Understanding crop responses to the light spectrum is critical for optimal indoor production. Far-red light is of particular interest, because it can accelerate growth through both physiological and morphological mechanisms. However, the optimal amount of supplemental far-red light for indoor lettuce production is yet to be quantified. Lettuce ‘Cherokee’, ‘Green SaladBowl’, and ‘Little Gem’ were grown under 204 µmol·m⁻²·s⁻¹ warm-white light-emitting diodes (LEDs) with supplemental far-red ranging from 5.3 to 75.9 µmol·m⁻²·s⁻¹. Supplemental far-red light increased canopy light interception 5 days after the start of far-red light treatment (DAT) for ‘Green SaladBowl’ and ‘Little Gem’ and 7 DAT for ‘Cherokee’. The increase in light interception was no longer evident after 12 and 16 DAT for ‘Green SaladBowl’ and ‘Little Gem’, respectively. We did not find evidence that supplemental far-red light increased leaf-level photosynthesis. At the final harvest, shoot dry weights of ‘Cherokee’ and ‘Little Gem’ increased by 39.4% and 19.0%, respectively, while ‘Green SaladBowl’ was not affected. In conclusion, adding far-red light in indoor production increased light interception during early growth and likely increased whole plant photosynthesis thus growth, but those effects were cultivar-specific; the increase in dry weight was linear up to 75.9 µmol·m⁻²·s⁻¹ far-red light.

Far-red photons (700–750 nm) can accelerate crop growth during indoor production through both physiological and morphological processes. A previous study showed that far-red photons can drive photosynthesis with efficiency similar to that of traditionally defined photosynthetically active photons (400–700 nm) if they are provided together with shorter-wavelength photons. Far-red photons also promote leaf and canopy expansion, which can increase light interception and growth. This study aimed to distinguish the contribution of morphological and physiological changes to crop growth induced by substituting red photons with far-red photons. We studied the long-term effects of substituting red photons with far-red photons on canopy light interception and whole-plant photosynthesis. ‘Little Gem’ lettuce (Lactuca sativa) seedlings were grown under four light spectrums of the same total photon flux density (400–750 nm). In addition to a background of a mixture of white and blue photons of 150 μ mol⋅m ⁻² ⋅s ⁻¹ , we provided 51 μ mol⋅m ⁻² ⋅s ⁻¹ red photons, far-red photons, or mixtures of red and far-red photons. In the first run, plants were harvested twice. The first harvest was at canopy closure, and the second harvest was when plants reached full size. In the second run, we harvested lettuce plants more frequently to minimize leaf overlap and interplant competition. We found that far-red photon substitution promoted leaf and canopy expansion and increased light interception. The effect of far-red photon substitution on leaf and canopy expansion was stronger in the second run than in the first run, likely because of lower plant density in the second run when plants were harvested more frequently. Far-red photon substitution of red photons decreased the amount of extended photosynthetically active radiation (ePAR) photons (400–750 nm) absorbed by leaves because of the lower leaf absorptance of far-red photons. The greater effect on canopy expansion in the second run of far-red photons substitution was able to exceed the reduction of ePAR photon absorption by leaves; therefore, we observed an increased crop gross photosynthetic rate (P _g ) between the second and third harvests during the second run. However, during the first run, lower absorptance of ePAR completely offset the effect of the greater canopy size and light interception, and crop P _g was decreased in the first run before the first harvest. The changes in light interception and crop P _g resulting from far-red photon substitution did not affect dry weight. Far-red photons had photosynthetic activity when applied with a blue and white light mixture, but their efficiency was approximately half that of red photons, potentially because of the lower absorptance of far-red photons. In conclusion, far-red photon substitution of red photons increased canopy size but decreased ePAR photons absorbed by leaves and did not increase the final dry weight. Because far-red light-emitting diodes (LEDs) have higher efficacy for converting electricity into photons, including far-red LEDs in fixtures for sole-source lighting can reduce energy costs without decreasing lettuce yields.

Biostimulants play an active role in sustainable crop production. While biostimulants are thought to have long-term effects on plant growth, little research has been conducted to confirm this hypothesis. In this study, we investigated the long-term residual effects of biostimulants applied exclusively during the onion seedling stage on subsequent plant growth. Three onion cultivars (‘Carta Blanca’, ‘Don Victoro’, and ‘Sofire’) were evaluated with the application of nine microbial biostimulants (LALRISE Mycorrhizae, LALRISE Bacillus velezensis, Mighty Mycorrhizae, MycoApply, Spectrum, Spectrum DS, Spectrum Myco, Tribus Original, and Tribus Continuum), one seaweed extract (Kelpak), and two fertilizer types (conventional and organic fertilizer). Plant morphology and biomass were investigated during the early bulb stage of onion growth. Parameters such as plant height, neck diameter, bulb diameter, and the fresh and dry weights of the shoot, bulb, and root were measured. The results indicated significant cultivar-specific effects of microbial biostimulant and fertilizer type, as well as their interactions, on onion early bulb growth. While seaweed extract exhibited minimal residual impact, specific microbial biostimulants, such as Mighty Mycorrhizae and MycoApply, significantly enhanced bulb growth in the red onion ‘Sofire’. Tribus Continuum was found to increase bulb growth of the yellow onion ‘Don Victoro’. Positive effects of microbial biostimulants on onion growth were also observed with LALRISE Bacillus velezensis, Spectrum Myco, Spectrum, and LALRISE Mycorrizae. Furthermore, microbial biostimulants demonstrated more significant positive effects on onion growth when applied in conjunction with organic fertilizer. In conclusion, microbial biostimulants exhibited long-term positive effects on onion plant growth even when applied solely during the seedling stage prior to transplanting. However, these effects were significantly influenced by onion cultivar and fertilizer type, with the greatest benefits observed when combined with organic fertilizer. We recommend MycoApply and Mighty Mycorrhizae for growers seeking to enhance onion productivity, particularly in organic cultivation, as the two products enhanced bulb and leaf growth in ‘Sofire’ and ‘Don Victoro’.

Fertilization guidelines for organic watermelon transplant production are rare. We investigated the effect of four commercial organic fertilizers and seven organic fertilizer blends, along with one conventional fertilizer (Peter’s Professional 20-20-20) on watermelon transplants. The four organic fertilizers were Nature Safe (fertilizer label: 7-7-7), Miracle-Gro (8-8-8), Dr. Earth fertilizer tea (4-4-4), and Drammatic (2-4-1). The seven blended organic fertilizers were created by supplementing Drammatic with nitrogen (N)-rich and/or potassium (K)-rich fertilizers to balance its N:phosphorus (P):K ratios. Watermelon ‘Jubilee’ was sown in organic substrate, and fertilizer treatments were applied weekly with a total of 0.4 g nitrogen/L substrate. Miracle-Gro and Drammatic had the highest N mineralization rate after 21 days and the highest inorganic N concentration, respectively, and resulted in the highest shoot dry weight among organic fertilizers. Miracle-Gro also resulted in the highest root dry weight. Dr. Earth fertilizer tea supplied the lowest N and P, and resulted in stunted transplants. Our results indicated that nitrate concentration was the most important factor influencing both shoot and root growth. Supplementing Drammatic with N-rich and/or K-rich fertilizers to balance its N:P:K ratio did not affect shoot and root dry weight. Combined, we concluded that nitrogen availability rather than nutrient balance is the key factor influencing watermelon transplant growth.

Conventional wheat breeding in the field is limited to two generations per year and is susceptible to environmental fluctuations. Speed-breeding in a plant factory offers a solution; however, recommendations for lighting strategies remain limited. To identify the optimal daily light integral (DLI) for the seedling stage, we tested three light intensities (300, 500, and 700 μmol m⁻² s⁻¹) and four photoperiods (10, 14, 18, and 22 h d⁻¹), resulting in DLIs ranging from 10.8 to 55.4 mol m⁻² d⁻¹. Our results indicated that an optimal DLI of 39.6 mol m⁻² d⁻¹ was associated with the highest seedling index (0.26) and root-to-shoot ratio (0.42), as well as enhanced photosystem performance. Beyond this DLI, these parameters and shoot biomass plateaued as the DLI increased. Moreover, treating seedlings with this optimal DLI of 39.6 mol m⁻² d⁻¹ (using 500 μmol m⁻² s⁻¹ light intensity and a 22 h d⁻¹ photoperiod) resulted in heading and flowering 5.9 and 7.5 days earlier after transplanting, respectively, than those under the lowest DLI (10.8 mol·m⁻²·d⁻¹). This study established a lighting strategy to produce high-quality seedlings and accelerate heading and flowering, thereby offering a valuable physiological framework for advancing speed-breeding systems in wheat.