Steering basil development through irrigation scheduling and additional LED lighting

Consumers demand that the sweet basil (Ocimum basilicum L.) plants they purchase in plant containers are healthy, undamaged, compact, and green with a long shelf life. Growers have to meet these quality standards throughout the year. However, the variation in environmental factors that plants experience in open or greenhouse production makes it challenging to reach this goal. This drives the sector towards even more controlled cultivation systems. To pave the way towards year-round production of high-quality basil plants this research project investigated the possibilities to steer basil development through irrigation scheduling and LED lighting, both in greenhouses and growth chambers.

The use of commercial soil moisture sensors to effectively steer irrigation was evaluated. Based on the soil moisture measurements and the biometric plant measurements, both quantitatively and qualitatively, irrigation could be controlled leading to basil morphological and anatomical changes. Basil plants grown under drier soil conditions had smaller xylem vessels at higher density and a more developed stem collenchyma providing mechanical support. Furthermore, the density and size of the leaf stomata were found to acclimate to the soil moisture conditions. Plants grown in more dry conditions were found to have higher stomatal density and shorter stomatal length. By managing the irrigation schedule, plants can be triggered to invest in their transpiration system, which results in stems and leaves that are more robust against environmental variability and changes. These anatomical changes help the plant to optimize its water use efficiency.

Thanks to its high energy efficiency, LED technology has become popular in indoor plant cultivation. As LEDs emit over a narrow spectral band, this creates opportunities to combine different LEDs to optimize the light spectra to steer plant growth and development while minimizing energy consumption. To this end, the effect of adding blue, green, red, and far-red light to a broad white light spectrum on basil growth and resource use was evaluated in this study. Basil grown under additional red and far-red light yielded plants with increased biomass production and resource-use efficiency. Supplementing with blue light decreased biomass production and energy-use efficiency, while supplemental green light only caused stem elongation. Supplementing the natural sunlight spectrum with specific wavelengths allows growers to establish desired plant morphology and yield during greenhouse cultivation, but can also serve as a basis to build on for growth chamber cultivation.

The effect of the light spectrum on the light response curve was also studied to find the optimal light intensity in a certain setting. It was found that the light saturation point of basil plants could be increased by adding far-red light to a broad white light spectrum. This allows to produce of the same amount of biomass in a shorter cultivation period by adding far-red light to the spectrum. Furthermore, additional far-red light increased sugar and starch levels in the source leaves significantly. This effect was more pronounced when the far-red light was supplemented during the total photoperiod. Increased sugar and starch concentrations provide more building blocks for plant growth and carbon transport. Hence, a higher sugar and starch content in the source leaves is a great advantage for the basil plant.