Dose-response of tomato fruit yield to far-red fraction in supplementary lighting

Abstract

Supplementary LED lighting in greenhouse horticulture is typically rich in red light (R; 600-700 nm), while it lacks far-red light (FR; 700-800 nm), resulting in growing conditions with lower-than-solar far-red fractions [<0.46; FR/(R + FR)]. In these light environments, the addition of FR can improve tomato harvest index and fruit yield (ripe fruit fresh weight). While fruit yield increases linearly with the dose of FR at low FR fractions (0.1-0.28), it is unknown whether this relationship holds at higher FR levels, up to and above solar FR fractions. In this study, the relationship between tomato fruit yield and the FR fraction in supplementary lighting was quantified. Two cluster tomato cultivars 'Foundation' and 'Trevine' were grown in two greenhouse compartments for 20 weeks during the winter season (September to February). Different fractions of supplementary FR (0.22 to 0.49) were applied while maintaining a constant supplementary photosynthetic photon flux density of 250 µmol m^-2 s^-1 and 16-hour photoperiod. A yield component analysis was used to identify the key physiological drivers of the FR effect on yield. Additionally, fruit quality at harvest (total soluble solids, soluble sugars, and pH) and shelf-life were assessed. Additional FR increased fruit yield up to an FR fraction of 0.40, where the highest effect was recorded (+16% fruit yield for both cultivars). Fruit yield increases under additional FR were mostly associated with increased plant dry weight, with a small yet significant increase in the fraction of dry matter partitioned to the fruits. The radiation use efficiency (g fruit fresh weight mol^-1) and electricity use efficiency of supplementary lighting (g fruit fresh weight kWh^-1) decreased at higher FR fractions (0.44 and 0.49). Finally, additional FR had a minimal effect on fruit quality and shelf-life. We conclude that adding FR to supplementary lighting can increase tomato fruit yield linearly up to an FR fraction of 0.40, while at higher FR fractions, further increases in FR have limited or even negative effects on yield and decrease radiation and electricity use efficiency.

Keywords: electricity use efficiency; far-red light; fruit quality; photosynthesis; radiation use efficiency; tomato; vertical light distribution; yield component analysis.

Figure 1

Effects of FR fraction in supplementary light on fruit yield (ripe fruit fresh weight; A, B), ripe fruit dry weight (C), radiation use efficiency (D), and electricity use efficiency (E) for cv. Foundation and cv. Trevine. Trendlines are depicted to show a significant linear or quadratic relationship with the FR fraction (p < 0.1, averaged over both cultivars). For significant quadratic relationships, letters denote significant differences between treatments, as determined by Fisher’s protected LSD test. The radiation and electricity use efficiency were calculated as fruit yield per unit of PFD and kWh, respectively. Grey markers represent the simulated radiation and electricity use efficiency expected if the additional FR in each light treatment was replaced by PAR, based on Marcelis et al. (2006), for cv. Foundation (round marker) and cv. Trevine (diamond marker). The lowest FR treatment (FR_0.22) was used as baseline for the PAR simulation. Asterisks indicate a significant difference between radiation and electricity use efficiency measured and simulated for a specific FR treatment as determined by Fisher’s protected LSD test (*p < 0.1; **p < 0.05). Each data point represents the average of two experimental units ± SEM, where the value per experimental unit is the average of six plants. The data refer to cumulative values over a period of 10 weeks, from the first to last fruit harvest, 63 to 139 DAT. FR, far-red light; PFD, photon flux density; DAT, days after transplant.

Figure 2

Effects of FR fraction in supplementary light on fraction of dry matter partitioned to the fruits (A), potential fruit dry weight (B), flowering rate (C), and fruit ripening rate (D) for cv. Foundation and cv. Trevine. Trendlines are depicted to show a significant linear or quadratic relationship with the FR fraction (p < 0.1, averaged over both cultivars). For significant quadratic relationships, letters denote significant differences between treatments, as determined by Fisher’s protected LSD test. Measurements of potential fruit dry weight required a specific pruning protocol that was carried out only for three FR treatments (FR_0.22, FR_0.40, and FR_0.49) due to time constraints. Flowering (C) and fruit ripening (D) rates were determined as the number of trusses per week with all flowers reaching anthesis or all fruits reaching ripe stage, respectively. Each data point represents the average of two experimental units ± SEM, where the value per experimental unit is the average of six plants (A, C, D) or 15 fruits (B). FR, far-red light.

Figure 3

Effects of FR fraction in supplementary light on plant dry weight after 20 weeks of cultivation, 140–143 DAT (A), leaf photosynthesis rate measured between 128 and 134 DAT (B), fraction of PAR (C, D), and FR (E, F) light remaining at different canopy depths for cv. Foundation and cv. Trevine. A trendline is depicted to show a significant quadratic relationship between plant dry weight and FR fraction (p < 0.1, averaged over both cultivars), and letters denote significant differences between treatments, as determined by Fisher’s protected LSD test. Each data point represents the average of two experimental units ± SEM, where the value per experimental unit is the average of five (B) or six (A) plants or the average of two experimental units (C to F). FR, far-red light; DAT, days after transplant.

Figure 4

Yield component analysis representing the effects of FR_0.40, compared to FR_0.22, for cv. Foundation (A) and cv. Trevine (B). The effect of additional FR is represented through the percentage difference between FR_0.22 and FR_0.40. Relative leaf photosynthesis rate was obtained by dividing the leaf photosynthetic rate by the incident PPFD, and it was measured between 128 and 134 DAT. All other data derive from the final destructive harvest (140 DAT) or represent averages and cumulative sums across the entire experimental period. Asterisks indicate a significant effect of FR_0.40 as determined by Fisher’s protected LSD test (*p < 0.1; **p < 0.05; ***p < 0.01). FR, far-red light; PPFD, photosynthetic photon flux density; DAT, days after transplant.

Figure 5

Effects of FR fraction in supplementary light fruit dry matter content (A), total soluble solids content (B), and shelf-life (C) for cv. Foundation and cv. Trevine. Trendlines are depicted to show a significant linear relationship with the FR fraction (p < 0.1; dashed line for cv. Trevine, dotted line for cv. Foundation, and solid line when there is no significant cultivar effect). Each data point represents the average of two experimental units ± SEM, where the value per experimental unit is the average of 18 (A, B) and 27 (C) fruits. FR, far-red light.