Effects of illumination on the growth and histogeny of garden cress seedlings under altered gravity

Abstract

The study is focused on the effects of blue (450 nm), red (660 nm) and far red (735 nm) spectral components of light on the growth of garden cress (Lepidium sativum L.) seedlings when the pathway of the induction of gravitropic responses is isolated by a 50 rpm horizontal clinostat. Custom-built light modules with one blue, four red and one far red light emitting diodes were fitted for the first time in the centrifuge–clinostat complex. Hypocotyls, roots and leaves of seedlings grown under altered gravity applying blue, red or far red light separately and a combination of all were compared with those grown under normal gravity conditions in the light and in the dark, respectively. The obtained data showed that the impact of light wavelengths of 450, 660 and 735 nm applied at a comparatively low density of the photon flux (5, 13, 0.8–1 μmol m–2s–1, respectively) had a stronger inhibiting effect on the elongation of hypocotyls and leafstalks when gravitropic stimulation was eliminated by clinorotation as compared with normal gravity. In blue light, the elongation of 1-g and clinorotated hypocotyls was reduced by 16% and 24%, in red by 29% and 43%, in far red by 57% and 64%, respectively. Irradiation of six light diodes available in the used modules at a fluence rate of 50 μmol m–2s–1 stimulated a more effective differentiation of palisade cells of clinorotated but not of 1-g leaves. Keywords: light, horizontal clinostat, seedlings, garden cress, growth, leaf tissues