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Metal Halide Lamps for Supplemental Lighting in Greenhouses: Crop Response and Spectral Distribution¹

Light intensity levels and plant growth under metal halide (MH) and fluorescent-incandescent (FI) lamps were compared.

When crop plants were exposed to the higher light energy (380 to 750 nm, measured) of the MH lighting system, dry weight over a 6-week period averaged 38% more than for plants exposed to the FI lighting system. Reduced hypocotyl and cotyledon elongation was the initial effect of the MH lamps. Plants under the MH lamps remained darker green, had larger leaves and thicker stems than plants under FI lamps. For one crop, soybeans, total dry matter increased, and the height of the plants was reduced by 33% under MH lamps relative to FI lamps. All plants under the MH lighting systems were sturdier and closely resembled field-grown plants.

Key Words: Soybeans • Alfalfa • Timothy • Birdsfoot trefoil • Corn • Crop Growth • Fluorescent-incandescent lights

² Associate Professor, Research Agronomist, Experimentalist and Research Agronomist, respectively, Department of Agronomy, Cornell University and ARS-USDA, Ithaca, New York.

Received for publication April 6, 1974.