Reduced Daily Light Integral at the End of Production Can Delay Tipburn Incidence with a Yield Penalty in Indoor Lettuce Production

Authors:
John Ertle The Ohio State University, 224 Howlett Hall, 2001 Fyffe Court, Columbus, OH 43210, USA

Search for other papers by John Ertle in
This Site
Google Scholar
Close
and
Chieri Kubota The Ohio State University, 330 Howlett Hall, 2001 Fyffe Court, Columbus, OH 43210, USA

Search for other papers by Chieri Kubota in
This Site
Google Scholar
Close

Click on author name to view affiliation information

Abstract

Indoor vertical farms that grow lettuce commonly encounter tipburn, which is an environmental disorder caused by calcium (Ca) deficiency during the late head-forming stages of lettuce. Characterized by marginal leaf necrosis of young expanding leaves, tipburn reduces marketable yield because of the appearance of these necrotic lesions. Lowering the daily light integral (DLI) to slow the plant growth rate has been a widely practiced approach to avoid tipburn in lettuce, but it largely reduces the final yield. We assessed the effect of lowering the DLI only during the end of production, which is a critical time because it is when tipburn is typically observed. Lettuce plants of tipburn-sensitive cultivars Klee and Rex were grown under a tipburn-inducing condition in growth chambers. Sixteen days after transplanting, the DLI was varied to 100% (L100), 85% (L85), 70% (L70), or 55% (L55) of the original 17.4 mol⋅m−2⋅d−1 to grow the final 12 d. At harvest, tipburn severity was reduced by lowering the DLI, but the magnitude of reduction was cultivar-specific. For ‘Klee’, the lowest tipburn severity was found at L55 (8% ± 2.1% of leaves), but the severity was similar for all other DLI levels (33% ± 3.5% of leaves). For ‘Rex’, tipburn severity was highest in the control (L100; 14% ± 2.8% of leaves) but similar for all other DLI levels (2% ± 0.9% of leaves). Reducing the end-of-production DLI to 55% resulted in a linear decrease in yield by up to 22% and 26% for ‘Klee’ and ‘Rex’, respectively. When the increase in marketable yields and decrease in the electricity cost were considered, decreasing the end-of-production DLI yielded a profitable contribution only for ‘Klee’ (L55). For moderately tipburn-sensitive ‘Rex’, revenue losses attributable to the yield decrease were too large to justify this approach of end-of-production reduced DLI.

At vertical indoor farms (IFs) throughout the world, lettuce (Lactuca sativa) is the most widely grown crop. These IFs have the capacity to grow lettuce to a marketable size faster than open-field production. However, losses of marketable yield caused by a localized Ca deficiency called tipburn are common for IF-grown lettuce. Near the end of production, lettuce growth exponentially increases (Both et al. 1997), and the limited transpiration rate common to IFs restrict Ca transport to young expanding leaves (Ahmed et al. 2022). During this stage of rapid growth, the Ca supply is unable to meet the demand of leaves near the growing shoot tip, resulting in necrotic lesions on the leaf margin (Goto and Takakura 1992; Lee et al. 2013). Tipburn symptoms are irreversible, and affected plants are considered unmarketable, resulting in the loss of the entire crop or significant labor to remove symptomatic leaves.

Although IFs have the distinct advantage of quickly producing high-quality lettuce year-round, lettuce tipburn can cause large losses of marketable yield, which restricts the annual production and revenue. Lettuce exhibits tipburn symptoms on the margins of young leaves at IFs as the DLI surpasses 13 or 15 mol⋅m−2⋅d−1 under the continuous (24-h) or 16-h photoperiod, with higher severity seen with a greater DLI (Caplan 2018; Sago 2016). However, growers at IFs want to maximize light intensity to increase the yield of lettuce without the risk of tipburn.

Several strategies can be used to reduce the risk of encountering tipburn, but they are challenging to implement at IFs. Reducing the temperature, limiting CO2 supplementation, or lowering light intensity can limit the growth rate of lettuce, thus reducing the likelihood of encountering a Ca deficiency. Other solutions include increasing the air velocity to enhance transpiration of lettuce (Goto and Takakura 1992), supplying an exogenous source of Ca through foliar sprays (Samarakoon et al. 2020), and increasing nighttime relative humidity (>95%) to drive mass flow through elevated root pressure (Vanhassel et al. 2015). However, IFs typically have multitiered cultivation structures with limited space above the crop, many lights and electrical equipment, and high costs associated with humidity management. Therefore, it is challenging to place fans that supply adequate air flow in limited space, risky to apply foliar sprays, and create high-humidity conditions that may damage expensive electronics or incur the cost of removing the humidity through heating, ventilation, and air conditioning systems. Therefore, there is a need to develop alternative strategies to manage tipburn at IFs.

The rate of tipburn appears to be correlated with the growth rate of lettuce (Sago 2016), which increases exponentially over time (Both et al. 1997). Increasing light intensity can increase the net photosynthetic rate, thus accelerating the growth rate of lettuce and directly contributing to a higher tipburn incidence (Ahmed et al. 2022). Reducing the lettuce growth rate can limit the tipburn risk, but growers may want to avoid changing CO2 or temperature because these could also reduce the growth of other crops produced at IFs. Therefore, changing the lighting strategy seems to be the best way of manipulating lettuce growth because it can be constrained to a smaller area without affecting other crops. Various studies have examined the management of lettuce tipburn through lighting controls, usually by examining the effect of different light intensities on the entire cropping cycle (Caplan 2018). Cultivating lettuce under lower light intensities can lessen the tipburn risk, but it can result in much less biomass. With continuous lighting, growing lettuce at a DLI of 13 mol⋅m−2⋅d−1 resulted in 92% lower tipburn severity than that under 26 mol⋅m−2⋅d−1; however, it also resulted in 40% less fresh weight (Sago 2016). Because many growers at IF now have the benefits of dimmable light-emitting diodes and automated lighting controllers, it is possible to explore many more alternative lighting strategies not reported in literature.

The risk of tipburn leads many growers to produce lettuce plants under slow growth rates for the entire production period. These strategies offer limited profitability because slowing growth results in less annual yield. Although reduced light intensity for the entire cultivation period is common, few studies have examined changing the light intensity during different stages of cultivation. Xu et al. (2020) found that varying the DLI between 5.8 and 11.5 mol⋅m−2⋅d−1 during the last 8 d of cultivation produced similar tipburn incidence and yield. Although tipburn was not reported, Jin et al. (2023) found that lettuce biomass under an increasing or decreasing light intensity regime could be similar to that with a constant intensity with an average of 13.4 mol⋅m−2⋅d−1. Potentially, instead of limiting lettuce growth for the whole cropping cycle, yield losses could be reduced by limiting DLI at end-of-production, when the tipburn risk is highest. Previously, we found that under tipburn-inducing conditions, lettuce tipburn usually occurs within the last 10 d of a 28-d cropping cycle (Ertle 2023). Therefore, we chose to examine the effect of reduced DLI during the last 12 d of lettuce production before the onset of the deficiency. Lettuce plants were grown under 17.4 mol⋅m−2⋅d−1 for 16 d after transplant before reducing the DLI to 85% (L85), 70% (L70), or 55% (L55) of the original intensity (or the same as the control; L100). We hypothesized that reducing DLI with end-of-production lighting treatments will reduce vegetative growth, limit tipburn severity, and increase the total marketable yield, despite overall yield losses.

Materials and Methods

Growth chamber setup.

This experiment was conducted in two identical walk-in growth chambers (Conviron, Winnipeg, ON, Canada) located in Columbus, OH, USA. The floor area within chambers was 2.74 × 3.32 m, with a growing area (per chamber) of 2.83 m2 at 1 m above the floor. The growing area was split among four identical carts (wire-frame shelves). Four identical nutrient film technique (NFT) channels (CropKing Inc., Lodi, OH, USA) made of food-grade polyvinyl chloride were mounted per cart. Channels were 9.5 (width) × 4 (height) × 123 (length) cm, with six 2.5- × 2.5-cm square holes punched in the channel cover. Each chamber had dedicated heating, ventilation, and air conditioning controlled by an aspirated temperature sensor centrally located 1.4 m above the floor. The chambers had additive CO2 control based on aspirated sensor (EE872; E+E Elektronik, Engerwitzdorf, Austria). Two light-emitting diode modules (GPL PM 168 DRBWFR L120 G3.0 C4 NA; Phillips Signify, NB, Eindhoven, The Netherlands) were mounted in each cart 50 cm above the channel surface. The targeted light spectra for all treatments were [as a percentage of the total photosynthetic photon flux (PPFD)] 19% blue (400–499 nm), 10% green (500–599 nm), and 71% red (600–699 nm), with a small amount of far-red light (∼1%, 700–750 nm). The DLI was adjusted without changing the light spectra.

Environmental conditions were monitored throughout the experiment with one CR1000X datalogger (Campbell-Scientific, Logan, UT, USA) in each chamber. Air temperature was monitored at the canopy level at the center of each NFT-equipped cart with calibrated T-type thermocouples (gauge, 36). Additional air temperature and relative humidity were measured with a fan-aspirated HMP-60 probe (Vaisala, Vantaa, Helsinki, Finland) located centrally in the chamber. The datalogger controlled additive humidity with an ultrasonic humidifier (Optimus U-31002; Optimus, Anaheim, CA, USA).

Plant materials and pretransplant growing conditions.

Lettuce cultivars Klee and Rex (Rijk Zwaan BV, De Lier, the Netherlands) were used because they were classified as highly and moderately sensitive to tipburn during our previous experiments, respectively (Ertle 2023). The seeds were initially germinated in 3.8- × 3.8-cm rockwool cubes (Grodan Inc., Ontario, Canada) for 2 d at a temperature of 20 °C under continuous lighting (PPFD 120 μmol⋅m−2⋅s−1) in a growth chamber (E15, Conviron, Winnipeg, Manitoba, Canada). Then, the seedlings were uncovered and left to grow in the same chamber for the next 12 d with a photoperiod of 16 h and day/night temperatures of 23 °C and 19 °C, respectively. The lights used were T8 white fluorescent lamps, and the seedlings were watered once per day with a half-strength nutrient solution.

Growing conditions after transplanting.

Two-week old seedlings with three or four fully expanded leaves were transplanted at a density of 34 plants/m2 in NFT channels described previously. Channels were rotated every other day to ensure light uniformity and limit the number of true border plants. Because of the alternating arrangement of plant spaces on the channels, one plant per channel was considered a border plant and removed from the analysis. Therefore, 160 of 192 total plants were included in data collection and analysis per experimental repeat (320 plants total).

During the first week of growth, root contact with nutrient solution was maximized by closing the drain and filling 80% of the channel volume (3.74 L). Every day, the solution was drained and refreshed to maintain dissolved oxygen levels (>5 mg⋅L−1). After 1 week, channels were opened and nutrient solution was constantly circulated as a typical NFT style until the end of the experiment. The nutrient solution was a modified leafy green hydroponic formula containing the following (in mg⋅L−1): 182 N (176 NO3–N, 6 NH4–N), 42 P, 152 K, 190 Ca, 42 Mg, 62 S, 87 Cl, and micronutrients. The pH and electrical conductivity (EC) of the solution were maintained at 6.0 ± 0.5 and 2.0 ± 0.2 dS⋅m−1. The nutrient solution was entirely changed once per week.

The growth chamber environmental conditions were selected based on our previous experiment to induce tipburn in lettuce (Ertle 2023). Setpoints and experimental mean ± SD are listed in Table 1. The additive CO2 injection began at the start of each day and required between 90 and 120 min to reach the 1000 μmol⋅mol−1 setpoint. The SD of CO2 reported in Table 1 includes this ramping period from ambient conditions, resulting in the large reported variation. After CO2 stabilized, the SD throughout the course of the day was typically within 50 μmol⋅mol−1 of the reported mean. The average horizontal and vertical air speeds were 0.16 ± 0.15 and 0.07 ± 0.02 m⋅s−1 when measured across the growing area using an anemometer (A004; Kanomax, Osaka, Japan) in both chambers (192 points; measured before and after the experiment). These conditions were chosen to maximize the lettuce growth rate and limit the transpiration rate, which resulted in tipburn of all lettuce cultivars in our previous study including Klee and Rex (10 cultivars tested) (Ertle 2023). During this experiment, the PPFD was 301.1 ± 2.2 μmol⋅m−2⋅s−1 across the growing surface for the first 16 d after transplanting (DLI = 17.4 mol⋅m−2⋅d−1). At the end of day 16, PPFD was reduced to target 100%, 85%, 70%, and 55% light output of the lighting modules, equating to approximately 300, 255, 210, and 165 μmol⋅m−2⋅s−1 (DLI = 17.3, 14.7, 12.1, and 9.5 mol⋅m−2⋅d−1) for the remaining 12 d before harvest (Table 2). The achieved light intensity is presented in Table 2 for each experiment, with an average light intensity before treatments of 17.4 ± 1.8 mol⋅m−2⋅d−1 across all growing surfaces. The cumulative DLIs for the entire experiment were 464, 431, 399, and 367 mol⋅m−2 for L100, L85, L70, and L55, respectively. The maximum reduction in the cumulative DLI for the entire experiment was approximately 21%.

Table 1.

Environmental conditions ± SD. Daytime setpoints were 23 °C, 75% relative humidity, 0.87 kPa VPD, and a CO2 concentration of 1000 μmol⋅mol−1. Nighttime setpoints were 19 °C, with venting to reduce the humidity, VPD, and CO2 concentration.

Table 1.
Table 2.

Mean ± SD of measured light treatment intensity [photosynthetic photon flux (PPFD)] and daily light integral (DLI) measured across the growing surface (192 points). Light quality and treatment names are reported for each chamber and experimental repeat.

Table 2.

Data collection.

Plants were assessed daily. They were considered to have tipburn when necrotic tissue lesions formed along the leaf margins of young expanding leaves near the shoot tip. The tipburn incidence is the percentage of plants that developed tipburn symptoms. Tipburn emergence is considered the number of days after transplanting (DAT) to visible symptomology. Tipburn severity was rated as the percent of leaves expressing tipburn symptoms at the time of harvest.

Leaf gas exchange rates were measured at 26 to 27 DAT using a Ciras-3 portable photosynthesis system (PP Systems, Amesbury, MA, USA). PPFD inside the leaf chamber was set to match the average light intensity and blue:green:red ratio of each treatment, except for far-red. For L100, L85, L70, and L55, this light intensity was set to 300, 255, 210, and 165 μmol⋅m−2⋅s−1, respectively. For each treatment, 12 plants (six of each cultivar) from each experimental repeat were selected for the gas exchange analysis, resulting in 96 total measurements of 320 total plants. The leaf selected for measurement on each plant was the most recently expanded leaf that was large enough for measurement.

At harvest, the number of leaves (>1 cm2; with and without tipburn) and shoot and root fresh weights of all plants (20 of each cultivar per treatment) were recorded. Plant samples were dried at 55 °C for at least 3 d before recording the dry weight. Six plants per treatment (three of each cultivar) were collected during each experimental repeat for tissue analysis. Tissue samples were sent away to undergo the mineral nutrient composition analysis (JR Peters, Allentown, PA, USA). Of 320 plants across both repeats, 48 had inner leaves (nearest the meristem; ∼6 leaves for ‘Rex’ and 10 leaves for ‘Klee’) and outer leaves (the remainder of the head) dried separately at harvest.

Experimental design and statistical analysis.

The experiment was repeated twice in 2022, from 9 Jun to 21 Jul and from 28 Jul to 8 Sep. The experimental model was a split-plot design, with four treatments replicated over two experimental repeats in each of two chambers. Each chamber had four carts and two treatments. All four treatments were replicated in both chambers during the two experimental repeats. During the second replication, treatments from the first chamber were applied in the second chamber, and vice versa, to replicate all four treatments in both chambers. Within each treatment, the same two cultivars (Klee and Rex) were grown. Treatments were assigned to the main plot (chamber) as a randomized complete block design with a blocking factor of the four carts in each chamber, and cultivars were assigned to the subplot (planting spaces) as a completely randomized design. Twenty plants of each cultivar were randomly assigned locations within each treatment. The experimental unit was considered individual plants of each cultivar in each treatment (or n = 160 total plants for a single experimental repeat). The experimental model included the experimental repeat [random effect; degrees of freedom (DF) = 1], the chamber (random; DF = 1), treatments (fixed effect; DF = 3), cultivars (fixed; n = 1), and residual error (DF = 153) of this split-plot design.

All statistical analyses were conducted using R and RStudio (R Core Team 2022; RStudio Team 2022). Linear regression and quadratic analyses were used to identify trends (α ≤ 0.05). Pearson’s and Spearman’s correlation coefficients were used for linear and nonlinear functions, respectively, to identify the relationship between measured variables and the finishing light reduction of PPFD (as a percentage). Polynomial regressions were applied when the P value was lower and R2 value was higher than the linear regressions.

Marketable yield and electricity costs.

The potential increase in marketable yield and decrease in lighting electricity costs were estimated. A wholesale price of $6.85⋅lb−1 ($15.07⋅kg−1) of lettuce was assumed (Simone Valle de Souza, personal communication). We assumed that lettuce with a tipburn severity less than 5% was fully marketable, whereas lettuce with higher severity had a loss of fresh weight proportional to the specific severity (e.g., 100 g shoot fresh weight with 10% tipburn severity yields 90 g of marketable fresh weight). However, this approach overestimates marketable yield losses because the “severity” recorded during our experiments was based on the number of leaves instead of the actual leaf fresh weight of young leaves showing tipburn symptoms. Yield per plant was converted based on g⋅m−2 using the planting density (34 plants/m2).

Electricity costs of our LED modules were calculated based on the manufacturer’s published electric energy consumption of the lights, assuming a photon capture efficiency of 80% (percentage of photons received in the plant growth area), lamp efficiency of 3 μmol⋅J−1, and electricity cost of $0.072⋅kWh−1, similar to the most recent industrial energy costs in the United States (US Energy Information Administration 2023). Reductions in light intensity were assumed to linearly reduce the electricity consumption. This linear relationship was validated using the reported energy use value given by the GrowWise control system (Phillips Signify, NB) used to operate the lights. A reduction of light intensity can also reduce the cooling load and costs. However, we assumed that only a small portion of the production area was subject to the low DLI; therefore, the cooling cost reduction was not considered during this analysis.

Results

Tipburn incidence, emergence, and severity.

For both cultivars, reduction in the DLI for the last 12 d of cultivation decreased the tipburn incidence (Table 3) and severity (Fig. 1), but the tipburn emergence (number of days after transplant) did not change (data not shown). For ‘Klee’ and ‘Rex’, tipburn symptoms emerged (became visible) at 21 ± 3.0 DAT and 23 ± 2.8 DAT, respectively. For ‘Klee’, tipburn severity reduced nonlinearly, with no significant difference in tipburn severity between the L70, L85, and L100 treatments. For ‘Rex’, the reduction in severity was also nonlinear, with no significant difference between the L55, L70, and L85 treatments. Compared with the control (L100), ‘Klee’ had 75% less tipburn and ‘Rex’ had no tipburn under the L55 treatment (Fig. 1). Tipburn incidence followed a similar trend as severity; ‘Klee’ had a similar tipburn incidence with all treatments but a large reduction under the L55 treatment, and ‘Rex’ had a reduced tipburn incidence for all lighting treatments compared to that under the L100 treatment (Table 3). Additionally, it was found that the earliest tipburn emergence for both cultivars occurred under the L55, L70, and L85 lighting treatments at 17 DAT, 1 d after the start of the reduced lighting treatments. However, the earliest tipburn emergence under the L100 treatment was 18 DAT for ‘Klee’ and 19 DAT for ‘Rex’.

Table 3.

Mean ± SE for all vegetative growth values.

Table 3.
Fig. 1.
Fig. 1.

Tipburn severity (% of leaves at time of harvest) across light intensity. Data are the means of two repetitions (n = 2) with 160 plants of each cultivar (n = 40 for each point). (A) ‘Klee’ and (B) ‘Rex’. DLI = daily light integral. Error bars indicate the SE.

Citation: HortScience 58, 10; 10.21273/HORTSCI17314-23

Plant growth.

Across the four lighting treatments, decreased DLI during the last 12 d of growth linearly reduced shoot and root fresh mass and dry mass for both cultivars (Table 3; Fig. 2). For ‘Klee’, the head fresh weight was reduced by 22% with the lowest light treatment (L55) compared with the control (L100), whereas ‘Rex’ was reduced by 26% (Fig. 2). Root fresh weight was reduced by 34% and 28% for ‘Klee’ and ‘Rex’, respectively, under the L55 treatment compared with L100. Root dry mass had a quadratic response to the reduction of the DLI within both cultivars and was lowest under the L55 and L100 treatments (Table 3). The number of leaves produced per plant was similar across all treatments for ‘Rex’ but declined linearly for ‘Klee’ by approximately 10% under the L55 treatment (approximately eight leaves) (Table 3).

Fig. 2.
Fig. 2.

Shoot fresh weight (g) across light intensity at the time of harvest. Data are the means of two repetitions (n = 2) with 160 plants of each cultivar (n = 40 for each point). (A) ‘Klee’. (B) ‘Rex’. DLI = daily light integral. Error bars indicate the SE.

Citation: HortScience 58, 10; 10.21273/HORTSCI17314-23

Gas exchange.

Plants of both cultivars at 26 DAT and 27 DAT had a linear reduction in net photosynthesis (Pn) with decreasing PPFD (Table 4). For ‘Rex’, E declined linearly with intensity, but ‘Klee’ had similar values across all treatments. Both cultivars had a linear reduction in water use efficiency (WUE; formula = Pn/E) with reduced light, but no change in stomatal conductance (gs) was found across the treatments (data not shown). Compared with the control (L100), ‘Klee’ plants in the L55 treatment had a 46% reduction in Pn and 41% reduction in WUE (Table 4). For ‘Rex’ grown under L55, Pn was reduced 50%, E was reduced 22%, and WUE was reduced 49% compared with plants under the L100 treatment (Table 4). The average E for all Klee plants was 2.4 ± 0.52 mmol⋅m−2⋅s−1, and cultivars Klee and Rex had average gs values of 262 ± 16.1 and 278 ± 19.4 mmol⋅m−2⋅s−1 across all treatments, respectively.

Table 4.

Means for tipburn incidence and mean ± SE for tipburn emergence, gas exchange measurements (26–27 DAT), and calcium (Ca) content in leaves (dry weight).

Table 4.

Tissue Ca.

The Ca concentrations of the Inner and outer leaf samples in both experimental repeats were assessed (Table 4). Among the treatments, there were no significant differences in the inner Ca or the ratio of inner-to-outer Ca (data not shown). The concentration of outer leaf Ca in ‘Rex’ linearly declined with reduced DLI (P = 0.097), but the difference in the Ca concentration was small (<12%) between all treatments (Table 4). There was a cultivar-specific difference in Ca concentrations of inner leaves (Table 4). The cultivar Rex had similar concentrations of Ca in inner leaves across all lighting treatments, whereas Klee had increased Ca concentrations in the inner leaf as the PPFD decreased.

Economics of DLI reduction.

Reducing DLI for the last 12 d of production provided an electricity cost-savings of $0.51⋅m−2 (L85) to a maximum of $1.53⋅m−2 per crop cycle (L55) (Fig. 3B). For ‘Rex’, reductions in overall plant growth (shoot fresh weight) under reduced DLI treatments resulted in less marketable yield, although tipburn severity was reduced (Fig. 3B). Compared with the control (L100), the marketable yield of ‘Rex’ was reduced, resulting in a loss between −$2.95 (L85) and −$14.96⋅m−2 (L55). For ‘Klee’, the same trend was observed under all treatments except the L55 treatment. Compared with the control (L100), ‘Klee’ had losses in marketable yield under the L85 and L70 treatments, resulting in losses of −$3.90 and −$4.99⋅m−2, respectively. However, for ‘Klee’ grown under L55, tipburn reduction was substantial enough to result in more marketable yield than was lost by biomass reduction, providing a profitable contribution of $2.55⋅m−2 (Fig. 3A).

Fig. 3.
Fig. 3.

Economic analysis of finishing light treatments using the marketable value ($⋅m−2) and electrical cost-savings ($⋅m−2) for (A) ‘Klee’, which is highly sensitive to tipburn, and (B) ‘Rex’, which is moderately sensitive to tipburn. DLI = daily light integral. Negative values indicate economic loss. Positive values indicate economic gain.

Citation: HortScience 58, 10; 10.21273/HORTSCI17314-23

Discussion

Plant growth is restricted by reducing the light intensity, generally as a linear function with DLI, as others have reported. For example, lettuce growth showed such linear decreases under light intensity of 9 to 19 mol⋅m−2⋅d−1 (Caplan 2018). We found that fresh shoot mass was linearly reduced with reduction in PPFD applied for the final 12 d of a 28-d post-transplanting production cycle, with maximum losses in yield of 22% and 26% for ‘Klee’ and ‘Rex’, respectively (Fig. 1). Shoot dry mass, root fresh weight, and the number of leaves produced by ‘Klee’ were also linearly reduced with lower DLI. Typically, lettuce under lower DLI initiate fewer leaves and have a smaller head fresh weight than those under higher PPFD (Kelly et al. 2020). The cultivar Rex developed fewer leaves than ‘Klee’, but the leaf number did not change with reductions in PPFD. Earlier initiation of reduced lighting treatments may reduce the number of developed leaves for ‘Rex’, but it did not occur during this experiment. Although plant growth is typically reduced linearly by DLI reductions, we found that limiting end-of-production DLI resulted in a quadratic relationship for root dry mass and the root-to-shoot ratio (based on dry mass) (Table 3). The lowest root dry mass and root-to-shoot ratio were found under the L55 and L100 treatments, whereas the two intermediate treatments (L70 and L85) had the highest value for both variables (Table 3). This could be a result of changes in the sink–source relationship. Because lighting was suddenly reduced, plants under the intermediate lighting treatments (L85, L70) partitioned more resources to root growth that increased the root-to-shoot ratio (Table 3). Under high light intensity, plants can photosynthesize at a faster rate, resulting in more above-ground biomass and below-ground biomass. Root biomass typically develops at a slower rate than shoot growth; therefore, a lower root-to-shoot ratio with the highest light treatment is possibly related to the exponential shoot growth characteristic of lettuce that outpaced root growth (van Holsteijn 1980). The low ratio found with the L55 treatment suggests that root development and shoot development were inhibited by the reduction in DLI, but not shoot growth outpacing root growth, as with the L100 treatment (Table 3).

Lower Pn resulted in less plant biomass at harvest for both cultivars, as expected (Tables 3 and 4). Others have found that the Pn of lettuce at an IF under 300 μmol⋅m−2⋅s−1 PPFD was 26% higher than that under 300 μmol⋅m−2⋅s−1, and it was correlated with a similar increase in shoot fresh weight at harvest (Ahmed et al. 2022). We found that the Pn of ‘Rex’ and ‘Klee’ were approximately 31% and 14% lower under the L70 treatment, respectively, than that under the L100 treatment (Table 4). In addition to Pn, differences in leaf area (although not measured) could contribute to differences in plant biomass (Fig. 1A) because a smaller leaf area would result in less light interception and biomass production. We are not sure if leaf area was reduced by lighting reduction, however. When canopy closure occurs, the leaf area effect is minimized. We did not record canopy closure during this experiment.

It was also found that transpiration rate (E) was similar under each treatment for ‘Klee’, but it was linearly reduced for ‘Rex’ under decreasing PPFD treatments (Table 4). Calcium is driven to leaf tissues relative to the magnitude of E, which can be greatly affected by the light intensity and spectrum. During our experiment, it appeared that cultivar-specific differences in E to light intensity can occur because E was decreased by lower intensity lighting for ‘Rex’ only (Table 4). Plants control E through the regulation of stomata, and gs can be modified by changes in environmental conditions. However, gs was not significantly different between lighting treatments for either cultivar. The gs were 262 ± 16.1 and 278 ± 19.4 mmol⋅m−2⋅s−1 for ‘Klee’ and ‘Rex’, respectively (data not shown). Environmental conditions across the chamber, including air speed, temperature, and humidity, did not have much temporal variation that likely would elicit changes in gs. Because of reductions in Pn, we found a linear decrease in WUE as DLI was reduced (Table 4). A higher WUE indicates less transpiration relative to photosynthesis and is generally related to the ability of a plant to support growth (Driesen et al. 2020). During this experiment, WUE was highest with the L100 treatment, and tipburn was also the most severe. We would expect that the lowest WUE would limit tipburn because the increased demand of Pn likely also increases the Ca required to maintain leaf expansion over a larger leaf area. Therefore, the tipburn risk would be highest among higher light treatments because of the increased demand for Ca resulting from higher Pn, which would first be limited by inadequate E to supply Ca to expanding leaves. However, WUE was not correlated with tipburn severity for either cultivar.

Lower concentrations of Ca in inner leaves are known to be linked to higher rates of tipburn because this is where tipburn generally occurs (Barta and Tibbitts 1986, 1991; Collier and Huntington 1983). Generally, these studies found that Ca accumulation decreased with each new leaf, was lower along leaf margins, and was lowest at the margins along the leaf tip. During our experiment, outer leaves had two- to three-times the amount of Ca found in inner leaves (Table 4), confirming this trend seen by others (Barta and Tibbitts 1991; Sago 2016). We sampled inner leaf and outer leaf Ca using a method similar to these reported studies and found that the concentrations of inner leaves and outer leaves were similar to those reported by Sago (2016). The transport of Ca is driven by E of leaf tissues to drive nutrient delivery and support growth. Under low transpiration conditions, like those during this experiment with low airflow, Ca transport is limited for leaves because the boundary layer resistance is high compared with faster airflow conditions. The boundary layer is created by gas exchange at the leaf surface increasing humidity, which limits transpiration because of reduced air mixing at the leaf surface. Under low airflow conditions, the resistance of this boundary layer increases because it is not broken by air movement (Aphalo and Jarvis 1993; Avissar et al. 1985). Although we reduced light in this experiment, there were no effects on the inner Ca concentrations in the leaves of ‘Rex’; however, ‘Klee’ accumulated more Ca (Table 4). Increased inner leaf Ca was not adequate to reduce the tipburn severity of ‘Klee’ except with the L55 treatment, however (Fig. 1A), indicating that the Ca requirement to support young leaf expansion is high and contributes to our finding that ‘Klee’ is highly sensitive to tipburn (Ertle 2023). Therefore, reducing DLI to limit the Ca demand may be a less effective strategy for moderately sensitive cultivars like Rex compared with highly tipburn-sensitive cultivars like Klee.

We observed that once tipburn emerges on one leaf of a lettuce plant, necrotic tissue typical of the Ca deficiency is expressed on all newly initiated leaves. This confirms the finding of others. Sago (2016) found that 88% of all newly initiated leaves express tipburn symptoms once the deficiency occurs on a given plant, regardless of the DLI during cultivation (13–26 mol⋅m−2⋅s−1) (Sago 2016). However, on the day following the initiation of the lighting treatment (17 DAT), some plants did have tipburn emergence with all treatments except the control (L100). Opposite of the typical trend that each following leaf also encountered tipburn, plants of both cultivars initiated at least one additional leaf before tipburn symptoms emerged again. It is likely that this sudden reduction in PPFD caused a shade-avoidance response that led to rapid leaf expansion and tipburn emergence. However, no other reports of this phenomena were found. Tipburn emerged an average of 2 d sooner for ‘Klee’ than for ‘Rex’, at 21 ± 3.0 DAT and 23 ± 2.8 DAT, respectively. This difference in emergence is likely attributable to the cultivar-specific differences in tipburn sensitivity and is within typical ranges (Ertle 2023). For ‘Rex’, a linear decrease in tipburn severity was seen with reduced DLI treatments (Fig. 1B), although tipburn emergence and the number of total leaves were the same (Table 3). As DLI was reduced, ‘Klee’ initiated fewer total leaves, although this difference was small (maximum of 10%), and tipburn emergence was the same (Table 3). However, tipburn severity was decreased when the DLI was reduced 45% under the L55 treatment. This difference in the tipburn severity relationship for the two cultivars could be a function of multiple factors discussed previously, including the initiation rate of new leaves, growth rate, total biomass production, and Ca supply. At harvest, ‘Klee’ had the same shoot mass as ‘Rex’, but it had more than twice as many leaves (Table 3). The Ca demand may be higher for the formation of new leaves than for the expansion of existing leaves because the initiation of new leaves is energetically more costly because of the formation of new tissues rather than cell enlargement of existing tissues (Micol and Hake 2003; Umoh et al. 2020). This difference between leaf initiation and expansion could contribute to the difference in tipburn severity seen between these similarly yielding cultivars. Under DLI ≥70% of the maximum, the Ca demand for ‘Klee’ was not reduced enough to elicit differences in tipburn severity because of the rate of leaf initiation, whereas the tipburn severity of ‘Rex’ declined linearly with reductions in light (Fig. 1B). This may explain why the tipburn severity of ‘Klee’ declined nonlinearly, and the initiation rate of leaves was high enough to result in a Ca deficiency. Regardless of the cultivar-specific tipburn risk, Ca deficiency will occur if the light intensity is too high to adequately limit the demand (Frantz et al. 2004). For ‘Rex’, the demand for Ca under 55% light was low enough to eliminate tipburn, whereas ‘Klee’ still had tipburn on 8.4% ± 2.1% of leaves (Fig. 1). Cultivar-specific differences in the leaf number and Ca demand to support growth likely resulted in these differences in the tipburn reduction response to decreasing light intensities.

For IFs, operating lights accounts for a significant portion of energy consumption costs (van Delden et al. 2021). A reduced DLI strategy applied near the end of cultivation resulted in a maximum saving in energy cost of $1.53⋅m−2 for the lowest light treatment (L55). During our experiment, reducing the DLI at end-of-production was targeted to maximize the production of tipburn-free biomass throughout cultivation and only reduce lighting to limit the Ca demand during the period of high tipburn risk based on our previous observations (Ertle 2023). This strategy was able to limit tipburn severity and yield loss caused by unmarketable tipburned leaves (Fig. 1). This strategy resulted in a net increase in the marketable biomass production of $2.55⋅m−2 for ‘Klee’ under the L55 treatment (Fig. 3A). However, ‘Rex’, a moderately tipburn-sensitive cultivar, had a loss of $14.96⋅m−2 with the same L55 treatment (Fig. 3B). These results suggest that reduced lighting is only beneficial for ‘Klee’ under the L55 treatment (Fig. 3A). None of the other DLI reduction treatments was beneficial for either cultivar, resulting in a net loss in marketable yield compared with the control (L100) treatment (Figs. 3A and 3B). Therefore, we recommend that highly sensitive cultivars like ‘Klee’ should be considered as good candidates for a low-intensity finishing light treatment to maximize marketable yield and reduce electrical lighting costs.

Although reducing light during the last 12 d of lettuce production can reduce tipburn, it was noted that the leaf texture seemed to be softer and more flexible to the touch under lower PPFD lighting. Although not quantitatively analyzed during this study, it has been reported that reducing PPFD reduces the firmness, leaf thickness, and textural quality of lettuce (Camejo et al. 2020; Mandizvidza 2017). Each of these quality characteristics is known to contribute to extended shelf life, and they are important for maintaining lettuce quality during postharvest (Zhang et al. 2007). Under decreasing light intensity during cultivation, reductions in postharvest quality and shelf life have been reported to be cultivar-specific (Gaudreau et al. 1994). Nevertheless, noticeable changes in leaf texture could be of concern to growers because this could suggest reductions in lettuce quality and shelf life after harvesting.

Conclusion

The tipburn risk for lettuce at vertical IFs is a commonly reported issue. During this experiment, we examined a reduced DLI lighting strategy to reduce the tipburn risk. We found that reducing the DLI for the last 12 d of a 42-d cropping cycle resulted in a decrease in plant growth and a decrease in or elimination of tipburn for two cultivars with high or moderate tipburn sensitivity. A simple economic analysis demonstrated that reducing the light intensity can offer a favorable profit contribution for the highly tipburn-sensitive cultivar Klee, but the yield reduction for the moderately sensitive cultivar Rex was too large to justify this approach. Cultivar-specific differences may make this finishing light strategy more favorable or less favorable for lettuce producers because reducing growth to avoid tipburn may also reduce farm profitability by limiting yields. Therefore, we recommend that reduced lighting should be considered for highly tipburn-sensitive cultivars within vertical IFs.

References Cited

  • Ahmed HA, Li Y, Shao L, Tong Y. 2022. Effect of light intensity and air velocity on the thermal exchange of indoor-cultured lettuce. Hortic Environ Biotechnol. 63(3):375390. https://doi.org/10.1007/s13580-021-00410-6.

    • Search Google Scholar
    • Export Citation
  • Aphalo PJ, Jarvis PG. 1993. The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air. Plant Cell Environ. 16(7):771783. https://doi.org/10.1111/j.1365-3040.1993.tb00499.x.

    • Search Google Scholar
    • Export Citation
  • Avissar R, Avissar P, Mahrer Y, Bravdo BA. 1985. A model to simulate response of plant stomata to environmental conditions. Agric Meteorol. 34(1):2129. https://doi.org/10.1016/0168-1923(85)90051-6.

    • Search Google Scholar
    • Export Citation
  • Barta DJ, Tibbitts TW. 1986. Effects of artificial enclosure of young lettuce leaves on tipburn incidence and leaf calcium concentration. J Am Soc Hortic Sci. 111(3):413416.

    • Search Google Scholar
    • Export Citation
  • Barta DJ, Tibbitts TW. 1991. Calcium localization in lettuce leaves with and without tipburn: Comparison of controlled-environment and field-grown plants. J Am Soc Hortic Sci. 116(5):870875. https://doi.org/10.21273/jashs.116.5.870.

    • Search Google Scholar
    • Export Citation
  • Both AJ, Albright LD, Langhans RW, Reiser RA, Vinzant BG. 1997. Hydroponic lettuce production influenced by integrated supplemental light levels in a controlled environment agriculture facility: Experimental results. Acta Hortic. 418:4551. https://doi.org/10.17660/actahortic.1997.418.5.

    • Search Google Scholar
    • Export Citation
  • Camejo D, Frutos A, Mestre TC, Del Carmen Piñero M, Rivero RM, Martínez V. 2020. Artificial light impacts the physical and nutritional quality of lettuce plants. Hortic Environ Biotechnol. 61(1):6982. https://doi.org/10.1007/s13580-019-00191-z.

    • Search Google Scholar
    • Export Citation
  • Caplan B. 2018. Optimizing carbon dioxide concentration and daily light integral combination in a multi-level electrically lighted lettuce production system (Thesis). University of Arizona, Tucson, AZ, USA.

  • Collier GF, Huntington VC. 1983. The relationship between leaf growth, calcium accumulation and distribution, and tipburn development in field-grown butterhead lettuce. Scientia Hortic. 21(2):123128. https://doi.org/10.1016/0304-4238(83)90157-7.

    • Search Google Scholar
    • Export Citation
  • Driesen E, Van den Ende W, De Proft M, Saeys W. 2020. Influence of environmental factors light, CO2, temperature, and relative humidity on stomatal opening and development: A review. Agronomy (Basel). 10(12):1975. https://doi.org/10.3390/agronomy10121975.

    • Search Google Scholar
    • Export Citation
  • Ertle J. 2023. Tipburn management through controlled environment for indoor vertical farm lettuce production. Dissertation. The Ohio State University, Columbus, OH, USA.

  • Frantz JM, Ritchie G, Cometti NN, Robinson J, Bugbee B. 2004. Exploring the limits of crop productivity: Beyond the limits of tipburn in lettuce. J Am Soc Hortic Sci. 129(3):331338. https://doi.org/10.21273/jashs.129.3.0331.

    • Search Google Scholar
    • Export Citation
  • Gaudreau L, Charbonneau J, Vézina L-P, Gosselin A. 1994. Photoperiod and photosynthetic photon flux influence growth and quality of greenhouse-grown lettuce. HortScience. 29(11):12851289. https://doi.org/10.21273/HORTSCI.29.11.1285.

    • Search Google Scholar
    • Export Citation
  • Goto E, Takakura T. 1992. Prevention of lettuce tipburn by supplying air to inner leaves. Am Soc. Agric Eng. 35(2):641645.

  • Jin W, Ji Y, Larsen DH, Huang Y, Heuvelink E, Marcelis LFM. 2023. Gradually increasing light intensity during the growth period increases dry weight production compared to constant or gradually decreasing light intensity in lettuce. Scientia Hortic. 311:111807. https://doi.org/10.1016/j.scienta.2022.111807.

    • Search Google Scholar
    • Export Citation
  • Kelly N, Choe D, Meng Q, Runkle ES. 2020. Promotion of lettuce growth under an increasing daily light integral depends on the combination of the photosynthetic photon flux density and photoperiod. Scientia Hortic. 272:109565. https://doi.org/10.1016/j.scienta.2020.109565.

    • Search Google Scholar
    • Export Citation
  • Lee JG, Choi CS, Jang YA, Jang SW, Lee SG, Um YC. 2013. Effects of air temperature and air flow rate control on the tipburn occurrence of leaf lettuce in a closed type plant factory system. Hortic Environ Biotechnol. https://doi.org/10.1007/s13580-013-0031-0.

    • Search Google Scholar
    • Export Citation
  • Mandizvidza TC. 2017. Influence of nutrient and light management on postharvest quality of lettuce (Lactuca sativa L.) in soilless production systems. Stellenbosch University, Stellenbosch, South Africa.

  • Micol JL, Hake S. 2003. The development of plant leaves. Plant Physiol. 131(2):389394. https://doi.org/10.1104/pp.015347.

  • R Core Team. 2022. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.

  • RStudio Team. 2022. RStudio: Integrated development environment for R. RStudio, PBC, Boston, MA, USA.

  • Sago Y. 2016. Effects of light intensity and growth rate on tipburn development and leaf calcium concentration in butterhead lettuce. HortScience. 51(9):10871091. https://doi.org/10.21273/HORTSCI10668-16.

    • Search Google Scholar
    • Export Citation
  • Samarakoon U, Palmer J, Ling P, Altland J. 2020. Effects of electrical conductivity, ph, and foliar application of calcium chloride on yield and tipburn of Lactuca sativa grown using the nutrient – film technique. HortScience. 55(8):12651271. https://doi.org/10.21273/HORTSCI15070-20.

    • Search Google Scholar
    • Export Citation
  • Umoh OT, Uyoh VE, Effiong EB. 2020. Review on the root, stem and leaf initiations in plants. Asian Plant Res J. 5(3):118. https://doi.org/10.9734/aprj/2020/v5i330106.

    • Search Google Scholar
    • Export Citation
  • US Energy Information Administration. Annual electric power industry report. https://www.eia.gov/electricity/annual/html/epa_02_10.html. [accessed 21 Feb 2023].

  • van Delden SH, SharathKumar M, Butturini M, Graamans LJA, Heuvelink E, Kacira M, Kaiser E, Klamer RS, Klerkx L, Kootstra G, Loeber A, Schouten RE, Stanghellini C, van Ieperen W, Verdonk JC, Vialet-Chabrand S, Woltering EJ, van de Zedde R, Zhang Y, Marcelis LFM. 2021. Current status and future challenges in implementing and upscaling vertical farming systems. Nat Food. 2(12):944956. https://doi.org/10.1038/s43016-021-00402-w.

    • Search Google Scholar
    • Export Citation
  • Vanhassel P, Bleyaert P, Van Lommel J, Vandevelde I, Crappé S, Van Hese N, Hanssens J, Steppe K, Van Labeke MC. 2015. Rise of nightly air humidity as a measure for tipburn prevention in hydroponic cultivation of butterhead lettuce. Acta Hortic. 1107(January):195201. https://doi.org/10.17660/ActaHortic.2015.1107.26.

    • Search Google Scholar
    • Export Citation
  • van Holsteijn HMC. 1980. Growth of lettuce: Quantitative analysis of growth. Mededelingen / Landbouwhogeschool Wageningen. Veenman. 80–13:124. https://edepot.wur.nl/287596.

    • Search Google Scholar
    • Export Citation
  • Xu W, Nguyen DTP, Sakaguchi S, Akiyama T, Tsukagoshi S, Feldman A, Lu N. 2020. Relation between relative growth rate and tipburn occurrence of romaine lettuce under different light regulations in a plant factory with LED lighting. Eur J Hortic Sci. 85(5):354361. https://doi.org/10.17660/eJHS.2020/85.5.7.

    • Search Google Scholar
    • Export Citation
  • Zhang FZ, Wagstaff C, Rae AM, Sihota AK, Keevil CW, Rothwell SD, Clarkson GJJ, Michelmore RW, Truco MJ, Dixon MS, Taylor G. 2007. QTLs for shelf life in lettuce co-locate with those for leaf biophysical properties but not with those for leaf developmental traits. J Expt Bot. 58(6):14331449. https://doi.org/10.1093/jxb/erm006.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    Tipburn severity (% of leaves at time of harvest) across light intensity. Data are the means of two repetitions (n = 2) with 160 plants of each cultivar (n = 40 for each point). (A) ‘Klee’ and (B) ‘Rex’. DLI = daily light integral. Error bars indicate the SE.

  • Fig. 2.

    Shoot fresh weight (g) across light intensity at the time of harvest. Data are the means of two repetitions (n = 2) with 160 plants of each cultivar (n = 40 for each point). (A) ‘Klee’. (B) ‘Rex’. DLI = daily light integral. Error bars indicate the SE.

  • Fig. 3.

    Economic analysis of finishing light treatments using the marketable value ($⋅m−2) and electrical cost-savings ($⋅m−2) for (A) ‘Klee’, which is highly sensitive to tipburn, and (B) ‘Rex’, which is moderately sensitive to tipburn. DLI = daily light integral. Negative values indicate economic loss. Positive values indicate economic gain.

  • Ahmed HA, Li Y, Shao L, Tong Y. 2022. Effect of light intensity and air velocity on the thermal exchange of indoor-cultured lettuce. Hortic Environ Biotechnol. 63(3):375390. https://doi.org/10.1007/s13580-021-00410-6.

    • Search Google Scholar
    • Export Citation
  • Aphalo PJ, Jarvis PG. 1993. The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air. Plant Cell Environ. 16(7):771783. https://doi.org/10.1111/j.1365-3040.1993.tb00499.x.

    • Search Google Scholar
    • Export Citation
  • Avissar R, Avissar P, Mahrer Y, Bravdo BA. 1985. A model to simulate response of plant stomata to environmental conditions. Agric Meteorol. 34(1):2129. https://doi.org/10.1016/0168-1923(85)90051-6.

    • Search Google Scholar
    • Export Citation
  • Barta DJ, Tibbitts TW. 1986. Effects of artificial enclosure of young lettuce leaves on tipburn incidence and leaf calcium concentration. J Am Soc Hortic Sci. 111(3):413416.

    • Search Google Scholar
    • Export Citation
  • Barta DJ, Tibbitts TW. 1991. Calcium localization in lettuce leaves with and without tipburn: Comparison of controlled-environment and field-grown plants. J Am Soc Hortic Sci. 116(5):870875. https://doi.org/10.21273/jashs.116.5.870.

    • Search Google Scholar
    • Export Citation
  • Both AJ, Albright LD, Langhans RW, Reiser RA, Vinzant BG. 1997. Hydroponic lettuce production influenced by integrated supplemental light levels in a controlled environment agriculture facility: Experimental results. Acta Hortic. 418:4551. https://doi.org/10.17660/actahortic.1997.418.5.

    • Search Google Scholar
    • Export Citation
  • Camejo D, Frutos A, Mestre TC, Del Carmen Piñero M, Rivero RM, Martínez V. 2020. Artificial light impacts the physical and nutritional quality of lettuce plants. Hortic Environ Biotechnol. 61(1):6982. https://doi.org/10.1007/s13580-019-00191-z.

    • Search Google Scholar
    • Export Citation
  • Caplan B. 2018. Optimizing carbon dioxide concentration and daily light integral combination in a multi-level electrically lighted lettuce production system (Thesis). University of Arizona, Tucson, AZ, USA.

  • Collier GF, Huntington VC. 1983. The relationship between leaf growth, calcium accumulation and distribution, and tipburn development in field-grown butterhead lettuce. Scientia Hortic. 21(2):123128. https://doi.org/10.1016/0304-4238(83)90157-7.

    • Search Google Scholar
    • Export Citation
  • Driesen E, Van den Ende W, De Proft M, Saeys W. 2020. Influence of environmental factors light, CO2, temperature, and relative humidity on stomatal opening and development: A review. Agronomy (Basel). 10(12):1975. https://doi.org/10.3390/agronomy10121975.

    • Search Google Scholar
    • Export Citation
  • Ertle J. 2023. Tipburn management through controlled environment for indoor vertical farm lettuce production. Dissertation. The Ohio State University, Columbus, OH, USA.

  • Frantz JM, Ritchie G, Cometti NN, Robinson J, Bugbee B. 2004. Exploring the limits of crop productivity: Beyond the limits of tipburn in lettuce. J Am Soc Hortic Sci. 129(3):331338. https://doi.org/10.21273/jashs.129.3.0331.

    • Search Google Scholar
    • Export Citation
  • Gaudreau L, Charbonneau J, Vézina L-P, Gosselin A. 1994. Photoperiod and photosynthetic photon flux influence growth and quality of greenhouse-grown lettuce. HortScience. 29(11):12851289. https://doi.org/10.21273/HORTSCI.29.11.1285.

    • Search Google Scholar
    • Export Citation
  • Goto E, Takakura T. 1992. Prevention of lettuce tipburn by supplying air to inner leaves. Am Soc. Agric Eng. 35(2):641645.

  • Jin W, Ji Y, Larsen DH, Huang Y, Heuvelink E, Marcelis LFM. 2023. Gradually increasing light intensity during the growth period increases dry weight production compared to constant or gradually decreasing light intensity in lettuce. Scientia Hortic. 311:111807. https://doi.org/10.1016/j.scienta.2022.111807.

    • Search Google Scholar
    • Export Citation
  • Kelly N, Choe D, Meng Q, Runkle ES. 2020. Promotion of lettuce growth under an increasing daily light integral depends on the combination of the photosynthetic photon flux density and photoperiod. Scientia Hortic. 272:109565. https://doi.org/10.1016/j.scienta.2020.109565.

    • Search Google Scholar
    • Export Citation
  • Lee JG, Choi CS, Jang YA, Jang SW, Lee SG, Um YC. 2013. Effects of air temperature and air flow rate control on the tipburn occurrence of leaf lettuce in a closed type plant factory system. Hortic Environ Biotechnol. https://doi.org/10.1007/s13580-013-0031-0.

    • Search Google Scholar
    • Export Citation
  • Mandizvidza TC. 2017. Influence of nutrient and light management on postharvest quality of lettuce (Lactuca sativa L.) in soilless production systems. Stellenbosch University, Stellenbosch, South Africa.

  • Micol JL, Hake S. 2003. The development of plant leaves. Plant Physiol. 131(2):389394. https://doi.org/10.1104/pp.015347.

  • R Core Team. 2022. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria.

  • RStudio Team. 2022. RStudio: Integrated development environment for R. RStudio, PBC, Boston, MA, USA.

  • Sago Y. 2016. Effects of light intensity and growth rate on tipburn development and leaf calcium concentration in butterhead lettuce. HortScience. 51(9):10871091. https://doi.org/10.21273/HORTSCI10668-16.

    • Search Google Scholar
    • Export Citation
  • Samarakoon U, Palmer J, Ling P, Altland J. 2020. Effects of electrical conductivity, ph, and foliar application of calcium chloride on yield and tipburn of Lactuca sativa grown using the nutrient – film technique. HortScience. 55(8):12651271. https://doi.org/10.21273/HORTSCI15070-20.

    • Search Google Scholar
    • Export Citation
  • Umoh OT, Uyoh VE, Effiong EB. 2020. Review on the root, stem and leaf initiations in plants. Asian Plant Res J. 5(3):118. https://doi.org/10.9734/aprj/2020/v5i330106.

    • Search Google Scholar
    • Export Citation
  • US Energy Information Administration. Annual electric power industry report. https://www.eia.gov/electricity/annual/html/epa_02_10.html. [accessed 21 Feb 2023].

  • van Delden SH, SharathKumar M, Butturini M, Graamans LJA, Heuvelink E, Kacira M, Kaiser E, Klamer RS, Klerkx L, Kootstra G, Loeber A, Schouten RE, Stanghellini C, van Ieperen W, Verdonk JC, Vialet-Chabrand S, Woltering EJ, van de Zedde R, Zhang Y, Marcelis LFM. 2021. Current status and future challenges in implementing and upscaling vertical farming systems. Nat Food. 2(12):944956. https://doi.org/10.1038/s43016-021-00402-w.

    • Search Google Scholar
    • Export Citation
  • Vanhassel P, Bleyaert P, Van Lommel J, Vandevelde I, Crappé S, Van Hese N, Hanssens J, Steppe K, Van Labeke MC. 2015. Rise of nightly air humidity as a measure for tipburn prevention in hydroponic cultivation of butterhead lettuce. Acta Hortic. 1107(January):195201. https://doi.org/10.17660/ActaHortic.2015.1107.26.

    • Search Google Scholar
    • Export Citation
  • van Holsteijn HMC. 1980. Growth of lettuce: Quantitative analysis of growth. Mededelingen / Landbouwhogeschool Wageningen. Veenman. 80–13:124. https://edepot.wur.nl/287596.

    • Search Google Scholar
    • Export Citation
  • Xu W, Nguyen DTP, Sakaguchi S, Akiyama T, Tsukagoshi S, Feldman A, Lu N. 2020. Relation between relative growth rate and tipburn occurrence of romaine lettuce under different light regulations in a plant factory with LED lighting. Eur J Hortic Sci. 85(5):354361. https://doi.org/10.17660/eJHS.2020/85.5.7.

    • Search Google Scholar
    • Export Citation
  • Zhang FZ, Wagstaff C, Rae AM, Sihota AK, Keevil CW, Rothwell SD, Clarkson GJJ, Michelmore RW, Truco MJ, Dixon MS, Taylor G. 2007. QTLs for shelf life in lettuce co-locate with those for leaf biophysical properties but not with those for leaf developmental traits. J Expt Bot. 58(6):14331449. https://doi.org/10.1093/jxb/erm006.

    • Search Google Scholar
    • Export Citation
John Ertle The Ohio State University, 224 Howlett Hall, 2001 Fyffe Court, Columbus, OH 43210, USA

Search for other papers by John Ertle in
Google Scholar
Close
and
Chieri Kubota The Ohio State University, 330 Howlett Hall, 2001 Fyffe Court, Columbus, OH 43210, USA

Search for other papers by Chieri Kubota in
Google Scholar
Close

Contributor Notes

We acknowledge and thank the members of the Kubota laboratory at The Ohio State University Department of Horticulture and Crop Science, the members of the Optimia project, and the US Department of Agriculture for their support of this research.

Funding was provided by the US Department of Agriculture National Institute of Food and Agriculture Specialty Crop Research Initiative grant (#2019-51181-30017).

C.K. is the corresponding author. E-mail: Kubota.10@osu.edu.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 930 930 40
PDF Downloads 721 721 31
  • Fig. 1.

    Tipburn severity (% of leaves at time of harvest) across light intensity. Data are the means of two repetitions (n = 2) with 160 plants of each cultivar (n = 40 for each point). (A) ‘Klee’ and (B) ‘Rex’. DLI = daily light integral. Error bars indicate the SE.

  • Fig. 2.

    Shoot fresh weight (g) across light intensity at the time of harvest. Data are the means of two repetitions (n = 2) with 160 plants of each cultivar (n = 40 for each point). (A) ‘Klee’. (B) ‘Rex’. DLI = daily light integral. Error bars indicate the SE.

  • Fig. 3.

    Economic analysis of finishing light treatments using the marketable value ($⋅m−2) and electrical cost-savings ($⋅m−2) for (A) ‘Klee’, which is highly sensitive to tipburn, and (B) ‘Rex’, which is moderately sensitive to tipburn. DLI = daily light integral. Negative values indicate economic loss. Positive values indicate economic gain.

 

Advertisement
Longwood Gardens Fellows Program 2024

 

Advertisement
Save