The Ratio of Red to Far-Red Light Affects Growth, Pigment Content, and Photosynthetic Rates in Cress Plants

Far-red light is electromagnetic radiation with wavelengths of 700–800 nm. Light of such wavelengths does not fall into the range of photosynthetically active radiation (PAR) but plays an informational role in plants and indirectly influences the photosynthetic rate. In this study, garden cress (Lepidium sativum L.) was grown under illumination with different red to far-red light ratios (RL/FRL) in the overall lighting spectrum. The treatments with RL/FRL ratios of 1.1, 0.8, and 0.5 were applied. Effects of RL/FRL ratio on plant height, the content of chlorophylls and carotenoids, and photosynthesis were examined. The decrease in RL/FRL ratio in grow light led to elongation of cress plants and reduced the content of photosynthetic pigments. Lighting with ratios RL/FRL = 0.8 and RL/FRL = 0.5 did not reduce the photosynthetic capacity (on the 14th day of the experiment), which can be regarded as a positive effect. Lighting with a ratio RL/FRL = 1.1 was accompanied by the reduction of photosynthesis, which was probably due to a significant decrease in the content of chlorophylls and carotenoids.