Powdery Mildew Resistance In Cucumber Seedlings Grown Under Different Light Sources

Recently, transplant production systems that use artificial light have been developed as a way to produce high quality transplants. In this study, we investigated the effects of light quality on growth and powdery mildew resistance in cucumber (Cucumis sativus) seedlings. Seedlings were grown under fluorescent lamps with high red-to-far-red (R:FR) light (R:FR = 7.0; FH) that have been commonly used in the transplant production systems or with low R:FR light (R:FR = 1.1; FL). The spectrum of FL was almost the same as that of FH, except in FR. The growth conditions were maintained at an air temperature of 28 ºC, a relative humidity of 50%, and a photosynthetic photon flux of 300 µmol m^-2 s^-1 at the canopy surface with a light:dark photoperiod of 12:12 h. When the cotyledons were fully expanded (5 or 7 d after seeding in FH or FL seedlings, respectively), powdery mildew fungus (Sphaerotheca cucurbitae; PM) were inoculated onto the adaxial leaf surface of seedlings by spraying PM spore suspension. The inoculated seedlings were then grown for 10 d under the same condition. Samples of non-inoculated seedlings in each group were taken to measure the morphological characteristics. The PM colonies were counted 8, 9 and 10 d after the inoculation. The PM colonies started to appear 8 d after the inoculation in both FH and FL seedlings. The number of PM colonies on FH seedlings was 0.6 times that on FL seedlings. Leaf dry mass per area (LMA) and percent dry matter (PDM) of FH seedlings were both 1.2 times those of FL seedlings. This indicates that the cotyledons of FH seedlings were thicker and tougher. From these results, The PM resistance in FH seedlings was probably improved by changes in leaf morphological characteristics such as thickness and toughness resulting from the higher R:FR illumination. On the other hand, the leaf expansion and the shoot elongation of FH seedlings were inhibited than that of FL seedlings. Therefore, there is a tradeoff between faster growth and improved resistance to PM. We need to consider which of the two advantages is more important when growing transplants under artificial light.