Height-convergence pattern in dense plant stands is affected by red-to-far-red ratio of background illumination

This study investigated the effects of illumination with different red-to-far-red ratios (R:FR) on height convergence pattern in cucumber (Cucumis sativus L.) seedling stands to obtain a basic knowledge for growing plant stands uniformly under artificial light sources. Seedlings of the same age but with different initial heights (30 or 15 mm) were grown under either metal-halide lamps with spectra similar to that of natural light (R:FR = 1.2) or fluorescent lamps with a high R:FR (R:FR = 11), at a photosynthetic photon-flux (PPF) of 300 μmol m−2 s−1 and a light:dark period of 16:8 h, until the first true leaf expanded. The shoot heights of the initially taller and shorter seedlings under the metal-halide lamps converged within 4 days. In contrast, the initially shorter seedlings under fluorescent illumination remained smaller than the initially taller seedlings throughout the experimental period (8 days), although the height difference decreased gradually. The rapid height-convergence under metal-halide illumination probably resulted from shade avoidance by the shorter seedlings due to the relatively low R:FR (=0.6) and PPF (=60 μmol m−2 s−1) at cotyledon level under shade from the taller seedlings. The R:FR at cotyledon level for the initially shorter seedlings under fluorescent illumination was 5.5, which was higher than the value (0.6) that caused the accelerated height growth response under metal-halide light. The fact that light quality affects the structure of plant community should be considered in selecting light sources for the plant production.