Duration, not timing during the photoperiod, of far-red application determines the yield increase in tomato

• Fruit yield increased linearly with the duration of far-red application. • Timing of far-red application during the photoperiod did not affect fruit yield. • Despite fruit yield increase, fruit quality was not reduced by far-red application. • Far-red increased fruit yield as efficiently as photosynthetic active radiation. Supplementary far-red light may increase fruit yield in greenhouse tomato cultivation. Apart from end-of-day applications, research has focussed on the effects of far-red light during the entire photoperiod, while few studies focused on the timing of far-red application. In this study, we assessed the impact of timing and duration of far-red application on fruit yield and quality in a high-wire tomato crop. Two commercial tomato cultivars were grown under 226 μmol m −2 s −1 of red and white supplementary LED lighting. Far-red light (63 μmol m −2 s −1 ) was additionally supplied during either the first or the second half of the photoperiod. These treatments were compared to a negative control, where no far-red light was supplied, and a positive control, where far-red light was supplied during the whole photoperiod. A yield component analysis was conducted to identify the parameters responsible for changes in fruit yield. We determined that fruit yield, total fruit dry weight and fruit production per unit of supplementary PAR light (400–700 nm) increased linearly with the duration of far-red application. Fruit yield was increased to a similar extent whether far-red was supplemented in the first or in the second half of the photoperiod. While different light treatments included 0, 14, or 28 % far-red in their cumulative supplementary photon flux density, there were no significant differences in their fruit production per unit of supplementary radiation (400–800 nm). Supplementary far-red increased fruit dry weight per unit of cultivation area by 23 % to 25 % as a result of increasing both plant dry weight (10 % to 12 %) and dry weight partitioning to the fruits (10 % to 14 %). The increase in fruit yield was less than the corresponding increase in total fruit dry weight due to a simultaneous increase in fruit dry matter content. Total soluble solids and pH of the fruits were hardly affected by far-red application. We concluded that supplementary far-red increased tomato fruit yield without compromising fruit quality, with this increase being determined by the duration of far-red application, not by its timing during the photoperiod.