vector (evidence for point (iii), below). Furthermore, epinastic bending occurred, when the isolated leaves were rotated on a clinostat (petioles parallel to the rotation axis or inclined to the rotation axis at an angle of 30°; 3 r.p.m.). Since a possible influence of the shoot was excluded, it is concluded that (i) perception and response are restricted to the leaf, (ii) gravity alone is sufficient to induce epinasty, (iii) a gravitropic component of the response can probably be excluded. The clinostat induced epinasty may not have been caused by nullifying the effect of gravity but due to continuous gravistimulation of the leaf.