Abstract
Soybeans (Glycine max [L.] Merr. cv. Williams) were grown to a standard developmental stage (6th trifoliate leaf c. 50% fully expanded) in a sunlit, temperature-controlled greenhouse (27±3°C) or in growth chambers (27±1°C) under microwave-powered (MP) E lamps (Fusion Systems, Rockville, MD) or a 50-50 mixture of high pressure sodium and metal halide (HID) lamps. Daily PAR in growth chambers was 44 mol m-2, provided either as a square-wave (HID; 875 μmol m-2s-1) or in steps (MP; peak irradiance c. 1650 μmol m-2s-1). Growth chamber experiments were conducted at 400 μl l-1 or 700 μl l-1 CO2“ambient” or “elevated”, respectively). Total dry matter was similar for all treatments at ambient CO2 but MP-grown plants were more like greenhouse plants with respect to Leaf Area Ratio, Specific Leaf Weight, and length of stem and petiole. Axillary growth, however, was much less under greenhouse conditions. Elevated CO2 resulted in a significant stimulation of plant growth under both HID and MP, but gains were greater under MP illumination. Enhanced growth of MP plants was marked by increased partitioning into roots. It is possible that morphological modifications in MP plants rendered them more efficient at conversion of PAR into dry matter.
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