208  2001 American Society for Photobiology 0031-8655/01 $5.00+0.00 Photochemistry and Photobiology, 2001, 73(2): 208–212 Evidence for Yellow Light Suppression of Lettuce Growth ¶ Tracy A. O. Dougher* and Bruce Bugbee Crop Physiology Laboratory, Department of Plants, Soils and Biometeorology, Utah State University, Logan, UT Received 24 July 2000; accepted 30 October 2000 ABSTRACT Researchers studying plant growth under different lamp types often attribute differences in growth to a blue light response. Lettuce plants were grown in six blue light treatments comprising five blue light fractions (0, 2, 6% from high-pressure sodium [HPS] lamps and 6, 12, 26% from metal halide [MH] lamps). Lettuce chlorophyll con- centration, dry mass, leaf area and specific leaf area un- der the HPS and MH 6% blue were significantly differ- ent, suggesting wavelengths other than blue and red af- fected plant growth. Results were reproducible in two replicate studies at each of two photosynthetic photon fluxes, 200 and 500 mol m -2 s -1 . We graphed the data against absolute blue light, phytochrome photoequili- brium, phototropic blue, UV, red:far red, blue:red, blue: far red and ‘yellow’ light fraction. Only the ‘yellow’ wavelength range (580–600 nm) explained the differences between the two lamp types. INTRODUCTION The use of different lamps in controlled environments has led to the discovery of gross changes in plant morphology caused by altered spectral quality. The photomorphogenic changes caused by altered phytochrome photoequilibrium (PPE)² are well documented (1). However, lamp types with similar PPE can significantly alter leaf and stem morpholo- gy. These studies suggest that morphological differences are caused by differences in the blue portion of the spectrum (2–7). Unfortunately, nonblue wavelengths also varied with lamp types in these studies, so definitive conclusions have to be questioned. In a companion paper (8), we compared lettuce growth under high-pressure sodium (HPS) lamps with metal halide (MH) lamps that were filtered to 6% blue. Chlorophyll concentration, dry mass, leaf area and specific leaf area (SLA) were sensitive to the remaining spectral out- put. Here we examine other known spectral responses to ¶Posted on the website on 30 November 2000. *To whom correspondence should be addressed at: Crop Physiology Laboratory, Department of Plants, Soils and Biometeorology, Utah State University, 4820 Old Main Hill, Logan, UT 84322- 4820, USA. Fax: 435-797-2605; e-mail: tracyaod@cc.usu.edu ² Abbreviations: B:FR, blue to far-red ratio; B:R, blue to red ratio; HPS, high pressure sodium; MH, metal halide; PPE, phytochrome photoequilibrium; R:FR, red to far-red ratio; SLA, specific leaf area. explain the morphological and growth differences we ob- served with this plant species. MATERIALS AND METHODS Lettuce (Lactuca sativa, cv. Grand Rapids) was grown in six blue light treatments (0, 2, 6% from HPS lamps and 6, 12, 26% from MH lamps) comprising five blue light fractions (Table 1, cols. 1–3) at a photosynthetic photon flux (PPF) of 200 and 500 mol m -2 s -1 and a 16 h photoperiod. There were two replicate studies at each PPF level. A comparison of HPS (Sylvania Lumalux) and MH (Syl- vania Metalarc) at 6% blue was used to determine if any parameter differences were caused by other wavelengths. Plants were grown hydroponically at 26/22°C day/night temperature, 70% relative hu- midity and elevated carbon dioxide (1000 mol mol -1 ). Other grow- ing procedures, environmental conditions and measurement tech- niques are described in the companion paper (8). RESULTS AND DISCUSSION As reported in the companion paper (8), the two 6% blue treatments produced significantly different chlorophyll con- centrations, dry masses, leaf areas and SLA in lettuce. This phenomenon was visually apparent in each replicate trial of this experiment and in preliminary trials before this experi- ment (9). It is unlikely that these differences were caused by other environmental differences between compartments because (1) treatments were randomized each time; and (2) atmospheric differences (carbon dioxide, humidity and tem- perature) between compartments were minimized by the use of a common air conditioning system. Apparently in lettuce, some wavelength(s) acts in conjunction with blue to affect plant growth. The data for chlorophyll concentration and to- tal dry mass (Figs. 1a and 2a) are representative of the re- sults for the other parameters. Here we analyze the effects of the other wavelengths we have considered. Thermal radiation Although the thermal radiation (700–10 000 nm) emitted by unfiltered HPS and MH lamps is considerable and different for the two lamp types, the thermal radiation in these ex- periments was equalized by recirculating, chilled water bar- riers. Amount of blue photons As discussed in the companion paper (8), some plant re- sponses are determined by the quantity of blue photons rath- er than the fraction of blue photons. However, both of the 6% blue treatments had the same amount of blue photons at each PPF: 12 mol m -2 s -1 blue at 200 mol m -2 s -1 or 30