Directional blue light irradiation triggers epidermal cell elongation of abaxial side resulting in inhibition of leaf epinasty in geranium under red light condition

Plant morphogenesis is strongly affected by light quality. We evaluated the effects of localized blue light irradiation from light-emitting diodes (LED) on the leaf shape of geranium plants, which show epinasty under red light conditions. Geranium (Pelargonium zonale Ait.) plants with five or six true leaves were transplanted into a growth chamber containing a metal halide lamp (MH) or a red LED panel (R-LED) that irradiated light at an intensity of 100 mu mol m(-2) s(-1). The fifth true leaves were then subjected to localized irradiation from small LED light sources. Under localized irradiation of blue light at a 50 mu mol m(-2) s(-1) intensity level, the leaf epinasty index, was 20% lower than under localized red irradiation. The extent of the inhibition of leaf epinasty by blue LED irradiation depended on the light intensity. Blue light irradiation to the abaxial sides of leaves did not inhibit leaf epinasty under red irradiation. In response to blue light irradiation to the adaxial side of the fifth true leaf, the leaf angle reached 225 degrees by the end of the experiment, which was larger than the angle reached under red irradiation. In fifth true leaves treated with localized blue irradiation for 6 h during the daytime, the leaf epinasty level was higher than with blue light irradiation for 12 h. The length of epidermal cells of the abaxial sides irradiated with blue light was 7-13% longer than under red irradiation in the absence of blue irradiation. These results suggest that in geranium, leaf epinasty is controlled by cell elongation in the abaxial epidermis triggered by blue light irradiation of the adaxial side of the leaf. (C) 2007 Elsevier B.V. All rights reserved.