Supplemental LED Lighting As a Potential Alternative to Plant Growth Regulators

Supplemental lighting is commonly used for greenhouse transplant production. High pressure sodium fixtures (HPS) are the common lighting technology to supplement solar light. However, light emitting diodes (LED) are an alternative to HPS. Current advertised LED fixtures for supplemental greenhouse lighting are more efficient (μmol J^-1) than HPS fixtures. LED fixtures can be built with customized spectrum providing the opportunity to manipulate plant morphology using light signals. Plant compactness is one of the most important morphological characteristics for transplant production. The use of chemical growth regulators (PGR) to decrease plant height is under high regulation as a result of their potential health risks. The use of supplemental lighting to increase plant compactness is a sound alternative to chemical PGRs. The objective of the present experiment was to compare plant growth and morphology under different supplemental lighting treatments and under PGRs in order to potentially reduce the use of PGRs in horticultural operations. Dianthus, geranium, pansy, petunia and salvia were grown in a greenhouse from seed until the plug stage under 9.2 ± 2.4 mol m^-2 d^-1 average solar daily light integral, 22.5 ± 2.4 °C average temperature, and 46 ± 16 % relative humidity. The six growing treatments consisted of: 1) 19%blue:81%red photon flux LED supplemental lighting , 2) 6%blue:5%green:89%red photon flux LED supplemental lighting, 3) HPS supplemental lighting, 4) no-supplemental lighting, 5) 6%blue:5%green:89%red photon flux LED + PGR and 5) no-supplemental lighting + PGR. All the supplemental lighting treatments delivered 100 μmol m^-2 s^-1 PF for 18h for a supplemental daily light integral of 6.5 mol m^-2 d^-1. Plant physiological responses such as dry mass, fresh mass, leaf number, leaf area, plant height, stem diameter, plant compactness, anthocyanin concentration, and chlorophyll concentration were measured for all the crops. The results comparing the different supplemental light treatments with the PGR treatments will be presented.