Effects of adding far-red light to the photosynthetic active spectrum on growth and morphological characteristics of basil (Ocimum basilicum L.) under controlled conditions

Light is an essential factor for growth and development in agricultural production. Vertical agriculture has been increasingly developed to produce leafy vegetables and herbs. High light consumption is the most important obstacle in the development of vertical farming system. Basil (Ocimum basilicum L.) is one of the most popular leafy vegetables and is suitable for growing in plant factories. Therefore, the aim of the present study was to reveal the yield and dry matter production response of basil to add far-red light to the photosynthetic active spectrum, and finally to investigate the interaction of far-red light and daily light integral (DLI) on growth, morphology and light use efficiency in basil. The experiment was performed in a split plot based on a randomized complete block design. Treatments included daily light integral (DLI) at two levels of 6.4 and 10.8 (mol m-2 d-1), and far-red light at three levels of 0, 5 and 20% based on the light recipe of the LEDs DLI. The measured traits included morphological traits (fresh and dry weight of leaves, stems and plants, plant height and leaf area), growth (specific leaf area (SLA), and partitioning to leaves), and determining the efficiency of light and radiation use efficiency. The results showed that the plant fresh and dry weight increased by far-red light at the level of 20% in high DLI conditions (10.8), which was due to the increase in stem fresh and dry weight. As a result, the partitioning to leaves decreased slightly, although it was not statistically significant. Similar to high DLI conditions, the plant fresh and dry weight increased with the addition of far-red light (5 and 20%) at low DLI conditions (6.4), but this increase was due to the increase in leaves fresh and dry weight, and eventually led to the more partitioning to leaves. SLA in plants grown in low DLI decreased significantly by decreasing the phytochrome stationary state (PSS) as a result of adding far-red light from 0.88 to 0.82, which was due to the increase in leaves dry matter. In this regard, in plants grown at high DLI, with decreasing the PSS as a result of adding far-red light from 0.88 to 0.85, the SLA did not change. In general, the response of shade avoidance syndrome (SAS) seems to be more associated with increased stem height when plants grown under high DLI, and is more associated with increased leaf area and stem height when plants grown under low DLI and high levels of far-red light.