Inversion of gravitropism by symmetric blue light on the clinostat

Abstract

Gravitropic stimulation of maize (Zea mays L.) seedlings resulted in a continuous curvature of the coleoptiles in a direction opposing the vector of gravity when the seedlings were rotated on a horizontal clinostat. The orientation of this response, however, was reversed when the gravitropic stimulation was preceeded by symmetric preirradiation with blue light (12.7 μmol photons·m⁻²). The fluence-response curve of this blue light exhibited a lower threshold at 0.5 μmol·m⁻², and could be separated into two parts: fluences exceeding 5 μmol·m⁻² reversed the direction of the gravitropic response, whereas for a range between the threshold and 4 μmol·m⁻² a split population was obtained. In all cases a very strong curvature resulted either in the direction of gravity or in the opposite orientation. A minor fraction of seedlings, however, curved towards the caryopsis. Furthermore, the capacity of blue light to reverse the direction of the gravitropic response disappeared with the duration of gravitropic stimulation and it depended on the delay time between both stimulations. Thistonic blue-light influence appears to be transient, which is in contrast to the stability observed fortropistic blue-light effects.