Several experiments were performed with impatiens, marigold, petunia, salvia, and tomato seedlings to quantify how different ratios of blue (B, peak=446 nm), green (G, peak=516 nm), orange (O, peak=596 nm), red (R, peak=634 nm), and hyper red (HR, peak=664 nm) from light-emitting diodes (LEDs) regulated plant growth while maintaining similar cultural and environmental conditions. Seedlings grown under O, R, and/or HR LEDs with background B and G light developed similar plant growth attributes including leaf size, stem length, and biomass accumulation. Therefore, selection of LEDs for horticultural lighting could be based on other factors such as economics. In another experiment, plants grown under at least 25% B light were 41 to 51% shorter and had 35 to 57% less fresh shoot weight than those grown under only R light at the same total photosynthetic photon flux. In a third experiment, plants grown under as little as 6.25% B light were 23 to 50% shorter, had 37 to 50% less fresh weight, up to 43% thinner leaves, and up to 49% less leaf area than plants grown under only R light. Seedlings under 50% G+50% R light were shorter than plants under only R light but taller than plants under only B light, suggesting that G light stimulated blue-light receptors (e.g., cryptochrome), but to a lesser extent than treatments with B light. Therefore, we postulate that a minimal quantity of B light (and to a lesser extent, G light) stimulates one or more B-light receptors that suppresses leaf and stem extension growth, which subsequently limits photon capture and constrains biomass accumulation of seedlings.
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