Fluorescent Illumination with High Red-to-far-red Ratio Improves Resistance of Cucumber Seedlings to Powdery Mildew

Abstract

We investigated the effects of fluorescent illumination with a high red-to-far-red ratio (R:FR) on the resistance of cucumber (Cucumis sativus) seedlings to powdery mildew fungus (Sphaerotheca cucurbitae; PM). Seedlings were grown at a photosynthetic photon flux (PPF) of 300 μmol m-2 s-1 providedby fluorescent lamps withhigh R:FR light(R:FR = 7.0; FLH) or low R:FR light (R:FR = 1.1; FLL) until cotyledons or the first foliage leaf were fully expanded. Spores of PM were then inoculated onto the leaves, and the seedlings were grown for 7 days (from cotyledon stage) or 9 days (from foliage-leaf stage) under FLH. The number of PM colonies on FLH seedlings was 0.80× (cotyledons) and 0.62× (foliage leaves) the number on FLL seedlings. The reduction on the FLH seedlings was probably the result of changes in leaf morphological characteristics such as a thicker epidermal tissue as a result of the higher R:FR illumination. The number of PM colonies on cotyledons of the FLH seedlings was also smaller than that on seedlings grown under metal-halide lamps providing a spectrum similar to that of natural light (R:FR = 1.2).