Growth and Acclimation of Impatiens, Salvia, Petunia, and Tomato Seedlings to Blue and Red Light

Plant growth is plastic and adaptive to the light environment; characteristics such as extension growth, architecture, and leaf morphology change, depending on the light spectrum. Although blue (B; 400–500 nm) and red (R; 600–700 nm) light are generally considered the most efficient wavelengths for eliciting photosynthesis, both are often required for relatively normal growth. Our objective was to quantify how the B:R influenced plant seedling growth and morphology and understand how plants acclimated to these light environments. We grew seedlings of three ornamental annuals and tomato under six sole-source light-emitting diode (LED) lighting treatments or one cool-white fluorescent treatment that each delivered a photosynthetic photon flux ( PPF ) of 160 µmol·m −2 ·s –1 for 18 h·d −1 . The following treatments were provided with B (peak = 446 nm) and R (peaks = 634 and 664 nm) LEDs: B 160 (160 µmol·m −2 ·s −1 of B light only), B 80 +R 80 , B 40 +R 120 , B 20 +R 140 , B 10 +R 150 , and R 160 . Seedlings of impatiens ( Impatiens walleriana ), salvia ( Salvia splendens ), petunia ( Petunia ×hybrida ), and tomato ( Solanum lycopersicum ) were grown for 31 to 37 days at a constant 20 °C. Plants with as little as 10 µmol·m −2 ·s −1 of B light were 23% to 50% shorter and had 17% to 50% smaller leaves than plants under only R light. Impatiens and salvia had 53% to 98% greater fresh shoot weight under treatments without B light than with ≥80 µmol·m −2 ·s −1 . Plants grown under fluorescent lamps had the greatest chlorophyll content but also had among the thinnest leaves of treatments. Blue-rich light increased flowering in impatiens and reduced incidence of intumescences on tomato. We conclude that, in sole-source lighting of propagules, B light inhibits leaf and stem expansion, which subsequently limits photon capture and constrains biomass accumulation. As little as 10 µmol·m −2 ·s −1 of B light in an R-dominant background can elicit desirable growth responses for the production of young plants and for other situations in which compact growth is desired.