Additional far-red increases fruit yield of greenhouse sweet pepper mainly through enhancing plant source strength

• Additional FR increased fruit yield of greenhouse sweet pepper. • Yield increase was mainly associated with increased plant source strength. • Additional FR led to higher fruit set and stronger fruit set fluctuation. • Sweet pepper fruits grown under additional FR had less cracking. Supplementary lighting is necessary for year-round greenhouse production of fruit vegetables in high-latitude regions. Far-red (FR) radiation can influence plant photomorphogenesis as well as photosynthesis. We aimed to identify the effects of supplementary FR on fruit set and yield of sweet pepper, and its underlying mechanisms via a yield component analysis. A 24-week greenhouse experiment was conducted with cultivars 'Gialte' and 'Margrethe', where FR was added to 190 μmol· m −2 · s −1 of white supplementary light in four treatments: 0, 50 or 100 μmol· m −2 · s −1 FR throughout the whole generative growth phase (since 8 weeks after transplanting), or 100 μmol· m −2 · s −1 FR for only four weeks (12 to 16 weeks after transplanting). Fruit yield increased linearly with the cumulative amount of FR provided in supplementary light. The increased fruit dry weight with additional FR was mainly associated with an increased plant dry weight, accompanied by a marginal increase in the fraction of dry matter partitioned to fruits. The increase in plant dry weight resulted from an increased light use efficiency (plant dry weight per unit of supplementary photosynthetic active radiation (PAR) incident on top of the canopy) and an increased incident PAR (due to taller plants closer to the lamps). However, additional FR reduced radiation use efficiency (plant dry weight per unit of supplementary PAR plus FR incident on top of the canopy), indicating that additional FR was used less efficiently than PAR for biomass production. Additional FR enhanced fruit set percentage and fruit set fluctuations over time, where both long-term and short-term FR application elevated the subsequent fruit set peak after the start of FR application. Without additional FR, 17 % fruits in 'Gialte' and 25 % in 'Margrethe' showed medium or severe cracking. Additional FR substantially reduced this percentage to 8 % in both cultivars. We conclude that additional FR improves sweet pepper fruit set and yield in greenhouses, mainly by enhancing plant source strength.