The influence of LED light photoperiod on growth and mineral composition of Brassica microgreens indoors

Photoperiod controls many physiological processes in plants, and the manipulation of day length can achieve the desired growth and development characteristics. The photoperiodic response depends on light signal perceived by plants' photoreceptors and can vary under controlled environment conditions where the artificial light sources are used. Many studies were focused on finding the specific light spectra and intensity to ensure healthy growth or manage metabolism, and less is known about the photoperiodic stimulus for vital processes in plants. This study was conducted to evaluate how photoperiod of light-emitting diodes (LEDs) illumination affects the growth and mineral composition of Brassica microgreens grown indoors. Mustard (Brassica juncea L., 'Red Lion'), red pak choi (Brassica rapa var. chinensis, 'Rubi F1') and tatsoi (Brassica rapa var. rosularis) microgreens were grown in walk-in growth chambers (21/17 +/- 2 degrees C day/night temperatures; 55 +/- 5% relative air humidity) for ten days. The day lengths of 8, 12, 16, 20 and 24 h were achieved from five solid-state illumination modules consisting of blue 447-nm, red 638-nm, 665-nm and far-red 731-nm LEDs. The total photon flux density (TPFD) was 300 mu mol m(-2) s(-1). The contents of mineral elements were determined by spectrometric ICP-OES method. The shortest 8-h photoperiod led to hypocotyl elongation and increased the leaf area and fresh weight of all investigated microgreens. The lengthening of plants was decreased due to day extension from 12- to 20-h, and 24-h photoperiod suppressed the growth process most. The better uptake of potassium, magnesium, phosphorus and zinc in mustard grown under 8-h photoperiod was observed. In tatsoi, the higher contents of all mineral elements (except iron) under 20-h photoperiod were determined. The 12- and 16-h photoperiods were the most proper for a higher uptake of calcium, magnesium and sulphur in red pak choi. The common tendency for fresh microgreens when the 24-h photoperiod increased the contents of investigated mineral elements were observed.