Growth of Lettuce, Spinach, Mustard, and Wheat Plants under Four Combinations of High-pressure Sodium, Metal Halide, and Tungsten Halogen Lamps at Equal PPFD

Abstract

Abstract Plants of lettuce (Lactuca sativa L. cv. Grand Rapids), spinach (Spinacia oleracea cv. Bouquet), white mustard (Sinapis alba L.), and wheat (Triticum aestivum L. cv. Karamu) were grown at 2 photosynthetic photon flux densities (PPFD 400 to 700 nm at 320 and 700 μmol s−1m−2 under 4 lamp treatments: metal halide lamps alone, high-pressure sodium lamps alone, metal halide plus tungsten halogen lamps (ca. 1:1 installed wattage), and metal halide plus high-pressure sodium lamps (ca. 1:1 installed wattage). Plants of all species grew well under all treatments and no growth abnormalities were apparent at harvest. It is concluded that dry-weight increase was determined by PPFD and not by spectral irradiance. However, lettuce, spinach, and mustard hypocotyl elongation was greater in young plants grown under the high-pressure sodium lamps in comparison with those grown under the metal halide or metal halide plus tungsten halogen treatments. A strong negative relationship between hypocotyl length and blue photon flux density (400–495 nm) was demonstrated. Anthesis of wheat occurred at the same time under all lamp treatments, but anthesis of mustard differed by 2 days at the higher PPFD and 4 days at the lower PPFD among lamp treatments. The time of anthesis for mustard was found to be weakly but positively correlated with the calculated phytochrome photoequilibrium. Chlorophyll concentrations in young lettuce and spinach plants growing under the high-pressure sodium lamps were 55% and 26% lower, respectively, than those in plants growing under metal halide lamps at the high PPFD level. However, final dry weight was unaffected by any of these morphological differences in the early growth stages.