Materials and Methods

Growing conditions.

Prevalent cultivars with red and yellow fruits (five cultivars of each) and large fruit sizes were selected based on interviews with seed companies and growers in Japan. These sweet pepper cultivars [Artega, Gialte, Maldonado, Mavera, and Kaite (Enza Zaden, Enkhuizen, the Netherlands); Nagano, Morbidelli, Sven, Keessie, and Jorit (Rijk Zwaan, Zuid-Holland, the Netherlands)] were hydroponically grown in a Venlo-type greenhouse (width, 18 m; length, 26 m; cultivated area, 448 m²) in Natori, Miyagi prefecture, Japan (lat. 38°10′N, long. 140°51′E).

Sweet pepper seeds were sown in Rockwool plugs (Kiemplug; Grodan, Limburg, the Netherlands) on 5 Nov 2021. Seeds were germinated in a germination chamber (Yasai-Hatsugaki; Keibunsha, Hiroshima, Japan). After germination, seedlings were placed in a growth chamber. The following environmental settings were used within the growth chamber: daylength, 16 h; day/night air temperature, 18/15 °C; CO₂ concentration in the atmosphere, 400 µmol·mol⁻¹; and photosynthetic photon flux density (PPFD), 250 µmol·m⁻²·s⁻¹. A fertigation system (NetaJet; Netafim, Tel Aviv, Israel) was used to dilute the concentrated stock solution to the target concentration using tap water. The electrical conductivity (EC) of the tap water was approximately 0.1 dS·m⁻¹. A concentrated stock solution was prepared by mixing commercially available straight fertilizers (Horgs, Tokyo, Japan). This concentrated stock solution was diluted using tap water and the fertigation system. Then, the seedlings were fertigated one to four times per day with a nutrient solution at an adjusted EC of 0.8 dS·m⁻¹. The frequency of fertigation was determined based on the plant age (i.e., one to four times per day when plants were young or close to transplanting). The macronutrients and micronutrients of this solution with an EC of 1.6 dS·m⁻¹ were as follows: 7.0 mmol nitrogen, 0.7 mmol phosphorus, 4.5 mmol potassium, 1.1 mmol magnesium, 2.5 mmol calcium, 1.6 mg·L⁻¹ iron, 0.6 mg·L⁻¹ boron, 0.6 mg·L⁻¹ manganese, 0.2 mg·L⁻¹ zinc, 0.15 mg·L⁻¹ copper, and 0.03 mg·L⁻¹ molybdenum. The seedlings were raised for 68 d after sowing.

On 12 Jan 2022 [68 d after sowing date; i.e., 0 d after transplanting (DAT)], the seedlings were transplanted into Rockwool cubes (10 × 10 × 6.5 cm; Delta 6.5 G; Grodan) and then placed onto Rockwool slabs (100 × 15 × 10 cm; Prestige; Grodan) in the greenhouse. The greenhouse was divided into nine rows, and the inter-row length was ∼1.5 m. Planting beds were arranged in a single-row planting system. Each plant was transplanted into a row every 25 cm. The 52 to 152 plants of each cultivar were transplanted into two or four replicates (two plots: Artega, Jorit, Maldonado, and Gialte; four plots: Mavera, Kaite, Nagano, Morbidelli, Sven, and Keessie). The numbers of replicates were determined based on the greenhouse structures and the plant numbers for the growth investigation of each cultivar (i.e., total of 44 plants). In four replicates, each plot comprised 8 to 20 plants. In two replicates, each plot comprised 8 and 36 plants.

Including 44 plants of each cultivar and guard plants, a total of 748 plants were transplanted in the greenhouse. The plants on both sides (8 plants: ‘Artega’, ‘Gialte’, ‘Mavera’, ‘Morbidelli’, ‘Sven’, ‘Keessie’, ‘Maldonado’, and ‘Jorit’; 92 plants: ‘Kaite’; 64 plants: ‘Nagano’) were grown as guards for the seven central rows. Additionally, the plants in the central row (40 plants: ‘Orandino’; 44 plants: ‘Nagano’) were grown as guards because they were grown under strong shadows with steel frames just above the plants. The guard plants (i.e., total of 308 plants in three rows) were excluded from the measurements. Therefore, except the guard plants from the whole plants, the 44 plants of each cultivar (i.e., total of 440 plants) were grown and used for growth investigations.

The planting density was ∼2.67 plants/m². The plants were trained on three main stems during cultivation (8.01 stems/m²). Plants were supplied with a sufficient nutrient solution using the fertigation system. The EC of the nutrient solution was maintained at 1.6 dS·m⁻¹ throughout the experiment. The greenhouse experiments also used the same concentrated stock solution supplied during the seedling period. The irrigation frequency was controlled based on outside solar radiation, and irrigation was performed every 0.6 to 0.8 MJ·m⁻². The drainage was discarded, and the daily drainage rate (drainage/supplied nutrient solution) was maintained at >30% throughout the experiment. During this experiment, we defined a weak lateral as a shoot that was shorter or thinner than other shoots. Weak laterals of each dichotomous branch were pruned once per week. Plants were trained on one leaf per node. The growth point of each main stem was pinched from 5 to 22 Aug 2022 (205–222 DAT). The experiment ended on 25 Oct 2022 (286 DAT).

The environmental conditions in the greenhouse were recorded and controlled every 5 min using an integrated environment control system (Connext; Priva. B.V., South Holland, the Netherlands). The operating set-point of the ventilation windows was frequently changed from 0 to 286 DAT. A heater (HB-1; House Boiler, Nepon, Tokyo, Japan) and heat pump (NGP1010T; Green Package, Nepon) were used to heat and cool the air, respectively. The CO₂ concentration in the greenhouse was maintained by exhausting the oil burner. Daytime CO₂ concentrations at 37 to 106, 107 to 118, 119 to 201, and 228 to 242 DAT were set at 1000, 650, 550, and 550 µmol·mol⁻¹, respectively. The CO₂ gas was not supplied during the other periods (0–36, 202–227, and 243–286 DAT). The place where the experiment was performed was unsuitable for high CO₂ enrichment during the spring and summer seasons because of the high outside air temperature. Therefore, until approximately 100 DAT, we set the CO₂ concentration at 1000 µmol·mol⁻¹ because of the low outside air temperature. After this period, we set the threshold at 550 to 650 µmol·mol⁻¹. We set these thresholds based on the work of Bosland and Votava (2012) and Russo (2012), who mentioned that the recommended CO₂ concentrations during the summer and winter seasons were approximately 400 and 800 µmol·mol⁻¹, respectively. Therefore, we set the thresholds for maintaining the CO₂ concentrations in the greenhouse to achieve values closer to these (Fig. 1). A shade curtain (TN-50, 50% shading rate; Sunsun Curtain, Nihon Wide Cross, Osaka, Japan) extended across the entire roof when the outside solar radiation reached 0.75 kW·m⁻² during 168 to 260 DAT. A thermal curtain (SN-10, 15% shading rate; Sunsun Curtain, Nihon Wide Cross) extended across the entire roof when the outside nighttime air temperature was lower than 20 °C during 0 to 156 and 269 to 286 DAT.

View Full Size

Changes in the daily average air temperature (A, dashed line), daily cumulative solar radiation in the greenhouse (A, solid line), and daytime average CO₂ concentration in the greenhouse (B, solid line) from 0 to 286 d after the transplanting date.

Citation: HortScience 59, 9; 10.21273/HORTSCI17904-24

• Download Figure
• Download figure as PowerPoint slide

The daily average air temperature, daily cumulative solar radiation in the greenhouse, and daytime average CO₂ concentration in the greenhouse are shown in Fig. 1. The daily average air temperature changed within the range of 20 to 28 °C. The daily cumulative solar radiation in the greenhouse gradually increased from 0 to 150 DAT and decreased from 150 to 286 DAT. The average daily average air temperature and average daily cumulative solar radiation in the greenhouse during 0 to 286 DAT were approximately 22.8 °C and 6.3 MJ·m⁻², respectively. The daytime average CO₂ concentrations in the greenhouse were 600 to 800 and 400 to 600 µmol·mol⁻¹ during 40 to 100 DAT and 100 to 286 DAT, respectively. The average daytime average CO₂ concentration in the greenhouse during 0 to 286 DAT was approximately 504 µmol·mol⁻¹.

Measurements.

Fresh and dry weights of each organ (g/plant; fruit, stem, and leaves) and leaf area (m²/plant) of three randomly sampled plants in each cultivar (30 plants in each sampling) were measured at 0, 64, 127, 183, and 276 DAT. The sampled plants were dried for 6 d at 100 °C using a large ventilation drier (DF1032; Fine Oven, Yamato Kagaku, Tokyo, Japan). A commercially available leaf area meter (LI-3100C; LI-COR, Lincoln, NE, USA) was used to obtain leaf area measurements. The LAI (m²·m⁻²) at the sample date was calculated by multiplying the observed leaf area (m²/plant) and planting density (2.67 plants/m²). Plants surrounding the sampled plants were grown as guards because these plants were grown with decreased planting density (<2.67 plants/m²) after the sampling date.

Four to five mature fruits of each cultivar were randomly sampled every month from 0 to 286 DAT (34–35 fruits per cultivar). The fresh and dry weights (g/fruit) of the sampled fruits were measured on each sampling date and finished drying date, respectively. The fruit DMC (g·g⁻¹) in each fruit was calculated by dividing the FDW by the FFW. The average of the individually calculated fruit DMC (n = 34–35) was defined as the fruit DMC in each cultivar (g·g⁻¹).

For FFW (g·m⁻²), 11 to 12 plants were randomly selected for ‘Artega’, ‘Jorit’, ‘Maldonado’, and ‘Gialte’, and 24 plants were randomly selected for ‘Mavera’, ‘Kaite’, ‘Nagano’, ‘Morbidelli’, ‘Sven’, and ‘Keessie’. The mature fruits of the selected plants were sampled weekly from 0 to 286 DAT. The total FFW at 0 to 276 DAT (FFW_total; g·m⁻²) was calculated by accumulating the immature FFW destructively sampled at 276 DAT and the harvested FFW measured at 0 to 276 DAT. Additionally, the harvested FFW at 0 to 276 DAT (FFW_harvest; g·m⁻²) was calculated by accumulating the harvested FFW measured at 0 to 276 DAT. The FDW during 0 to 276 DAT (g·m⁻²) was calculated by multiplying the FFW_total during 0 to 276 DAT (g·m⁻²) and the fruit DMC (g·g⁻¹). The TDM production during 0 to 276 DAT (g·m⁻²) was calculated by accumulating the dried weight in each organ at 276 DAT and the FDW during 0 to 276 DAT. The fruit DM_f at 276 DAT (g·g⁻¹) was calculated by dividing the FDW by the TDM production during 0 to 276 DAT.

Light extinction coefficients (k) for ‘Artega’, ‘Maldonado’, ‘Morbidelli’, ‘Gialte’, and ‘Jorit’ were measured during 180 to 183 DAT according to Homma et al. (2023) and Watabe et al. (2021). Three plants of each cultivar were destructively sampled for the measurements. The plant canopy was segmented into four layers every 50 cm from the growth point, and the PPFD in each layer was measured using a line quantum sensor (LI-191R; LI-COR) and a light meter (LI-190A; LI-COR). After PPFD measurements at 180 DAT, the leaf area was destructively measured at 183 DAT to estimate the cumulative LAI of each layer. The light extinction coefficient (k) for each cultivar was obtained from the slope of the linear regression analysis curve using the relative PPFD and cumulative LAI in each layer (Eq. [1]) (Monsi and Saeki 2005):

$\frac{{\mathrm{I}}_i}{{\mathrm{I}}_0}\hspace{0.17em}=\hspace{0.17em}1-{\mathrm{e}}^{-k\times \mathrm{LA}{\mathrm{I}}_i}$[1]

where I_i is the light intensity of each layer i in the plant canopy, I₀ is the light intensity above the plant canopy, and LAI_i is the cumulative LAI of each layer i of the plant canopy.

Significant light extinction coefficients (k) for ‘Nagano’, ‘Keessie’, ‘Mavera’, and ‘Sven’ were obtained using the previous year’s experiment [transplanting date: 17 Dec 2020; measurement date: 5 to 6 Aug 2021 (231–232 DAT); LAI at the measurement date: >8.0 m²·m⁻²; planting density: 2.67 plants/m²]. The location, cultivation practices, and methodology for measuring the light extinction coefficient (k) of the previous year’s experiment were the same as those in the present experiment. Therefore, we used these light extinction coefficients (k) (‘Nagano’, ‘Keessie’, ‘Mavera’, and ‘Sven’ were measured in 2021; ‘Artega’, ‘Maldonado’, ‘Morbidelli’, ‘Gialte’, and ‘Jorit’ were measured in 2022) during the analysis. The light extinction coefficient (k) for ‘Kaite’ could not be obtained because of a lack of data; therefore, this cultivar was excluded from the yield component analysis.

The intercepted light per plant at m DAT (IL_m; MJ·m⁻²) was predicted as follows (Eq. [2]) (Saito et al. 2020):

$\mathrm{I}{\mathrm{L}}_m\hspace{0.17em}=\hspace{0.17em}\left(1-{\mathrm{e}}^{-k\times \mathrm{LA}{\mathrm{I}}_m}\right)\times \left(1-{\mathrm{R}}_{\mathrm{c},m}\right)\times {\mathrm{T}}_{\mathrm{g}}\times \hspace{0.17em}{\mathrm{R}}_{\mathrm{p}}\times \mathrm{S}{\mathrm{r}}_m$[2]

where IL_m is the function of cumulative outside solar radiation at m DAT measured in 2022 (Sr_m; MJ·m⁻²), ratio of photosynthetically active radiation (PAR) to outside solar radiation (R_p; 0.5 MJ·MJ⁻¹) (Ohtani 1997), light transmissivity of the greenhouse (T_g; 0.47 MJ·MJ⁻¹; measured before the experiment), daily calculated ratio of solar radiation above the plant canopy to solar radiation above the shade curtain (R_c,m; MJ·MJ⁻¹), LAI at m DAT (LAI_m; m²·m⁻²), and the light extinction coefficient (k; dimensionless). The LAI_m was estimated using the obtained regression equation (r² = 0.96–0.99) between the measured LAI at the sampling dates and days after the transplanting date. The regression equations and parameters were as follows: ‘Nagano’, LAI_m = 0.038 × DAT; ‘Keessie’, LAI_m = 0.039 × DAT; ‘Mavera’, LAI_m = 0.033 × DAT; ‘Sven’, LAI_m = 0.035 × DAT; ‘Artega’, LAI_m = 0.045 × DAT; ‘Maldonado’, LAI_m = 0.036 × DAT; ‘Morbidelli’, LAI_m = 0.040 × DAT; ‘Gialte’, LAI_m = 0.034 × DAT; and ‘Jorit’, LAI_m = 0.038 × DAT. The daily changes in the R_c,m values are shown in Supplemental Fig. 1. The cumulative intercepted light (IL) per plant during 0 to 276 DAT (MJ·m⁻²) was calculated by accumulating the IL_m during 0 to 276 DAT.

LUE at 276 DAT (g·MJ⁻¹) was calculated by dividing the TDM production during 0 to 276 DAT (g·m⁻²) by the IL (MJ·m⁻²) as follows (Eq. [3]):

$\mathrm{LUE}\hspace{0.17em}=\frac{\mathrm{TDM}}{\mathrm{IL}}$[3]

Statistical analysis.

Cultivar differences in yield components were investigated using Tukey’s multiple comparison test. The significance of the obtained light extinction coefficient (k) was evaluated based on r² values and P values. Pearson’s correlation coefficients (r) among the yield components were estimated using the obtained growth and environmental data (five red cultivars, n = 15; four yellow cultivars, n = 12; total, n = 27). The correlation analysis was performed to analyze mutually relevant yield components (Watabe et al. 2021).

Results

Sweet pepper plants were grown hydroponically in a greenhouse under controlled environmental conditions (Fig. 1). The LAI (m²·m⁻²) at the sampled dates steadily increased to 7 to 9 m²·m⁻² during 0 to 183 DAT, and this increase saturated to 276 DAT for most cultivars (data not shown). The pinching growth point stopped the development of new leaves, resulting in LAI saturation. The TDM production (g·m⁻²) at the sampled dates increased during 0 to 183 DAT, and this increase saturated to 276 DAT for most cultivars (Fig. 2).

View Full Size

Changes in the total dry matter production of 10 sweet pepper cultivars grown in the greenhouse until 276 d after the transplanting date. Sampling dates were 0, 64, 127, 183, and 276 d after the transplanting date. Total dry matter production was calculated by accumulating observed dry matter at the sampling date and harvested fruit dry weight until the sampling date. Ar (A, ●), Na (A, ◆), Ma (A, ▲), Ke (A, ＋), Mv (A, ■), Ka (B, ○), Sv (B, ◇), Mo (B, △), Gi (B, ×), and Jo (B, □) in the figure represent the cultivars Artega, Nagano, Maldonado, Keessie, Mavera, Kaite, Sven, Morbidelli, Gialte, and Jorit, respectively. Values of each cultivar are shown as the mean (n = 3).

Citation: HortScience 59, 9; 10.21273/HORTSCI17904-24

• Download Figure
• Download figure as PowerPoint slide

The obtained yield components differed significantly (Table 1). Light extinction coefficients (k), LAI, LUE (g·MJ⁻¹), IL per plant (MJ·m⁻²), fruit DM_f (g·g⁻¹), TDM production (g·m⁻²), FDW (g·m⁻²), fruit DMC (g·g⁻¹), and total FFW (FFW_total; kg·m⁻²) were within the ranges of 0.58 to 0.77, 7.7 to 12.8 m²·m⁻², 2.5 to 3.5 g·MJ⁻¹, 759 to 874 MJ·m⁻², 0.43 to 0.57 g·g⁻¹, 1926 to 2570 g·m⁻², 972 to 1280 g·m⁻², 0.059 to 0.071 g·g⁻¹, and 14.6 to 21.6 kg·m⁻², respectively. The unique yield components of these cultivars were analyzed using Pearson’s correlation coefficient (r).

Table 1.

Total fruit fresh weight (FFW_total), fruit dry matter content (DMC), fruit dry weight (FDW), total dry matter production (TDM), fruit dry matter fraction (DM_f), intercepted light per plant (IL), light-use efficiency (LUE), leaf area index (LAI), and light extinction coefficient (k) of 10 sweet pepper cultivars at 276 d after the transplanting date (n = 3).

View Table

The FFW_total was determined by dividing FDW by fruit DMC. The FDW values showed a trend similar to that of the FFW_total (Table 1). For example, the high and low FFW_total values for ‘Gialte’ and ‘Mavera’ corresponded to the high and low FDW values, respectively. The FFW_total values were lower for ‘Nagano’ and ‘Artega’ than for ‘Gialte’ and ‘Morbidelli’; however, the FDW values of these cultivars did not show significant differences (Table 1). The fruit DMC values for ‘Nagano’ and ‘Artega’ were approximately 0.07 g·g⁻¹, but those of ‘Gialte’ and ‘Morbidelli’ were approximately 0.06 g·g⁻¹ (Table 1). Therefore, the approximately 17% lower fruit DMC values for ‘Gialte’ and ‘Morbidelli’ than those for ‘Nagano’ and ‘Artega’ determined the high FFW_total values for ‘Gialte’ and ‘Morbidelli’ in this case (Table 1). The FFW_total and FFW_harvest values were positively and negatively correlated with the FDW values (r = 0.83 and 0.74; P < 0.001) and fruit DMC values (r = −0.70 and −0.53; P < 0.01), respectively (Fig. 3A). The regression equation obtained from the scatter plots of the fruit DMC and FFW_total values showed significant r² values (r² = 0.50; P < 0.001) (Fig. 4). Therefore, both the FDW and fruit DMC significantly affected the FFW_total and FFW_harvest values. High FDW and low fruit DMC values were closely related to high FFW_total and FFW_harvest values.

View Full Size

Correlation coefficient (r) between yield components of sweet pepper cultivars at 276 d after the transplanting date. Nine cultivars (Nagano, Keessie, Mavera, Artega, Maldonado, Sven, Morbidelli, Gialte, and Jorit) were used for the analysis. (A, B, and C) The results of the analysis based on nine (n = 27), five (red: Nagano, Keessie, Mavera, Artega, and Maldonado; n = 15), and four (yellow: Sven, Morbidelli, Gialte, and Jorit; n = 12) cultivars, respectively. Solid and dashed lines represent analyzed yield components and those that were not analyzed, respectively. *P < 0.05, **P < 0.01, ***P < 0.001, N.S., not significant.

Citation: HortScience 59, 9; 10.21273/HORTSCI17904-24

• Download Figure
• Download figure as PowerPoint slide

View Full Size

Relationship between the fruit dry matter content and total fruit fresh weight (FFW) of 10 sweet pepper cultivars grown in the greenhouse at 276 d after the transplanting date (n = 30). Cultivars Artega, Jorit, Maldonado, Gialte, Mavera, Kaite, Nagano, Morbidelli, Sven, and Keessie were used for the analysis. For each cultivar, fruit dry matter content was calculated as the average of harvested fruits (n = 34–35). The total FFW was calculated by accumulating the immature FFW per plant measured at 276 d after the transplanting date and the mature harvested FFW per plant measured during 0 to 276 d after the transplanting date from the randomly investigated plants. Open (○) and closed (●) circles represent yellow and red cultivars (n = 15 of each color), respectively. ***P < 0.001.

Citation: HortScience 59, 9; 10.21273/HORTSCI17904-24

• Download Figure
• Download figure as PowerPoint slide

Red fruit cultivars did not show significant correlations between FFW (FFW_total or FFW_harvest) and fruit DMC; however, yellow fruit cultivars showed significant correlations (P < 0.05) (Fig. 3B and C). However, both red and yellow fruit cultivars showed significant correlations between FFW (FFW_total or FFW_harvest) and FDW (P < 0.01) (Fig. 3B and C). Using the FFW with immature fruits (FFW_total) or without immature fruits (FFW_harvest) barely affected the results of the correlation analysis during this study. Overall, for red and yellow fruit cultivars, common positive correlations between FFW and FDW were observed, whereas significant correlations between FFW and fruit DMC were observed for the red, but not for the yellow, cultivars. Using the combined data of red and yellow cultivars, both FFW_total and FFW_harvest showed positive and negative correlations with FDW and fruit DMC, respectively (Fig. 3).

The FDW was determined by multiplying fruit DM_f and TDM production. The fruit DM_f values were within the range of 0.48 to 0.57 g·g⁻¹, except for ‘Maldonado’ (Table 1). The fruit DM_f values of yellow cultivars did not show significant differences (0.53–0.57 g·g⁻¹) (Table 1). The TDM production values were within the range of 1926 to 2570 g·m⁻² (Table 1). A comparison of both components showed that the variation of the TDM production was greater (1926–2570 g·m⁻²) than that of the fruit DM_f (0.43–0.57 g·g⁻¹). The TDM production values were positively and significantly correlated with FDW (r = 0.50; P < 0.01); however, the fruit DM_f values were not (r = 0.18; not significant) (Fig. 3A). In summary, the effects of TDM production on FFW_total values were greater than those of fruit DM_f. High FDW values were closely related to high TDM production values.

Specifically, TDM production was calculated by accumulating the observed dry matter of the whole plant in this study. Additionally, IL can be estimated using Eq. [2] in this study. Then, the LUE can be estimated by dividing the TDM production by the IL. Therefore, the TDM production comprised two factors, i.e., IL and LUE. By comparing both components, the variations in the LUE (2.5 to 3.5 g·MJ⁻¹) of the cultivars were greater than the variations in the IL (759–874 MJ·m⁻²) (Table 1). Additionally, the TDM values were positively correlated with the LUE (r = 0.93; P < 0.001) and IL values (r = 0.42; P < 0.05) (Fig. 3A). The correlation coefficient was higher for LUE (r = 0.93) than for IL (r = 0.42). Therefore, although the LUE and IL values affected the TDM values, the LUE values showed a closer relationship with the TDM values. Overall, high LUE values were closely related to the high TDM values, high TDM values were closely related to the high FDW values, and, finally, high FDW values and low fruit DMC values were closely related to the high FFW_total and FFW_harvest values.

Discussion

The following yield components were measured: FFW, FDW, fruit DMC, TDM, fruit DM_f, LUE, IL per plant, LAI, and light extinction coefficient (k). These components showed significant differences among cultivars (Table 1). Both the total FFW (FFW_total; g·m⁻²) and harvested FFW (FFW_harvest; g·m⁻²) were negatively and positively correlated with the fruit DMC (g·g⁻¹) and FDW (g·m⁻²), respectively. The FDW values were positively correlated with the TDM production (g·m⁻²), and the TDM values were positively correlated with the LUE (g·MJ⁻¹). The highest FFW_total values were observed in the yellow cultivar Gialte, which was characterized by considerably low fruit DMC and high LUE values. Therefore, low fruit DMC and high LUE values were closely related to high FFW_total and FFW_harvest values.

As demonstrated by our results, high LUE values were closely related to high FFW_total and FFW_harvest values (Fig. 3A). This result is consistent with those of previous studies (Homma et al. 2023; Watabe et al. 2021), indicating a positive correlation between LUE and fresh yields. Focusing on the TDM components showed that LUE values had more effects on the TDM values than the IL values (Fig. 3). This result can be explained by light penetration of the plant canopy using the Beer–Lambert model (Eq. [1]) (Monsi and Saeki 2005). This model showed good TDM predictions for sweet pepper (Homma et al. 2022, 2024) and indicated that ∼90% to 95% of solar radiation is intercepted by the plant canopy when the LAI (m²·m⁻²) is 3 to 4 m²·m⁻² and the light extinction coefficient (k) is 0.75 (Higashide 2022). During this experiment, the LAI values of all tested cultivars were more than 4 m²·m⁻² during 127 to 286 DAT (data not shown). Additionally, the obtained light extinction coefficients (k) were in the range of 0.58 to 0.77. Therefore, the IL values at 127 to 286 DAT were similar among the cultivars (data not shown). Heuvelink and Kuerkels (2015) reported that the LAI values of sweet pepper often exceeded 6 m²·m⁻² at production sites. Therefore, the LUE differences had a greater effect on the TDM values of the year-round sweet pepper production because the LAI was maintained at high values; therefore, the IL values were similar among cultivars.

Although the fruit DM_f (g·g⁻¹) values were not closely related to the FDW values in this study, the TDM values were (Fig. 3). Similar results have also been reported by Nabeshima et al. (2019). The fruit DM_f values were similar among cultivars; however, the TDM values differed considerably (Table 1). Specifically, the fruit DM_f values were unaffected by the TDM values. Homma et al. (2023) reported similar fruit DM_f values among four red cultivars; these values did not contradict our results. For tomatoes, yield increases in Dutch cultivars have been accomplished with no increase in fruit DM_f values but with LUE values (Higashide and Heuvelink 2009) that partially corresponded with those of our study of Dutch sweet pepper cultivars. The photosynthetic ability of the plant canopy is related to the LUE values. Therefore, high-yielding cultivars of the prevalently grown greenhouse tomato and sweet pepper cultivars bred in the Netherlands may be characterized by the high photosynthetic ability of the plant canopy (i.e., LUE).

Our results indicated a negative correlation between fruit DMC and FFW (FFW_total and FFW_harvest) values (Fig. 3). Previous studies have not observed this correlation (Homma et al. 2023; Watabe et al. 2021). We believe that this inconsistency may be attributed to the fruit color of the tested cultivars. These previous studies have investigated only red cultivars; however, our study investigated red and yellow cultivars (Table 1, Fig. 3). Most of the prevalent yellow cultivars showed markedly lower fruit DMC values than those of the red cultivars (Table 1). Therefore, the average fruit DMC values of the five yellow cultivars were considerably lower than those of the red cultivars (data not shown). Based on the observed data, the reason why commercially prevalent yellow cultivars have low fruit DMC values is unknown. However, for tomatoes, the fruit DMC values are closely related to the total soluble solids (Itoh et al. 2020). Specifically, sweet tomatoes had high fruit DMC values. Similarly, Homma et al. (2023) reported that the high Brix sweet pepper cultivar Trirosso had notably higher fruit DMC values (>0.10 g·g⁻¹) than those of the other three red cultivars. Rathnayaka et al. (2021) reported that the total soluble solids of sweet pepper fruits were related to the fruit DMC values. In other words, similar to tomatoes, the sweetness of sweet pepper fruits is related to the Brix and fruit DMC values. Therefore, the tested yellow cultivars with low fruit DMC values may be characterized by low Brix values. Additionally, Marcelis et al. (1998) reported that the fruit DMC values of cucumbers changed from 2.5 to 4.0% during cultivation, indicating the effects of environmental conditions on fruit DMC values. Therefore, further research is necessary to reveal the relationship between fruit DMC values and total soluble solids or environmental conditions and clarify the effects of fruit DMC on FFW_total and FFW_harvest. These findings may support our results, especially the finding that fruit DMC is a critical determinant of the FFW_total and FFW_harvest of sweet peppers.

Although our results showed that the yellow cultivars were characterized by low fruit DMC values, Ferreira et al. (2012) reported that the total soluble solids of the yellow cultivar Eppo were higher than those of the red cultivar Margarita. These data did not support our results. We believe that the inconsistencies between our results and those of Ferreira et al. (2012) can be partially explained by the investigation of Dan et al. (1996), who reported that recently released sweet pepper cultivars had lower fruit sugar contents than those of formerly released cultivars. The fruit DMC values of the previously released yellow cultivars in the Netherlands may have been reduced by breeding. Additionally, Higashide et al. (2012b) reported that the breeding strategy may be dependent on regions or countries. This study used the cultivars bred in the Netherlands. Therefore, a comparison of fruit DMC values may be valid only for cultivars bred during the same period or close regions.

The results showed that the fruit DMC and LUE values were closely related to the FFW_total and FFW_harvest values (Fig. 3). Therefore, if controlling both components is possible, then growers can control fresh yields appropriately. In conventional practice, for example, the fruit DMC values of tomatoes can be manipulated by controlling the water supply or changing the fertility of the nutrient solution (Itoh et al. 2020, 2023; Saito and Matsukura 2014). Regarding sweet pepper, Tadesse et al. (1999) reported that the fruit DMC values increased from 0.075 to 0.10 g·g⁻¹ when fertility in substrates increased from 2.0 to 10.0 dS·m⁻¹; however, the high fertility considerably decreased the TDM production, fruit firmness, and fresh yields. In contrast, Bae and Kim (2004) reported that the fruit DMC values decreased when fertility in substrates decreased to less than 2.0 dS·m⁻¹; however, the low fertility considerably decreased the TDM production and fresh yields. Therefore, controlling the fruit DMC values with environmental controls may be impractical for sweet peppers, unlike tomatoes. Appropriate environmental controls and selecting cultivars characterized by low fruit DMC (e.g., Gialte) (Table 1) may contribute to the achievement of high fresh yield.

The LUE values are closely related to crop differences, substrate fertility, air temperature, water supply, and CO₂ concentrations in the atmosphere (Sinclair and Muchow 1999). Underground and aboveground environments can be controlled in hydroponic greenhouse production using environmental controls to achieve optimal conditions for plant growth. For underground environments, appropriate water supply and fertilizer applications increase the TDM production and fresh yields of sweet pepper (de Souza et al. 2019; Maas 1987; Tadesse et al. 1999). Although the effect of the underground environment on LUE values was not mentioned in these studies, the increased TDM values must have resulted from an increase in the IL or LUE values. Sinclair and Muchow (1999) reported that unfertilized and underground drought conditions decreased LUE. Therefore, a sufficient supply of a nutrient solution with an appropriate EC (approximately 1.5–2.5 dS·m⁻¹) (Bae and Kim 2004; Maas 1987; Tadesse et al. 1999) probably results in high and stable LUE values for sweet pepper production. The experiment in this study met these conditions (EC in the nutrient solution, 1.6 dS·m⁻¹; daily drainage rate, >30%).

Atmospheric CO₂ enrichment in greenhouses increases LUE values for sweet pepper (Homma et al. 2024; Watabe et al. 2021). To achieve high fresh yields at production sites, Russo (2012) recommended increasing the CO₂ concentration in greenhouses to 400 and 800 µmol·mol⁻¹ during summer and winter, respectively. In this case, for example, the LUE values of sweet pepper cultivar Artega at 400 and 800 µmol·mol⁻¹ were approximately 2.0 to 3.0 and 4.0 to 5.0 g·MJ⁻¹, respectively (Homma et al. 2024; Watabe et al. 2021). The increment (approximately 2.0 g·MJ⁻¹) resulting from the CO₂ enrichment was considerably larger than the LUE variations (2.5–3.5 g·MJ⁻¹) (Table 1). This experiment set CO₂ concentrations in the greenhouse with high and low periods according to conventional practice. Therefore, differences in the CO₂ responses of cultivars to produce dry matter may have affected the LUE variations in this study (2.5–3.5 g·MJ⁻¹) (Table 1). Additionally, previous studies indicated that optimal daytime and nighttime air temperature of sweet pepper were approximately 23 to 27 °C and 18 to 20 °C, respectively (Bakker 1989; Russo 2012), and these temperatures may improve the photosynthetic rate (Hanying et al. 2001; Oh and Koh 2019). Although the effect of air temperature on LUE was not mentioned in these studies, an increased photosynthetic rate with an optimal air temperature may lead to an increase in LUE (Sands 1996). Appropriate supplemental lighting increases the IL as well as the LUE of some horticultural crops (tomato, cucumber, and sweet pepper) (Kim et al. 2013; Schouten et al. 2024); however, long-term use of continuous light (i.e., 24-h photoperiod) is detrimental to plant growth (Demers and Gosselin 2000). Therefore, air temperatures within the optimal range, appropriate supplemental lighting, and high CO₂ concentrations in the greenhouse probably maintain LUE at high values. Overall, unlike fruit DMC, LUE manipulation using environmental controls is somewhat practical for sweet peppers. Selecting cultivars with high LUE (e.g., Mavera) (Table 1) and improving the LUE with appropriate environmental controls may achieve high fresh yields.

In summary, fruit DMC manipulation using environmental controls may be impractical for sweet pepper production, whereas LUE manipulation may be somewhat practical. In addition, our results showed that low fruit DMC and high LUE values were closely related to high FFW_total and FFW_harvest values. Considering this knowledge, the directions for further improvements in production are as follows: cultivars characterized by low fruit DMC and high LUE values should be selected and plants should be grown in appropriate greenhouse environments (appropriate water and nutrient supplies, high CO₂ concentration, appropriate supplemental lighting, and optimal daytime and nighttime air temperatures) to maintain high LUE values. Introducing these suggestions to production sites can improve sweet pepper productivity.

The LUE and fruit DMC values are different among cultivars, and these two factors considerably affected the FFW_total and FFW_harvest values in this study. Underlying physiological processes of the differences in LUE were implied by a model presented by Zepeda et al. (2023). Although we calculated LUE values by dividing the TDM production by the IL during this study, LUE values were alternatively interpreted as the subtraction of maintenance and growth respiration from the efficiency expressing the potential CO₂ conversion to dry matter (Zepeda et al. 2023). Maintenance and growth respiration were mainly determined by the temperature and whole structure plant dry weight. The efficiency expressing the potential CO₂ conversion to dry matter was determined by the carboxylation conductance (i.e., one of components of the canopy conductance) and CO₂ compensation point, both of which were determined with temperature. Additionally, this efficiency was determined by the LUE at very high CO₂ concentrations and the CO₂ concentration within the leaves, which was affected by boundary layer conductance and stomatal conductance. Overall, LUE differences observed during this study may have been related to the cultivar-dependent physiological responses to CO₂ concentrations and temperatures; therefore, each cultivar may have different biological coefficients (e.g., carboxylation conductance, CO₂ compensation point). Differences in these coefficients among cultivars may have contributed to cultivar-dependent LUE values observed during this study.

Underlying physiological processes of the differences in fruit DMC may have been the increased fruit DMC values during fruit growth and accumulations of natural pigments or sucrose contents within fruit. Focusing on the individual growth of sweet pepper fruits, Wubs et al. (2012) revealed that fruit DMC values gradually increased from 600 to 1000 °C·d after anthesis because of the gradual increase of FDW, whereas the FFW increased to almost 600 °C·d after anthesis. Therefore, cultivar-dependent increases in fruit dry matter at the latter growth stage may be related to the cultivar-dependent fruit DMC values. Additionally, the major red and yellow colors of fruits are attributable to carotenoid, capsanthin, capsorubin, β-carotene, and violaxanthin (Bosland and Votava 2012). Biosynthetic pathways of these pigments were investigated and revealed in previous studies (Jang et al. 2022; Wang et al. 2023). Different types of pigments within fruits may be related to the fruit DMC values that were considerably different between red and yellow colors during this study. Additionally, the total soluble solids of sweet pepper fruits were related to the fruit DMC values (Rathnayaka et al. 2021). Therefore, cultivar-dependent increases in fruit dry matter at the latter growth stage may also be related to the fruit DMC values, and these increments may be related to the accumulations of natural pigments and sucrose contents within fruits.

This study had some limitations. First, we used the light extinction coefficient (k) obtained during different years in this study. Higashide (2022) and Monsi and Saeki (2005) reported that the light extinction coefficient (k) represents specific values of crops and cultivars. Although extremely different light conditions in the greenhouse resulted from 45% and 65% of greenhouse light transmissivity in the same region, the light extinction coefficient of sweet pepper cultivar Artega showed similar values (Homma et al. 2023; Watabe et al. 2021). Therefore, we hypothesized that using the light extinction coefficient (k) obtained during different years had validity in the analysis. However, we cannot deny the possibility that morphological differences between experiment years might occurred during this experiment; therefore, this may have affected the values of the light extinction coefficient (k). Second, a limited number of cultivars were tested. Even though 10 prevalent red and yellow cultivars with large fruit sizes were used in this study, cultivars with small fruit sizes or different fruit colors (orange, white, and black) were excluded. Additionally, the cultivars were selected based on their prevalence in the Japanese market; specifically, the cultivars bred in the Netherlands were selected. Even though red and yellow colors are prevalent (Frank et al. 2001), major cultivars may differ by country or region (e.g., Nagano in Japan and Aristotle X3R in the United States) (Boyhan et al. 2020; Homma et al. 2023). Therefore, it is recommended that further studies should investigate yield components of selected cultivars with common indicators (genotypes or phenotypes). Third, our results were obtained from plants grown in optimal environments above and below the ground (Fig. 1); specifically, the effects of severe environmental conditions and the spread of diseases or insects on FFW were not considered. Practical production often suffers from fungal or viral diseases, which decrease fresh yields (Reddy 2016). The spread of diseases or viruses is partially related to biomass parameters (e.g., stem and root dry weights) (Mohammadbagheri et al. 2021) or the expression of virus-resistant genes (e.g., PMMoV-resistant L3 gene) (Matsunaga and Saito 2018), respectively. Additionally, tolerance to high air temperatures is partially related to the genotype (Saha et al. 2010; Yamazaki et al. 2020). Therefore, our results do not guarantee the anti-disease or anti-environmental stress effects on plants. Further studies of the effects of environmental stresses, diseases, or viruses on FFW that focus on yield components may be necessary.

However, greenhouse sweet peppers with prevalent cultivar are usually grown in optimal environments (e.g., optimal air temperatures, appropriate water and nutrient supplies, and low disease spread) to achieve high FFW and profit. Additionally, environmental (air temperature, CO₂ concentration, and fertigation) and growth (stem training, pinching, and stem density) conditions were similar to those at the production sites during this experiment. This study highlights the broad range of LUE and fruit DMC values among sweet peppers, and we successfully showed that this range strongly affected the fruit fresh yield. Therefore, we believe that the results obtained during this study have sufficient practical applicability at production sites.

Conclusion

Our results showed that the total FFW was positively (r = 0.83; P < 0.001) and negatively (r = –0.70; P < 0.001) correlated with the FDW and fruit DMC, respectively. The FDW was positively correlated with the TDM production (r = 0.50; P < 0.01), and the TDM production was positively correlated with the LUE (r = 0.93; P < 0.001). The cultivars with a high total FFW were characterized by the notably low fruit DMC and high LUE (e.g., Gialte). Therefore, high-yielding sweet peppers are characterized by low fruit DMC and high LUE. This knowledge is expected to support growers and breeders in exploring high-yield plants and increasing productivity.