Production of organic potted herbs with LED supplementary lighting: effect on plant biomass and phenols

M. Dorais, A. Brégard, C. Ménard, B. Dansereau, N. Zyromski, S. Pepin
The LED technology for greenhouse horticulture offers the possibility of using a blend of monochromatic light wavelengths that can stimulate or inhibit specific morphological and physiological responses and thus act as growth regulators. This is particularly interesting for organic herbs and ornamentals as few products are available to control plant growth. Our study hypothesis was that LED supplemental lighting (SL) is a good tool for organic greenhouse ornamental growers to increase plant biomass and product quality such as leaf phenol concentration. By using randomized complete block designs with three replicates, two light treatments (with and without LED SL) were compared with 21 herb species and cultivars grown in seven sets of three species (called trios 1 to 7; Trio 1: Salvia officinalis - Origanum vulgare - Rosmarinus officinalis; Trio 2: Persicaria hydropiper - Persicaria odorata - Mentha piperita; Trio 3: Majorana syriaca - Thymus vulgaris - Rungia klossii; Trio 4: Tagetes lucida - Artemisia arbotanum - Thymus citriodorus; Trio 5: Mentha spicata-Ocimum herbalea - Majorana syriaca; Trio 6: Mentha piperita f. citrate - Mentha suaveolens - Mentha 'Hilary's Sweet Lemon'; Trio 7: Artemisia arborescens - Lippia dulcis - Micromeria fruticosa). Trials were carried out in 2017 at Laval University (Quebec City, QC, Canada) during 9 weeks (three successive harvests). The LED SL PPFD at the plant level was 193 µmol m‑2 s‑1 for a 12-h photoperiod (6 to 18h) (15% blue and 85% red). Our results showed that growth indices were not affected by the number of harvest nor the light treatment. However, the visual quality declined for most trios at the third harvest (9 weeks) and tended to be lower under SL. On the other hand, the total dry biomass of all trios produced over the whole experiment increased by 34% under LED SL compared with ambient light, while their leaf phenol concentration increased on average by 10.5%. In conclusion, LED SL may be a good tool for organic producers in order to increase the plant biomass and the phenol concentration of a large number of herb species, although inconclusive response was observed for plant visual quality.
Dorais, M., Brégard, A., Ménard, C., Dansereau, B., Zyromski, N. and Pepin, S. (2020). Production of organic potted herbs with LED supplementary lighting: effect on plant biomass and phenols. Acta Hortic. 1286, 47-54
DOI: 10.17660/ActaHortic.2020.1286.8
organic farming, light-emitted diode, aromatic herbs, health compounds

Acta Horticulturae