Six light/dark cycles within 24 h reduce tomato plant growth primarily resulting from a short uninterrupted dark period

Light is the driving force for photosynthesis and plant growth. In nature, one light/dark cycle occurs within 24 h, while in vertical farms multiple light/dark cycles within 24 h can be implemented e.g. to reduce energy costs based on dynamic electricity prices. The consequences of more light/dark cycles within 24 h for plant growth have long been studied. In general, more cycles affect plant growth negatively, but not always. Furthermore, it is unclear whether negative effects are caused by the frequency of light/dark cycles or rather the short duration of the uninterrupted dark period. Young tomato plants (Solanum lycopersicum L. cv. Moneymaker) were grown in a climate room with mixed red and blue light (photon ratio, red: blue = 6) under one, two, three, and six light/dark cycles in 24 h (18/6 h, 9/3 h, 6/2 h, and 3/1 h, respectively) as well as under reduced dark periods when there was one light/dark cycle in 24 h (21/3 h and 23/1 h). In all treatments, the same daily light integral (13 mol m⁻² d⁻¹) was maintained. After 25 days of treatment, multiple light/dark cycles in 24 h resulted in larger specific leaf area and lower chlorophyll and carbohydrate contents, reduced leaf conductance, net photosynthetic rate, and photosynthetic capacity compared to one cycle. Relative growth rate linearly decreased with the increasing number of light/dark cycles, mainly caused by a reduced net assimilation rate rather than changed morphology. The short uninterrupted dark period was the predominant factor behind the negative effect of six light/dark cycles.