Plant response from the interaction between far-red and orange photons were rarely known, compared to that of far-red and red photons. Recent previous studies have found application of supplemental orange photons increases the openness of tomato plant architecture, resulting in improved dry weight than supplemental blue, green and red photons. However, limited information is available on the effects of orange photons on plant growth, morphology, and photosynthetic efficiency. Thus, our objective was to quantify the effects of orange photons on growth and photosynthetic responses during long term crop cultivation, compared to red photons. Dwarf tomatoes ‘Red Robin’ were grown in a walk-in chamber with controlled environmental conditions for 96 days after sowing (day temp. 24.3 ± 0.4℃ / night temp. 19.8 ± 0.5℃ and RH 60.5 ± 3.5%). Four light spectral treatments were applied as follows: 1) B25G25O200 (orange), 2) B25G25R200 (red), 3) B25G25O165FR35 (O FR), and 4) B25G25R165FR35 (R FR) (subscripts denote photon flux densities in µmol m² s-1). All spectral treatments had the same total light intensity of 250 µmol m² s-1 with a 18-h photoperiod. Leaf photosynthetic rate was measured before the fruit stage under sole-source orange or red light, as well as under combination lights (RGB or OGB), in a random order. Plant height and main stem length significantly increased under the two spectral treatments with far-red photons (i.e., O FR or R FR), compared to treatments without far-red photons (i.e., orange and red treatments). In comparison between orange and red treatments (without far-red), total dry weight of orange treatment was significantly higher than in red. However, the trend was opposite in the treatments with far-red photons (O FR treatment was lower than R FR treatment). In comparison between with and without far-red photons, total leaf area and fruit dry weight under far-red photons were significantly higher than those in the treatments without far-red photons, whereas stem weight was lower. Brix° under with far-red photons was higher than the treatment without far-red, and that of orange treatment was higher than in the red. Photosynthesis rate under sole-source orange photons was higher than under red photons, but no significant difference was observed among under combination lights. Overall, our results indicated that application of orange photons instead of red photons led to improved biomass and fruit yields in dwarf tomato, resulting in enhanced openness in the canopy structure; however, the trends were reversed with the application of far-red photons.
