Effect of Radiation Quality and Relative Humidity on Intumescence Injury and Growth of Tomato Seedlings

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Marlon Retana-CorderoEnvironmental Horticulture Department, University of Florida, Institute of Food and Agricultural Sciences (IFAS), 1549 Fifield Hall, Gainesville, FL 32611-0670

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Samson HumphreyEnvironmental Horticulture Department, University of Florida, Institute of Food and Agricultural Sciences (IFAS), 1549 Fifield Hall, Gainesville, FL 32611-0670

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Celina GómezDepartment of Horticulture and Landscape Architecture, Purdue University, 625 Agriculture Mall Drive, West Lafayette, IN 47907-2010

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Intumescence is a physiological disorder that affects some tomato (Solanum lycopersicum) cultivars grown in environments lacking ultraviolet radiation. Both far-red (FR) radiation and blue light have been shown to help mitigate this disorder. Thus, the objectives of this study were to characterize and compare intumescence injury and growth of various tomato cultivars propagated under different radiation qualities (Expt. 1) and to evaluate plant responses to the interactive effect of radiation quality and relative humidity (RH) (Expt. 2). Seedlings of six cultivars in Expt. 1 were grown under broad band white light (W), W and blue with (WBFR) or without (WB) FR radiation, and blue and red light with FR radiation (BRFR). Seedlings of four cultivars in Expt. 2 were grown under W or WBFR and a low (≈50%) or high (≈95%) RH. In both experiments, seedlings were grown under a daily light integral of ≈13 mol·m‒2·d‒1 (200 ± 4 μmol·m‒2·s‒1 for 18 h·d−1). FR radiation was provided using 20 ± 2 μmol·m−2·s−1 delivered throughout the entire photoperiod or at the end-of-day (EOD) in Expts. 1 or 2, respectively. Intumescence was generally suppressed when seedlings in Expt. 1 were grown under BRFR and WBFR, which also corresponded with the general response to stomatal conductance (gs). In contrast, seedlings grown under W had the highest incidence of intumescence, ranging from 23% to 69% across cultivars. The high blue photon flux (PF) ratio in WB was not effective at suppressing intumescence injury without FR radiation, although incidence and severity were lower compared with W. In Expt. 2, intumescence incidence was generally lower in seedlings grown under WBFR, and RH had small effects on intumescence. In both experiments, younger leaves were relatively less affected by intumescence, suggesting that a developmental factor is associated with the disorder. As expected, providing FR radiation resulted in a general increase in stem height across cultivars and in both experiments. The high RH provided in Expt. 2 also resulted in an increase in stem height. However, seedlings under low RH produced larger leaves, lower specific leaf area, and more shoot dry mass than those under high RH. Overall, our findings show that applying FR radiation helps suppress intumescence, but strategies are needed to minimize issues with excessive stem elongation.

Abstract

Intumescence is a physiological disorder that affects some tomato (Solanum lycopersicum) cultivars grown in environments lacking ultraviolet radiation. Both far-red (FR) radiation and blue light have been shown to help mitigate this disorder. Thus, the objectives of this study were to characterize and compare intumescence injury and growth of various tomato cultivars propagated under different radiation qualities (Expt. 1) and to evaluate plant responses to the interactive effect of radiation quality and relative humidity (RH) (Expt. 2). Seedlings of six cultivars in Expt. 1 were grown under broad band white light (W), W and blue with (WBFR) or without (WB) FR radiation, and blue and red light with FR radiation (BRFR). Seedlings of four cultivars in Expt. 2 were grown under W or WBFR and a low (≈50%) or high (≈95%) RH. In both experiments, seedlings were grown under a daily light integral of ≈13 mol·m‒2·d‒1 (200 ± 4 μmol·m‒2·s‒1 for 18 h·d−1). FR radiation was provided using 20 ± 2 μmol·m−2·s−1 delivered throughout the entire photoperiod or at the end-of-day (EOD) in Expts. 1 or 2, respectively. Intumescence was generally suppressed when seedlings in Expt. 1 were grown under BRFR and WBFR, which also corresponded with the general response to stomatal conductance (gs). In contrast, seedlings grown under W had the highest incidence of intumescence, ranging from 23% to 69% across cultivars. The high blue photon flux (PF) ratio in WB was not effective at suppressing intumescence injury without FR radiation, although incidence and severity were lower compared with W. In Expt. 2, intumescence incidence was generally lower in seedlings grown under WBFR, and RH had small effects on intumescence. In both experiments, younger leaves were relatively less affected by intumescence, suggesting that a developmental factor is associated with the disorder. As expected, providing FR radiation resulted in a general increase in stem height across cultivars and in both experiments. The high RH provided in Expt. 2 also resulted in an increase in stem height. However, seedlings under low RH produced larger leaves, lower specific leaf area, and more shoot dry mass than those under high RH. Overall, our findings show that applying FR radiation helps suppress intumescence, but strategies are needed to minimize issues with excessive stem elongation.

Intumescence is a physiological disorder that affects some solanaceous plant species such as pepper (Capsicum annuum) and eggplant (Solanum melongena) and has been widely documented in tomato (S. lycopersicum) (Craver et al., 2014; Eguchi et al., 2016a; Massa et al., 2008; Williams et al., 2016). This abiotic-induced disorder is typically characterized by blisters or gall-like tumors on the surface of leaves, petioles, or stems of susceptible plants. Intumescence injury develops from parenchymal and epidermal cells that expand beyond their normal size, leading to tissue rupture and collapse that sometimes causes necrosis and leaf abscission (Craver et al., 2014). Severe intumescence can negatively affect plant growth directly as a result of limiting leaf area for photosynthesis, or indirectly caused by pathogen infections through the abscission zone (Cruz and Gómez, 2022).

Although research on intumescence injury dates to the early 1900s (Dale, 1901), the underlying mechanisms that regulate this disorder are not well understood. Studies have shown that intumescence develops in environments that lack ultraviolet radiation (100 to 400 nm). Accordingly, Lang and Tibbitts (1983) and Kubota et al. (2017) showed that ultraviolet-B radiation (280 to 320 nm) effectively suppresses intumescence in tomato seedlings. However, the exact mechanisms of control remain uncertain. It is likely that changes in epidermal thickness and the accumulation of photoprotective compounds in response to ultraviolet help protect plants against intumesce injury (Escobar-Bravo et al., 2019). Fluorescent lamps, some of which emit sufficient ultraviolet radiation to suppress intumescence, are continuously being replaced with light-emitting diode (LED) fixtures for indoor plant production. Most LED fixtures do not provide ultraviolet radiation and thus, issues with intumescence are expected to increase. In addition, the increasing availability of compact tomato cultivars that can be grown in controlled environments calls for a better understanding of strategies to minimize issues with intumescence (Cruz et al., 2022).

FR (700 to 800 nm) radiation and blue (400 to 500 nm) light have been shown to help reduce intumescence injury in tomato seedlings grown under narrow band LED fixtures. Wollaeger and Runkle (2014, 2015), and Hernández et al. (2016) reported reductions in intumescence when tomato seedlings were grown under a high blue PF ratio. Further, Eguchi et al. (2016a) concluded that a small dose of FR provided as EOD radiation combined with a high blue PF ratio helps minimize intumescence and counteracts undesirable increases in stem height often caused by FR. Accordingly, others have shown that blue light helps inhibit stem elongation in tomato and other plant species (Nanya et al., 2012; Shimizu et al., 2006; Snowden et al., 2016) and attenuates the effect of FR radiation on extension growth (Park and Runkle, 2019). Eguchi et al. (2016b) suggested that the dose of EOD-FR required to suppress intumescence might change depending on cultivar susceptibility and environmental conditions. The role of FR radiation and blue light at controlling intumescence is unknown, but implies that photoreceptor proteins such as phytochromes and cryptochromes are involved at regulating the development of this disorder, which is consistent with observations made by Morrow and Tibbitts (1988).

RH also has been shown to be an important factor affecting intumescence development (Douglas, 1907; Eisa and Dobrenz, 1971). Lang and Tibbitts (1983) reported that although tomato seedlings under different RH developed similar injuries, the injury was more severe under high RH. The authors speculated that RH-induced effects on cuticle thickness could influence the development of intumescence. This corresponds with observations made by Williams et al. (2016), who suggested that plant-water status and its effect on cell turgor pressure and gas exchange can aggravate intumescence in plants. It is likely that the response to intumescence is determined by a synergistic effect of radiation quality, RH, and other environmental conditions that affect plant-water relations.

Most studies on intumescence have focused on evaluating suppression strategies using tomato rootstock cultivars primarily derived from the wild species Solanum habrochaites or Solanum hirsutum (Eguchi et al., 2016b; Zhao et al., 2008). Recently, Cruz et al. (2022) and Cruz and Gómez (2022) identified various susceptible tomato cultivars with large variability in their level of susceptibility to intumescence injury. Furthermore, intumescence control with FR radiation and high blue PF has not been evaluated alongside broad band white (400 to 700 nm) LEDs, which are often preferred to narrow band LEDs because of their higher color-rendering index that enables proper assessment of plant health status. Therefore, the objectives of this study were to characterize and compare intumescence injury and growth of various tomato cultivars propagated under different radiation qualities using broad band and narrow band LEDs (Expt. 1) and evaluate plant responses to the interactive effect of radiation quality and RH (Expt. 2). We hypothesized that intumescence would be lower in seedlings grown under FR radiation compared with white light only, particularly under low RH.

Materials and Methods

Plant material and growing conditions.

Compact tomato cultivars Patio (Seminis, St. Louis, MO), Sweet ‘n’ Neat Yellow (Syngenta, Basel, Switzerland), Little Bing (PanAmerican Seed Co., West Chicago, IL), Yellow Canary (Sakata Seed Co., Yokohama, Japan), Little Napoli (PanAmerican Seed Co.), and Camaro (Sakata Seed Co.) were used in Expt. 1. These cultivars have been identified as having different levels of susceptibility to intumescence (Cruz and Gómez, 2022; Cruz et al., 2022). Seeds were sown in industry-standard 84-cell propagation trays (25 mL individual cell volume; Blackmore Co., Belleville, MI) filled with a horticultural-grade substrate composed of 79% to 87% peatmoss, 10% to 14% perlite, and 3% to 7% vermiculite (v/v) (Pro-Mix BX general purpose; Premier Tech Horticulture, Quakertown, PA). Trays were cut into 4 × 6-cell partial trays and seedlings were grown only in the innermost eight cells within a partial tray. Tomato cultivars Yellow Canary, Little Napoli, Camaro, and Maxifort (De Ruiter, St. Louis, MO) were used in Expt. 2 following the same procedures described previously, except that industry-standard 72-cell propagation trays (20 mL individual cell volume; Blackmore Co.) were used as 2 × 3-cell partial trays where each cell had a seedling. In both experiments, trays were placed inside plastic humidity domes (54 cm × 28 cm × 15 cm; Acro Dome, Acro Plastics, LTD., Edmonton, Alberta, Canada) and kept under darkness for 3 d. After seedling emergence, domes were removed and the substrate surface of each partial tray was covered with a thin layer of parboiled rice hulls (Sungro, Agawam, MA) to minimize complications with fungus gnats (Bradysia sp.) (Cloyd et al., 2009). Seedlings were sub-irrigated as needed with a fertilizer solution (15N–2.2P–12.5K, Peters Cal-Mag Special Fertilizer; ICL Specialty Fertilizers, Dublin, OH) providing 150 mg·L‒1 nitrogen, with an electrical conductivity of 1.7 ± 0.2 dS·m‒1.

Controlled environment set up.

In Expt. 1, seedlings were grown in a walk-in growth chamber (C6 Control System with ECoSysTM Software; Environmental Growth Chambers, Chagrin Falls, OH) at the University of Florida (Gainesville, FL). The chamber had two opposite shelving units regarded as blocks, each with four growing compartments (304-cm length × 91-cm width × 60-cm height) that served as treatment replications. Each compartment held a single partial tray of each cultivar. A black fabric curtain (4.5-m length × 2-m width) hung from the center of the chamber to block radiation leakage between opposite shelves (<3 µmol·m‒2·s‒1). Black and white polyethylene plastic (0.13-mm) was also used to cover the back and sides of each compartment to minimize radiation leakage between vertical treatment compartments, with the white side facing the plants. The experiment was conducted twice, with two treatment replications within each experimental run.

In Expt. 2, seedlings were grown in a 12-m2 air-conditioned growth room at the University of Florida, which had four growing compartments (180-cm length × 90-cm width × 100-cm height) on top of benches placed on both sides of the room. Two adjoining compartments on each side of the room were regarded as a block. Each block had two radiation-quality treatments and two RHs as described later in this article, each of which held a single partial tray of each cultivar. To prevent radiation leakage between opposite compartments, a black fabric curtain (4.0-m length × 2-m width) hung from the center of the growth room. The back and sides of all compartments were covered with a 0.3-mm thick black and white polyethylene film to minimize radiation leakage (≤1 μmol·m−2·s−1) within the experimental area. The experiment was conducted twice, for a total of two radiation-quality × RH replications within each experimental run.

Treatments.

In Expt. 1, seedlings were grown under four radiation-quality treatments. Broad band white light (W) was provided by two horticultural-grade LED fixtures (Greenpower; Philips Lighting, Somerset, NJ; 150-cm long) with peak wavelengths of 450 and 660 nm. Fixtures were manually raised or lowered to achieve target photosynthetic PF densities (PPFDs). White supplemented with blue light (WB) was provided by two dimmable broad band white LED fixtures (RAY66 PhysioSpec Indoor; Fluence Bioengineering, Austin, TX) with peak wavelengths of 446, 599, and 664 nm and a single blue LED fixture (RAY66; Fluence Bioengineering) with peak wavelength of 446 nm. White supplemented with blue and FR (WBFR) was provided by the same fixture configuration described previously with the addition of a single dimmable FR LED fixture (RAY66 PfrSpec; Fluence Bioengineering) with peak wavelength of 730 nm. Last, blue and red light supplemented with FR (BRFR) was provided by dimmable dichromatic LED fixtures (RAY66 AnthoSpec; Fluence Bioengineering) with peak wavelengths of 446 and 664 nm and a single FR LED fixture (RAY66 PfrSpec; Fluence Bioengineering). The spectral characteristics of all treatments are described in Table 1. All treatments provided a PPFD of 200 ± 4 µmol·m‒2·s‒1 for 18 h·d−1 (0500 to 1100 HR), which resulted in a daily light integral (DLI) of ≈13 mol·m‒2·d‒1. Seedlings under WBFR and BRFR received 20 ± 2 µmol·m−2·s−1 of FR radiation throughout the entire photoperiod.

Table 1.

Spectral characteristics of four radiation-quality treatments used to evaluate intumescence injury and growth in tomato seedlings.

Table 1.

In Expt. 2, W and WBFR were also evaluated as radiation-quality treatments using the same fixture configurations and DLI described previously, except that 20 ± 2 μmol·m−2·s−1 of FR radiation was provided for 4 min at the EOD using a single fixed-output FR LED fixture with peak wavelength of 730 nm (FGI 730 NM; Forever Green Indoors, Seattle, WA). In both experiments, radiation maps were recorded to achieve target PF densities in each treatment compartment using a spectroradiometer (SS-110; Apogee Instruments Inc., Logan, UT) placed at midcanopy height across eight points within the experimental area. Partial trays within each compartment were rotated daily to minimize location effects within the experimental area.

Two RHs were also evaluated in Expt. 2 using transparent plastic containers (77-cm length × 52-cm width × 36-cm height; 2565507 Project Source; LF, LLC., Findlay, OH) to create a dome effect. For a low RH (L) effect, the four lateral sides of each container were cut open in a rectangle shape (24-cm length × 17-cm width × 17-cm height) to maintain RH at approx. 50% by using an electric dehumidifier (DH5022K1W; Hisense, Suwanee, GA). A high RH (H) effect (≥ 95%) was maintained by placing seedling trays inside intact containers. In both cases, a 0.5-cm opening was left between the base of the container and the bench surface to minimize temperature buildup within the growing environment.

Environmental conditions.

In Expt. 1, the setpoints for air temperature and CO2 concentration of the chamber were day/night 22/18 °C and constant 800 µmol·mol‒1, respectively. In Expt. 2, the room temperature was set at 19 °C and CO2 was kept at ambient levels. Temperature and RH were monitored using shielded dataloggers (Elitech dataloggers; Elitech, Milpitas, CA) placed at canopy height inside each plastic container. In Expt. 1, the average daily air temperature and RH were (mean ± standard deviation) 22.4 ± 0.2 °C and 78.8 ± 4.7%, respectively. In Expt. 2, the average daily air temperatures were 21.1 ± 0.7 °C and 22.8 ± 1.3 °C under low and high RH, respectively; average daily RHs were 50.5% ± 3.7% and 96.4% ± 2.9% under low RH and high RH, respectively. Average CO2 concentration was logged every 15 min using a built-in datalogger (DL1 Datalogger; Environmental Growth Chambers) or a digital controller (Atlas 8; Titan Controls, Vancouver, WA) in Expts. 1 and 2, respectively. The average daily CO2 was 812 ± 28 µmol·mol‒1 and 434 ± 19 µmol·mol‒1 in Expts. 1 and 2, respectively.

Data collection and analyses.

In Expt. 1, seedlings were destructively harvested 26 to 31 d after the start of each experimental run. In Expt. 2, seedlings of ‘Maxifort’, ‘Camaro’, ‘Little Napoli’, and ‘Yellow Canary’ were harvested at 12, 18, 24, and 27 d, respectively, after the start of each experimental run.

Data collected were the same in both experiments. One to 2 days before each destructive harvest, intumescence incidence was calculated by dividing the number of leaves with signs of intumescence by the total number of leaves per plant. On the same day, intumescence severity was visually evaluated on the first, second, and third true leaves from each plant using a subjective scale ranging from 0 to 10, where the percentage indicates the leaf area covered by intumescence with 0 = no intumescence; 1 = 1% to 10%; 2 = 11% to 20%; 3 = 21% to 30%; 4 = 31% to 40%; 5 = 41% to 50%; 6 = 51% to 60%; 7 = 61% to 70%; 8 = 71% to 80%; 9 = 81% to 90%; and 10 = 91% to 100%.

The gs was measured 2 to 4 d before each destructive harvest from the first true leaf per plant using a steady-state leaf porometer (Model SC-1; Decagon Devices Inc., Pullman, WA). Four days before each destructive harvest, SPAD index was measured on three leaves per plant using a chlorophyll meter (SPAD-502; Konica Minolta Sensing Inc., Ramsey, NJ); data were averaged based on three measurements per plant.

Before each destructive harvest, stem height was measured with a ruler and leaves were counted. Stems were then cut at the base of the substrate and leaves were cut from the stems. A leaf area meter (LI-3100C; LI-COR Biosciences, Lincoln, NE) was used to measure total leaf area per plant. Leaves and stems were subsequently placed into the same bag for drying. Samples were oven-dried for 2 d at 70 °C. Stem and leaf dry mass were measured separately using an electronic balance. Specific leaf area (SLA) was used as a measure of leaf thickness and calculated by dividing leaf area by leaf dry mass.

Data analyses.

In both experiments, partial trays with the different cultivars were arranged in a randomized complete block design. Each shelf in the growth chamber and two adjoining compartments within each growth room were regarded as blocks in Expts. 1 and 2, respectively. For each cultivar, data from all seedlings per tray were averaged and treated as a single data point per replication. Data were pooled between replications over time as the variances between experiments were not different based on Levene’s test and the statistical interactions between treatment and replications over time were not significant (P ≥ 0.05). The influence of the different categorical independent variables (i.e., treatments and cultivars) and their possible interaction on each of the continuous dependent variables were analyzed using a two-way analysis of variance (ANOVA). In both experiments, data are presented by cultivar to illustrate unique cultivar trends because most dependent variables showed a treatment × cultivar interaction (P ≤ 0.05). In Expt. 1, data were subjected to ANOVA using R version 3.6.1 (R Core Team, 2020) and all treatment means were compared with each other using Tukey’s honestly significant difference (HSD) test (P ≤ 0.05) with the Agricolae package in R (n = 4). In Expt. 2, data were subjected to ANOVA and when the radiation-quality × RH interaction was not significant (P ≥ 0.05), data were pooled for main effect treatment means and compared using Tukey’s HSD test (P ≤ 0.05) (n = 8 for main effects; n = 4 for interactions).

Results and Discussion

Intumescence and physiological responses to radiation quality and RH.

In Expt. 1, intumescence was completely suppressed when seedlings were grown under BRFR and WBFR except for ‘Camaro’, which had 9% of intumescence incidence under WBFR (Table 2). Overall, seedlings grown under W had the highest intumescence incidence, ranging from 23% to 69% across cultivars. This was followed by seedlings under WB, for which intumescence incidence ranged from 7% to 54% across cultivars. In Expt. 2, ‘Camaro’, ‘Little Napoli’, and ‘Yellow Canary’ seedlings grown under WBFR had a lower intumescence incidence (5%, 5%, and 11%, respectively) than those under W (58%, 59%, and 41%, respectively). Surprisingly, there was no RH effect on intumescence incidence except for ‘Maxifort’, for which intumescence was highest under WB-H and completely suppressed under WBFR-L (Table 3). In contrast, Suzuki et al. (2020) reported 2% to 3% intumescence incidence in tomato leaves grown in a greenhouse under 60% to 70% RH, whereas plants under 90% RH had 77% intumescence incidence. The authors suggested that intumescence is affected by a poor development of the cuticular layer under high RH and low ultraviolet radiation.

Table 2.

Intumescence incidence and severity in six tomato cultivars grown under four different radiation qualities in Expt. 1.

Table 2.
Table 3.

Intumescence incidence and severity measured in four tomato cultivars grown under different radiation quality and relative humidity treatments in Expt. 2.

Table 3.

Our findings support results from studies illustrating the positive effect of FR radiation to suppress intumescence in susceptible tomato cultivars (Tables 2 and 3). Hernández et al. (2016) reported an up to 31% reduction of intumescence incidence when tomato seedlings were treated with EOD-FR radiation compared with using narrow band blue and red LEDs only. Similarly, Eguchi et al. (2016b) reported a 19% to 55% reduction in the number of leaves affected by intumescence in tomato seedlings treated with EOD-FR compared with no exposure to FR radiation. Although mechanisms behind the FR-suppression of intumescence are unclear, others have suggested that a phytochrome-mediated effect caused by increasing the red:FR ratio is likely responsible for this response (Lang and Tibbitts, 1983; Morrow and Tibbitts, 1988).

Morrow and Tibbitts (1987) showed that tomato leaf discs treated with red light developed intumescence, but immediate exposure to FR radiation suppressed the disorder. Eguchi et al. (2016b) hypothesized that there is a specific FR dose required to suppress intumescence that is likely cultivar-specific and more effective at night. Our findings from Expt. 1 illustrate the potential to reduce intumescence injury with FR radiation provided throughout the photoperiod in combination with either narrow band blue and red LEDs or broad band white LEDs. Similarly, our findings from Expt. 2 support results from others showing the potential to suppress intumescence with EOD-FR. Nonetheless, although studies have shown that blue light helps attenuate the excessive stem elongation often caused by FR radiation (Eguchi et al., 2016a, 2016b; Park and Runkle, 2019), recommendations still need to be developed to completely suppress this undesirable morphological response (Tables 4 and 5). In addition, more research is needed to determine the most effective and energy-efficient control strategy for intumescence suppression. Considering that broad band white LED fixtures enable a better assessment of plant health status, they provide a key advantage when growing susceptible plants indoors compared with narrow band LEDs used as background light.

Table 4.

Growth and physiological parameters measured in six tomato cultivars grown under different radiation qualities in Expt. 1.

Table 4.
Table 5.

Growth and physiological parameters measured in four tomato cultivars grown under different radiation quality and relative humidity treatments in Expt. 2.

Table 5.

Results from Expt. 1 show that the high blue PF ratio in WB was not effective at suppressing intumescence injury without FR radiation (Table 2). Although seedlings grown under WBFR and WB received 51% and 54% of blue light, respectively (Table 1), those under WBFR generally had a lower intumescence severity. Others have shown that a high blue PF ratio provided by narrow band LED fixtures helps decrease intumescence injury (Hernández et al., 2016; Wollaeger and Runkle, 2014, 2015). However, only monochromatic blue light was reported to suppress the disorder in those studies, sometimes at the expense of increasing stem height. Similar to our findings, Massa et al. (2008) reported that intumescence in pepper plants was not mitigated when using high blue PF ratios. Eguchi et al. (2016b) hypothesized that blue light photoreceptors mediate various plant responses that affect intumescence development. One control mechanism could be affected by the blue light–induced inhibition of cell enlargement and division (Dougher and Bugbee, 2004), which may limit the development of symptoms such as blisters or gall-like tumors. Another mechanism that may affect intumescence development is the interaction and coaction of photoreceptors such as phytochrome and cryptochrome (Más et al., 2000; Wang et al., 2018). There is likely a synergy between providing FR radiation and a high blue PF ratio, which could regulate the interactive response of these photoreceptors and could explain the high suppression of intumescence when both radiation strategies are used.

Our findings suggest that intumescence is more responsive to FR radiation than to high blue PF (Table 2). Phytochromes have the largest absorption peaks in the red and FR region of the spectrum, but both active and inactive forms of phytochromes have small absorption peaks under blue light and ultraviolet-A (380 to 408 nm) radiation (Meng and Runkle, 2017). Others have shown that blue light and FR radiation have some overlapping physiological control mechanisms in plants. For example, Meng and Runkle (2014, 2017) described the potential to regulate flowering induction of ornamental plants with blue light. However, their findings show that the PF required to induce a similar response to FR radiation is higher with blue light. Moreover, studies have shown that both blue light and FR radiation have positive rooting effects on plants (Christiaens et al., 2019; Kurilčik et al., 2008; Ramírez-Mosqueda et al., 2017). Results from these studies illustrate the potential to control plant responses that are typically regulated with FR radiation using blue light. Although our findings show potential to minimize issues with intumescence using blue light, further studies are needed to determine the blue PF ratio that can plausibly cause the same intumescence suppression response to FR radiation.

In Expt. 1, younger leaves developed less intumescence than older leaves, with values across cultivars ranging on the first, second, and third leaf, respectively, from 3.8 to 0.8, 3.0 to 0.6, and 0.9 to 0.2 in seedlings grown under W, and from 1.2 to 0.3, 1.0 to 0.1, and 0.5 to 0 in those under WB. In Expt. 2, there was a radiation-quality × RH interaction for intumescence severity on the first and second leaf in ‘Camaro’ seedlings, where those grown under W-L generally had the highest intumescence severity (2.9 and 2.3, respectively), followed by those under W-H (1.3 and 1.0, respectively), WBFR-H (0.3 and 0.1, respectively) and WBFR-L (0.1 and 0.1, respectively). There was also a radiation-quality × RH interaction in the intumescence severity measured on the third leaf for ‘Little Napoli’ seedlings grown under W-L, which had higher values (1.3) than those under W-H (0.7), WBFR-H (0), and WBFR-L (0). Interestingly, there was no intumescence on the third leaf of ‘Maxifort’ seedlings except for those grown under W-H, which had an intumescence severity of 0.2.

Overall, our results suggest that a developmental factor is associated with intumescence injury (Tables 2 and 3). Similar to our findings, Cruz and Gómez (2022) and Mohmmed et al. (2020) reported a decrease in intumescence severity as plants grew and continued to develop new leaves. Considering that the whole-plant capacity for gas exchange increases with plant growth, the lower intumescence severity in younger leaves could be plausibly explained by changes in the whole-plant capacity to regulate water, likely reducing the risk of cell rupture and collapse, which is a common symptom of intumescence injury (Craver et al., 2014).

In Expt. 1, seedlings grown under BRFR and WBFR generally had a higher gs than those under WB and W (Table 4). For example, ‘Camaro’, ‘Patio’, and ‘Sweet ‘n’ Neat Yellow’ seedlings grown under BRFR and WBFR had up to 24%, 16%, and 17% higher gs, respectively, than those under WB and W. For ‘Yellow Canary’, gs was similar among seedlings grown under BRFR, WBFR, and WB, but up to 20% lower in those under W. No differences were measured in gs for ‘Little Bing’ and ‘Little Napoli’ seedlings grown under the different radiation-quality treatments, but the trend was similar to that in other cultivars. In Expt. 2, seedlings grown under WBFR and high RH generally had the highest gs except for ‘Little Napoli’, in which gs was lowest in seedlings grown under W-L (Table 5).

The terms intumescence and edema are often used interchangeably to refer to the same physiological disorder, as both cause similar symptoms, such as water-soaked lesions, epidermal cell hypertrophy, and translucent outgrowths on the surface of leaves (Craver et al., 2014). According to Williams et al. (2016), intumescence affects tomato in response to changes in radiation quality, whereas edema affects plants in response to problems associated with water relations. Kang et al. (2009) explained that although intumescence development could be affected by changes in water relations within plant cells, the disorder is primarily regulated by photoreceptors. Considering that a high blue PF ratio can increase stomatal opening (Inoue and Kinoshita, 2017) and gs (Izzo et al., 2021), it likely affects transpiration (Devi and Reddy, 2018). We postulate that although a high blue PF ratio might increase (to a point) gs in plants, a low vapor-pressure deficit under high RH could hinder leaf transpiration, plausibly leading to excessive turgor pressure within leaf cells, increasing the incidence of intumescence (Driesen et al., 2020; Sagi and Rylski, 1978).

In Expt. 1, seedlings grown under WB generally had the highest SPAD index across cultivars, most times followed by those under W (Table 4). In contrast, ‘Camaro’, ‘Maxifort’, and ‘Yellow Canary’ seedlings grown in Expt. 2 under W-H had the lowest SPAD index, but the highest treatment mean for this variable was inconsistent across cultivars. ‘Little Napoli’ seedlings grown under WBFR and low RH had the highest SPAD index in Expt. 2. SPAD index is often used as an indicator of leaf chlorophyll concentration (Azia and Stewart, 2001) and both phytochrome and cryptochrome photoreceptors are known to be involved in chlorophyll biosynthesis (Okamoto et al., 2020). A low red:FR ratio has been shown to reduce chlorophyll concentration in leaves by affecting transcription factors that regulate biosynthesis of chlorophyll intermediates (Meng et al., 2019). Conversely, blue light has been shown to increase chlorophyll concentration in tomato and other plant species (Snowden et al., 2016). The contrasting responses in SPAD index between the two experiments are likely attributed to the RH effect in Expt. 2. As shown by others, RH affects transpiration, which drives mass flow, uptake, and translocation of nutrients like nitrogen, which is a key component of chlorophyll molecules (Masood et al., 2020; Song et al., 2021).

Growth and morphological responses to radiation quality and RH.

As expected, providing FR radiation resulted in an increase in stem height across cultivars, which is typically undesirable during seedling propagation (Tables 4 and 5). For example, ‘Camaro’, ‘Patio’, ‘Sweet ‘n’ Neat Yellow’, and ‘Yellow Canary’ seedlings grown in Expt. 1 under BRFR and WBFR were 44% to 69%, 52% to 76%, 49% to 71%, and 38% to 53% taller, respectively, than those under WB and W. Seedlings in Expt. 2 grown under WBFR and high RH were generally taller than those under any other treatment. For example, ‘Camaro’ seedlings under WBFR-H were 22% taller than those under W-H, and up to 75% taller than those under W-L and WBFR-L. Similarly, there was a radiation quality × RH interaction for ‘Little Napoli’, which indicated that seedlings under WBFR-H were 18% taller than those under WBFR-L, and up to 38% taller than those under W-H and W-L.

Others have shown that FR radiation causes an increase in stem elongation in plants such as tomato (Hwang et al., 2020; Zhang et al., 2019), lettuce (Lactuca sativa) (Lee et al., 2015), and various ornamental species (Park and Runkle, 2019). It is widely accepted that a shade avoidance response mediated by a low phytochrome photostationary state produced under a low red:FR ratio is responsible for this response (Kalaitzoglou et al., 2019; Zhen et al., 2022), which corresponds with our results for stem height (Tables 1, 2, and 3). Similar to FR, others have reported that high RH (≥90%) results in an increase in stem height (Azuma et al., 1997; Fjeld, 1985; Hirai et al., 2000). Vu et al. (2013) showed that grafted tomato seedlings grown for 10 d under 90% RH were 11% taller than those under 70% RH. Considering that stem elongation is partly driven by the enlargement of plant cells caused by turgor pressure, plants grown under high RH with low transpiration rates may build up turgor pressure, which could be responsible for the increase in stem height measured in our study in response to high RH (Azuma et al., 1997).

In Expt. 1, seedlings grown under BRFR, WBFR, and WB generally produced larger leaves than those under W (Table 4). Furthermore, leaves of ‘Little Napoli’ and ‘Sweet ‘n’ Neat Yellow’ seedlings under WBFR were up to 38% and 70% larger, respectively, than those in all other treatments. In Expt. 2, ‘Camaro’, ‘Little Napoli’, and ‘Yellow Canary’ seedlings grown under low RH produced 58%, 91%, and 54% larger leaves, respectively, than those under high RH, but there were no differences in leaf area in response to radiation quality (Table 5). For ‘Maxifort’, leaf area was lowest under W-H, which is likely explained by its high susceptibility to intumescence which caused severe leaf abscission (Tables 2 and 3; Fig. 1). Similar to our results, Miyama and Yasui (2021) reported leaf abscission in several tomato cultivars grown under 90% RH, which ultimately affected leaf area of seedlings.

Fig. 1.
Fig. 1.

‘Maxifort’ seedlings in Expt. 2 showing signs of leaf abscission caused by intumescence after 12 d of growth under broad band white light-emitting diode fixtures and high relative humidity.

Citation: HortScience 57, 10; 10.21273/HORTSCI16712-22

In Expt. 1, ‘Camaro’ and ‘Little Napoli’ seedlings grown under BRFR had 20% to 27% and 57% to 67% higher SLA, respectively, than those under WBFR, WB, and W (Table 4). Similarly, ‘Patio’, ‘Sweet ‘n’ Neat Yellow’, and ‘Yellow Canary’ seedlings under BRFR and WBFR had 42% to 59% higher SLA than those under W. In Expt. 2, SLA was generally higher in seedlings grown under high RH (Table 5). For ‘Maxifort’, SLA was 9% higher under WBFR than W, whereas SLA for ‘Yellow Canary’ was highest under W-H and WBFR-H, and lowest under W-L.

Similar to height, leaf area expansion is considered a common shade avoidance response in plants grown under FR radiation (Legendre and van Iersel, 2021). Increasing leaf area is often accompanied by a reduction in leaf thickness, sometimes quantified as SLA. Stem height, leaf area expansion, and SLA have also been shown to respond to blue light (Mitchell and Stutte, 2017). Monochromatic blue light has been shown to increase leaf area and reduce leaf thickness in some plant species (Hernández and Kubota, 2016; Park et al., 2022). In contrast, increases in the blue light PF ratio generally result in smaller, thicker leaves (Dougher and Bugbee, 2004). Our findings show that providing both FR and a high blue PF ratio can increase leaf area expansion and SLA compared with white light only (Tables 4 and 5). However, it appears that although high RH causes a similar increase in stem height to FR, it does not have the same effect at increasing leaf area expansion. This could be attributed to changes in transpiration in response to RH, as high RH can limit the uptake of nutrients that support active plant growth and leaf area expansion in plants (Cramer et al., 2009). Correspondingly, Del Amor and Marcelis (2004) reported a 16% reduction in the leaf area of tomato seedlings when RH increased from 70% to 95%.

In Expt. 1, ‘Camaro’ and ‘Sweet ‘n’ Neat Yellow’ seedlings grown under WBFR produced 25% and 50% higher shoot DM, respectively, than those in all other treatments (Table 4). Similarly, ‘Little Bing’ seedlings grown under WBFR and W produced 40% higher shoot DM than those under WB. For ‘Little Napoli’ and ‘Yellow Canary’, shoot DM was higher under WBFR than WB, whereas for ‘Patio’, shoot DM was higher under WBFR than BRFR. In Expt. 2, ‘Camaro’ seedlings grown under W-L produced 20% higher shoot DM than those under WBFR-L, and twice the shoot DM than those under W-H and WBFR-H (Table 5). ‘Little Napoli’ and ‘Yellow Canary’ seedlings grown under low RH produced 167% and 100% higher shoot DM, respectively, than those under high RH. Similarly, ‘Maxifort’ seedlings under W-L produced higher shoot DM than those under W-H, likely attributed to the leaf abscission caused by intumescence (Tables 2 and 3).

Others have shown increases in shoot DM of tomato and other plant species grown under FR radiation (Cao et al., 2016; Kalaitzoglou et al., 2019; Lee et al., 2016). Zhen et al. (2022) explained that the combination of white light and FR radiation promotes a synergistic effect in leaf photochemical efficiency and photosynthetic rate, which consequently increases fresh and dry biomass production of plants. Zhen and Bugbee (2020) showed that lettuce plants grown under either narrow band blue and red LEDs or broad band white LEDs supplemented with 15% FR radiation had larger leaves and up to 31% more biomass than those without FR. However, to minimize issues with excessive stem elongation, no more than 20% of the total PF has been recommended to be used as FR (Zhen et al., 2021). In agreement with our findings for shoot DM in response to RH, Del Amor and Marcelis (2004) reported 11% higher total DM in tomato seedlings grown under 50% RH compared with those under >95% RH. In that study, dry mass partitioning to stems increased by 14% when RH increased from 70% to 95%, likely caused by changes in stem height.

Cultivar responses.

‘Camaro’ and ‘Maxifort’ had the highest susceptibility to intumescence in Expts. 1 and 2, respectively (Tables 2 and 3). Furthermore, ‘Maxifort’ seedlings grown in Expt. 2 under W-H had the highest intumescence incidence (69%), followed by those under W-L and WBFR-H (49% and 44%, respectively). ‘Maxifort’ seedlings under high RH also had higher intumescence severity on the first leaf (4.9) than those under low RH (0.9). ‘Maxifort’ is a vigorous rootstock often grafted to scions of indeterminate tomato cultivars for high-wire production in greenhouses. Various studies focused on intumescence have shown that ‘Maxifort’ has a high degree of susceptibility to the disorder, even in greenhouses that use glazing materials that often block ultraviolet radiation from sunlight (Williams et al., 2016). Based on our findings and those of others, there is a clear genetic component associated with the susceptibility to intumescence (Craver et al., 2014; Wu et al., 2017; Zhao et al., 2008). Accordingly, Prinzenberg et al. (2022) recently identified two to eight quantitative trait loci for intumescence and concluded that the susceptibility to this disorder is highly genotype dependent, with heritability ranging from 54% to 83%. ‘Maxifort’ is a cross of the wild tomato species S. habrochaites (formerly known as Lycopersicon hirsutum) with cultivated tomato (S. lycopersicum) (Avila et al., 2019). S. habrochaites is native to South America and commonly found at high elevations where ultraviolet radiation is abundant (de Vries et al., 2016). It is plausible that ‘Maxifort’ is highly susceptible to intumescence under low ultraviolet radiation due to the drastic environmental differences from its point of origin, particularly considering differences in availability of ultraviolet radiation (Eguchi et al., 2016a).

In conclusion, both WBFR and BRFR were effective treatments at suppressing intumescence in Expt. 1, which also corresponded with the general response to gs. This is useful information for growers who may need to produce susceptible tomato cultivars indoors but prefer to use broad band white LEDs, which enable a better assessment of plant health status. The high blue PF ratio in WB was not effective at suppressing intumescence injury without FR radiation, although incidence and severity were lower compared with W. In Expt. 2, intumescence incidence was generally lower in seedlings grown under WBFR, and RH had small effects on intumescence. In both experiments, younger leaves were relatively less affected by intumescence than older leaves, suggesting that a developmental factor is associated with this disorder. Further research is needed to elucidate the role of plant age on intumescence injury and to understand how other susceptible species are affected by this disorder. As expected, providing FR radiation resulted in a general increase in stem height across cultivars and in both experiments. Interestingly, the high RH provided in Expt. 2 also resulted in an increase in stem height. However, seedlings under low RH produced larger leaves with lower SLA and higher shoot DM than those under high RH. Overall, our findings show that applying FR radiation helps suppress intumescence at the expense of increasing stem height in tomato seedlings. With the large availability of new compact tomato cultivars suitable for indoor plant production, further studies are needed to evaluate alternative strategies that can help minimize issues with intumescence while maintaining plants compact.

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  • Williams, K.A., Miller, C.T. & Craver, J.K. 2016 Light quality effects on intumescence (oedema) on plant leaves 275 286 Kozai, T., Fujiwara, K. & Runkle, E.S. LED Lighting for Urban Agriculture. Springer Singapore https://doi.org/10.1007/978-981-10-1848-0

    • Search Google Scholar
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  • Wollaeger, H.M. & Runkle, E.S. 2014 Growth of impatiens, petunia, salvia, and tomato seedlings under blue, green, and red light-emitting diodes HortScience 49 734 740 https://doi.org/10.21273/HORTSCI.49.6.734

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  • Wollaeger, H.M. & Runkle, E.S. 2015 Growth and acclimation of impatiens, salvia, petunia, and tomato seedlings to blue and red light HortScience 50 522 529 https://doi.org/10.21273/HORTSCI.50.4.522

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  • Wu, Q., Park, S., Kirkham, M.B. & Williams, K.A. 2017 Transcriptome analysis reveals potential mechanisms for inhibition of intumescence development by UV radiation in tomato Environ. Exp. Bot. 134 130 140 https://doi.org/10.1016/j.envexpbot.2016.11.006

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  • Zhang, Y.T., Zhang, Y.Q., Yang, Q.C. & Tao, L.I. 2019 Overhead supplemental far-red light stimulates tomato growth under intra-canopy lighting with LEDs J. Integr. Agric. 18 1 62 69 https://doi.org/10.1016/S2095-3119(18)62130-6

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  • Zhao, X., Edwards, J.D., Kang, B.H., Simonne, E.H., Koch, K.E., Hochmuth, R.C., Olson, S.M. & Scott, J.W. 2008 Plant tumor development on tomato derived from Lycopersicon hirsutum Proc. Florida State Hortic. Soc. 121 167 169

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  • Zhen, S. & Bugbee, B. 2020 Substituting far-red for traditionally defined photosynthetic photons results in equal canopy quantum yield for CO2 fixation and increased photon capture during long-term studies: Implications for re-defining PAR Frontiers in Plant Sci. 1433 https://doi.org/10.3389/fpls.2020.581156

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  • Zhen, S., van Iersel, M. & Bugbee, B. 2021 Why far-red photons should be included in the definition of photosynthetic photons and the measurement of horticultural fixture efficacy Front. Plant Sci. 12 1158 https://doi.org/10.3389/fpls.2021.693445

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  • Zhen, S., Kusuma, P. & Bugbee, B. 2022 Toward an optimal spectrum for photosynthesis and plant morphology in LED-based crop cultivation 309 327 Kozai, T., Genhua, N. & Masabni, J. Plant Factory Basics, Applications and Advances. Academic Press https://doi.org/10.1016/B978-0-323-85152-7.00018-5

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Contributor Notes

Financial support was received from the U.S. Department of Agriculture National Institute of Food and Agriculture, Multistate Research Project NE1835: Resource Optimization in Controlled Environment Agriculture. We thank industry partners of the Research on Urban Gardening (RUG) consortium for supporting this research, including PanAmerican Seed Co., Syngenta Flowers, BioWorks, and Scotts Miracle Gro Co. We also thank our partners from the Floriculture Research Alliance at the University of Florida for their support and Sakata Seed Co. for donating seed for these trials.

C.G. is the corresponding author. E-mail: cgomezva@purdue.edu.

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    Fig. 1.

    ‘Maxifort’ seedlings in Expt. 2 showing signs of leaf abscission caused by intumescence after 12 d of growth under broad band white light-emitting diode fixtures and high relative humidity.

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  • Zhang, Y.T., Zhang, Y.Q., Yang, Q.C. & Tao, L.I. 2019 Overhead supplemental far-red light stimulates tomato growth under intra-canopy lighting with LEDs J. Integr. Agric. 18 1 62 69 https://doi.org/10.1016/S2095-3119(18)62130-6

    • Search Google Scholar
    • Export Citation
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  • Zhen, S. & Bugbee, B. 2020 Substituting far-red for traditionally defined photosynthetic photons results in equal canopy quantum yield for CO2 fixation and increased photon capture during long-term studies: Implications for re-defining PAR Frontiers in Plant Sci. 1433 https://doi.org/10.3389/fpls.2020.581156

    • Search Google Scholar
    • Export Citation
  • Zhen, S., van Iersel, M. & Bugbee, B. 2021 Why far-red photons should be included in the definition of photosynthetic photons and the measurement of horticultural fixture efficacy Front. Plant Sci. 12 1158 https://doi.org/10.3389/fpls.2021.693445

    • Search Google Scholar
    • Export Citation
  • Zhen, S., Kusuma, P. & Bugbee, B. 2022 Toward an optimal spectrum for photosynthesis and plant morphology in LED-based crop cultivation 309 327 Kozai, T., Genhua, N. & Masabni, J. Plant Factory Basics, Applications and Advances. Academic Press https://doi.org/10.1016/B978-0-323-85152-7.00018-5

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