Root-zone Temperature Effects on Spinach Biomass Production Using a Nutrient Film Technique System

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  • 1 National Agriculture and Food Research Organization, Institute of Vegetable and Floriculture Science, 3-1-1, Kannondai, Tsukuba, Ibaraki, 305-8519, Japan
  • | 2 Meiji University, 2060-1, Kurogawa, Kawasaki, Kanagawa, 215-0035, Japan
  • | 3 National Agriculture and Food Research Organization, Institute of Vegetable and Floriculture Science, 3-1-1, Kannondai, Tsukuba, Ibaraki, 305-8519, Japan

Root-zone temperature (RZT) is closely related to nutrient transportation and biomass production. However, its influence on biomass production and dry matter distribution remains unknown, especially in year-long production greenhouses. We explore the potential of RZT as an environmental control method to promote spinach field production by quantifying the effects of RZT to increase spinach production. Three RZT treatments using a nutrient film technique (NFT) system quantified and evaluated the effects of spring, summer, and winter spinach cultivation. We investigated the growth characteristics, total aboveground dry matter, and fraction of dry matter distribution to the leaf and root (which corresponded with yield). The RZT effects on total aboveground dry matter varied with the average air temperature inside the greenhouse. The total aboveground dry matter correlated positively with RZT in optimal air temperature conditions (15–20 °C). The dry matter-to-leaves ratio of the spinach did not correlate significantly with RZT in suboptimal (5 °C < air temperature < 15 °C) or supraoptimal (20 °C < air temperature) conditions. Therefore, RZT can promote biomass accumulation. We suggest RZT provides a feasible method for controlling the dry matter distribution fraction. Further research into the functional role of RZT will support hydroponic growers in improving crop yield.

Abstract

Root-zone temperature (RZT) is closely related to nutrient transportation and biomass production. However, its influence on biomass production and dry matter distribution remains unknown, especially in year-long production greenhouses. We explore the potential of RZT as an environmental control method to promote spinach field production by quantifying the effects of RZT to increase spinach production. Three RZT treatments using a nutrient film technique (NFT) system quantified and evaluated the effects of spring, summer, and winter spinach cultivation. We investigated the growth characteristics, total aboveground dry matter, and fraction of dry matter distribution to the leaf and root (which corresponded with yield). The RZT effects on total aboveground dry matter varied with the average air temperature inside the greenhouse. The total aboveground dry matter correlated positively with RZT in optimal air temperature conditions (15–20 °C). The dry matter-to-leaves ratio of the spinach did not correlate significantly with RZT in suboptimal (5 °C < air temperature < 15 °C) or supraoptimal (20 °C < air temperature) conditions. Therefore, RZT can promote biomass accumulation. We suggest RZT provides a feasible method for controlling the dry matter distribution fraction. Further research into the functional role of RZT will support hydroponic growers in improving crop yield.

RZT is a crucial factor that influences plant growth and biomass accumulation in an NFT system. RZT affects several physiological processes, including biosynthesis, ion uptake, respiration, accumulation, and distribution (Atkin and Tjoelker, 2003; Yasushi et al., 2014; Zhang et al., 2008). Moreover, the low running cost required to control RZT makes it a feasible approach to maintain plant growth and improve field production under greenhouse cultivation (Yasushi et al., 2013). In tomato, fruit yield increased when the RZT was cooled to 25 °C in high air-temperature conditions. The increases in growth and fruit yield were caused by phycological activities, such as root activity, stomatal conductance (gS), and dry matter partitioning (Dodd et al., 2000; Ye et al., 2003).

RZT is one of the many controlled environmental factors in an NFT system. It is necessary to quantify the effects of RZT to determine the most effective range for optimal growth and production. In principle, the mean air temperature (MAT) and RZT are two important factors affecting the growth, accumulation, and distribution of both the aerial and underground segments of plants. For most horticultural crops, there is an optimal MAT and RZT range that supports growth and division. Especially in greenhouse production, MAT and RZT are usually controlled with planting schedules and climate variations. When the MAT and RZT are less than the threshold, the suboptimal environment is not sufficient for optimal plant growth. Conversely, when the MAT and RZT are more than the optimal range, plant growth is supraoptimal, which increases plant stress. So, creating optimal conditions has focused on the optimal range of MAT as an important index to control and enable year-round production. For example, in Solanum lycopersicum (tomato) cultivation, the daily temperature range is 25 to 28 °C during the day, with nighttime temperatures of 10 to 15 °C.

In contrast, the effect of RZT is much less specific, and previous quantification experiments demonstrated it is insufficient for used in production systems (Baumbauer et al., 2019). Conversely, recent research has confirmed that RZT can affect shoot growth significantly, and roots play an important role in the adaptation of plants to temperature fluctuations (Ye et al., 2003; Zhang et al., 2008). Several studies investigating root morphology have observed that controlling RZT thermotolerance of certain temperate vegetable crops can reduce the effects of fluctuating diurnal ambient temperatures (Jie et al., 2019; Zhang et al., 2008). Previous research has also indicated that roots may respond to root temperature to regulate the growth of shoots and photosynthesis in different growing conditions (Carotti et al., 2021; Jie et al., 2019; Malcolm et al., 2014). The influence of supraoptimal RZTs on growth is related to gS. Previous data revealed the factors controlling leaf initiation and development were independent of the factors controlling gS in aeroponically grown plants at different root temperatures (Dodd et al., 2000).

The effect of RZT is strongly influenced by other environmental factors in greenhouse conditions, especially seasonal variations. Despite the known impact of RZT on several physiological processes, the potential of optimizing RZT for improved crop production in different seasons has not been evaluated previously. In most year-round production commercial greenhouses, solar radiation is still the main energy resource. The MAT inside the greenhouse varies with variations in outside climate. In addition, energy use efficiency, including light use efficiency (LUE), varies with the changes in the environment in greenhouses (Solbach et al., 2021). The cultivation of plants is just in such a synthetic environment. Understanding seasonal variation RZT effects on crop production is the aim of this work (Fig. 1). We selected spinach as our experimental species because it is cultivated commercially using NFT systems for year-round production. To determine the optimal RZT in year-round cultivation environments, we analyzed the effects of RZT on plant performance, biomass production, and LUE. The results of this study provide a basis for further optimization of the optimal growing environment and also widen the applications of seasonal cooling and heating of the root system during greenhouse production.

Fig. 1.
Fig. 1.

Schematic illustration of the effects of root-zone temperature on spinach growth. Blue represents the effects of environmental factors aboveground on the growth of plants that have been confirmed in other experiments; orange indicates the promotion effect of root-zone temperature and mean air temperature defined in this experiment. CO2 = carbon dioxide.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Materials and Methods

Three experiments were conducted at the Institute of Vegetable and Floriculture Science, National Agriculture and Food Research Organization, Japan (lat. 36.04N, long. 140.03E) in 2019 and 2020. The first trial was carried out from 7 to 31 Aug. (middle summer), the second from 4 to 30 Oct. (middle autumn), and the third from 11 Dec. to 21 Jan. (middle winter). The experimental greenhouse is a standard even-span greenhouse with a polyethylene cover. The environment in the experimental greenhouse was based on spinach production greenhouses. In winter, the glasshouse was maintained at 12-/12-h day/night temperatures of 15/10 °C. In autumn, the glasshouse was maintained at 12-/12-h day/night temperatures of 25/15 °C. In summer, the natural ventilation system and inside shading were used, and the glasshouse was maintained at a 12-/12-h day/night temperature of 30/25 °C. In each trial, seeds of spinach (NPL8; Mitsubishi Chemical Agri Dream Co., Ltd., Tokyo, Japan) were sown in 288-cell trays, and seedlings were grown for 10 d in a growth chamber with a day/night air temperature of 22/19 °C. The photoperiod was 12/12 h (day/night). The nutrient solution had an electrical conductivity (EC) of 1.5 dS⋅m–1 and a carbon dioxide concentration of 1000 ppm. The seedlings were transplanted into a nutrient film hydroponic system with a planting density of 75 plants/m2 (Fig. 2).

Fig. 2.
Fig. 2.

Growth of spinach in the production by the nutrient film technique system. Seedlings were cultured in a growth chamber with a day/night air temperature of 22/19 °C. The seedlings were transplanted and grown in an experimental greenhouse.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

RZT was controlled after transplanting, with the nutrient solution temperature maintained at the target temperature, which was either 15 °C (RZT-15), 20 °C (RZT-20), or 25 °C (RZT-25) (Fig. 3). During the winter and autumn growing cycles, RZTs were maintained using water heaters in each nutrient tank (Fig. 3A). During the summer growing cycle, RZTs were achieved by mixing the water in cooling tanks and nutrient solution in each tank (Fig. 3B). The water quantity in the tanks was adjusted based on temperature sensors and electromagnetic valves. The nutrient solution was maintained at an EC of 2.6 to 3 m–1 by weekly review of the EC and pH; the results were used to alter the nutrient solution to the optimal level for spinach absorption. Air temperature, RZT, light intensity, and humidity were monitored continuously from the day of transplanting until the total length of spinach reached 25 cm, which is the standard length for harvesting. Standard commercial spinach management practices were used throughout the experiment. Figure 4 shows the MATs for each trial. The average temperatures were 13.6 °C, 21.0 °C, and 27.8 °C in winter, autumn, and summer, respectively. The temperature in autumn was close to the optimum for spinach development. In all temperature treatments, the RZTs were maintained within an average of ±0.7 °C of the target temperature. The temperature variations between RZT and MAT in each treatment are shown in Fig. 5. The temperature difference between the RZT and MAT in the NFT system was affected by season. During the summer, the maximum temperature difference was more than 15 °C; in winter, the maximum temperature difference was –14 °C.

Fig. 3.
Fig. 3.

Schematic representation of the experimental setup. (A) In the winter and autumn growing cycles, root-zone temperature (RZT) was achieved by using water heaters in each nutrient tank. (B) In the summer growing cycle, the RZT was achieved by mixing the water in the cooling tanks with groundwater.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Fig. 4.
Fig. 4.

Temperature variations inside the experimental greenhouse in the winter, autumn, and summer growing cycles.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Fig. 5.
Fig. 5.

Temperature differences between root-zone temperature and mean air temperature during the seasonal experiments using the nutrient film technique system.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

During each trial, representative plants were assessed on days 0, 7, 14, and 21 of the RZT treatments (n = 6). Plant length, leaf area, fresh weight, and dry weight (dried in a ventilated oven for 72 h at 80 °C) were recorded. For each treatment, the leaf area of each replicate was measured (LI31000C; LI-COR Biosciences, Lincoln, NE) once a week.

The regression analysis between dry weight, length, fresh weight, leaf area, and RZT involved linear fitting tools based on 12 sample specimens for each replicate of each treatment. Analysis of variance was used to determine the dry matter ratio to provide the specific leaf area (SLA) ratio, which indicates the size of the leaf area the plant creates for the most efficient light use for a given volume of leaf biomass. These data were used to compare the three experiments using a two-way analysis of variance with Tukey’s test. Significance was determined at P < 0.05 (OriginPro 2021b; OriginLab Corp., Northampton, MA).

SLA was expressed as the quotient of LA of the dry mass of the leaves.
SLA=LA/W

where LA is the area of all leaves (cm2) and W (g) is the dry mass of those leaves.

The light use efficiency (LUE, g⋅MJ−1) of a canopy was expressed as the quotient of the dry weight (DW, g) of the plant to G_total, with total light energy (MJ) at intermediate and final harvests.
LUE=DW/G_total

The dry matter ratio (DMR, g/g) was expressed as the quotient of the dry matter weight (g) to the fresh weight (FW, g).

The G_total was the accumulated light quality (MJ).
DMR=DW/FW

Results

Morphological characteristics of hydroponic spinach in seasonal climates.

To examine the effect of seasonal climate on spinach growth, the data were grouped by season. The increase in dry weight, fresh weight, leaf area, and length of plants showed an increasing pattern in all seasons, but the speed of increase varied (Fig. 6). The fastest increase in dry weight over the whole year was in winter and autumn and reached 2 ± 0.5 g/plant at harvest (Figs. 7 and 8). The increase in dry weight was faster in winter than in autumn. At harvest, the winter dry weight reached 1.0 ± 0.5 g/plant and the summer dry weight reached 0.8 ± 0.5 g/plant (Figs. 7 and 9). The increase in leaf area followed a similar trend. In autumn, the greatest individual leaf area was 840 cm2/plant and the least was only 400 cm2/plant. The average leaf area in autumn was 620 ± 100 cm2/plant. The leaf area in summer was 230 ± 100 cm2/plant and the leaf area in winter was 200 ± 100 cm2/plant.

Fig. 6.
Fig. 6.

Changes in total plant dry weight, fresh weight, leaf area, and leaf length in different seasons. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each season.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Fig. 7.
Fig. 7.

Winter variations in total plant dry weight, fresh weight, leaf area, and leaf length. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each root-zone temperature treatment (15, 20, and 25 °C).

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Fig. 8.
Fig. 8.

Autumn variations in total plant dry weight, fresh weight, leaf area, and leaf length. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each root-zone temperature treatment (15, 20, and 25 °C).

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Fig. 9.
Fig. 9.

Summer variations in the total plant dry weight, leaf area, fresh weight, and leaf length. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each root-zone temperature treatment (15, 20, and 25 °C).

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

The average fresh weight was greatest in autumn at 31 ± 10 g/plant at harvest. In contrast to dry weight, the leaf area and fresh weight were similar in summer and winter, with small amounts of 14 ± 5 and 13 ± 5 g/plant, respectively. This trend suggests the fresh weight in summer and winter did not vary. In winter, the air temperatures were suboptimal (5 °C < air temperature < 15 °C); in summer, the air temperatures were supraoptimal (20 °C < air temperature).

Plant leaf length increased with accumulation in temperature, which is the sum of the average air temperature from transplant day to harvest day, with the greatest increase in autumn. In autumn, the largest average leaf length was ≈27 ± 4 cm, at the accumulation temperature of 500 °C. In summer and autumn, the average leaf length was 21 ± 2 and 17 ± 5 cm, respectively, at an accumulation temperature of 500 °C. Therefore, the necessary accumulated temperature is more than 500 °C for year-round harvesting.

RZT treatment effects on growth efficiency.

To examine the effects of RZT on the seasonal growth of spinach, data were grouped by RZT in each season. For each season, the RZT treatments had different effects on the dry weight, fresh weight, leaf area, and leaf length (Fig. 10).

Fig. 10.
Fig. 10.

Variations in (A) dry matter ratio, (B) specific leaf area, and (C) light use efficiency in each root-zone temperature treatment (15, 20, and 25 °C) under seasonal climates summer, autumn, and winter). The data in the upper row are mean ± se of the means (n = 6). Results of the least significant difference multiple range test are presented in the bottom row. Significant at *P ≤ 0.05, **P ≤ 0.01, or ***P ≤ 0.001.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

In winter, MAT was less than RZT, and the aerial sections of the spinach were in a suboptimal environment. Neither the high RZT treatment (RZT-25; average dry weight, 0.62 ± 0.4 g/plant) or the low RZT treatment (RZT-15; average dry weight, 0.76 ± 0.4 g/plant) improved the results. The middle RZT treatment (RZT-20) had the greatest dry weight. A similar pattern occurred with fresh weight, with the greatest fresh weight of 17 ± 5 g/plant in the RZT-20 treatment. Spinach grown at RZT-20 had more leaves and a greater leaf area than the RZT-25 and RZT-15 spinach. The lengths of the aerial sections of the spinach were not statistically different among all three RZT treatments (P ≥ 0.05).

In autumn, the MAT was ≈21 °C, which is the optimal temperature for spinach growth. The dry and fresh weights were similar in all three RZT treatments. The dry weight was 1.60 ± 0.4, 1.65 ± 0.4, and 1.70 ± 0.4 g/plant in RZT-25, RZT-20, and RZT-15, respectively. Leaf area for RZT-25 and RZT-20 was similar at harvest, which was greater than the leaf area in RZT-15. The RZT treatment did not affect leaf length significantly during the experimental period in both RZT-20 and RZT-25 treatments.

In the summer experiment, the MAT was more than 27 °C, which is a supraoptimal condition for spinach growth. The low RZT treatments improved spinach growth in the hot summer environment. In RZT-15, dry weight reached 1.5 ± 0.4 g/plant; in RZT-20, the dry weight was 1.0 ± 0.4 g/plant. The lowest dry weight occurred in RZT-25, which was 0.7 ± 0.4 g/plant. Variations in the leaf area and fresh weight showed a similar pattern, with the greatest leaf area and fresh weight in RZT-15. The leaf area in RZT-15 reached 420 ± 100 cm2/plant. The height increase also improved in the decreased RZT. The length of the spinach in RZT-15 and RZT-20 was longer than in RZT-25, but there was no significant difference between RZT-15 and RZT-20.

Dry matter ratio.

The dry matter ratio varied with the seasons. The greatest dry matter ratio occurred in winter, with a corresponding value of 0.07 g/g. The dry matter ratios in summer and autumn were less than those in winter, and ranged from 0.004 to 0.06 g/g. The low RZT treatment improved the dry matter ratio under superoptimal conditions. In summer, the dry matter ratio was significantly different between RZT-15 and RZT-25. The average dry matter ratio was 0.04 in RZT-25 and 0.06 in RZT-15. There was no significant differences among any RZT treatments in autumn or winter.

Specific leaf area.

SLA is an index that determines how much new leaf area is involved in each unit of biomass produced. Seasonal climate changes affected SLA. In our study, the lower RTZ limited the leaf area expansion in the high-air temperature environment. The SLA indexes were greater in winter and autumn than in summer. However, there were no significant differences among RZT treatments in winter. In summer, the SLA indexes were significantly different among all RZT treatments. In autumn, there was a significant difference between RZT-15 and RZT-25.

Light use efficiency.

LUE had seasonal climate variations. The LUE in winter was less than in summer and autumn. The average LUE in winter was ≈0.3, 0.9 g⋅MJ–1 in autumn, and 1.0 g⋅MJ–1 in summer. In winter, there were no significant differences in LUE among any of the RZT treatments. A slightly lower LUE occurred in the RZT-25 treatment compared with summer and autumn.

Discussion

Effect of seasonal variations on morphological characteristics of hydroponic spinach.

Temperature is a key environmental factor influencing plant growth. Our results demonstrate that RZT affects plant growth and partitioning of dry mass (Dodd et al., 2000; Gent, 2017; Malcolm et al., 2014; Ye et al., 2003). Leaf area, plant biomass, shoot-to-root ratio, and the relative growth rate varied significantly among the different RZT treatments. Our results suggest that RZT induced chemical signals and increased the tolerance of aerial sections to environmental stress. Our results also support previous research, in which spinach growth was promoted by low RZTs during a hot summer (Jie et al., 2019).

Our results add to this, demonstrating that seasonal variation is critical for evaluating the effect of RZT. Total dry weight, leaf area, fresh weight, and length of leaves varied with seasonal climates. Spinach growth in autumn was faster than in summer and winter, demonstrating the fastest growth occurs in optimal MAT conditions. A greater production of spinach could be cultivated in autumn after the same accumulated temperature as the other seasons. Dry matter per plant was 20% greater in autumn compared with the other seasons. Stem elongation and leaf expansion were the greatest in relatively optimal MAT conditions. Dry weight, leaf area, fresh weight, and leaf length were strongly affected by season. Therefore, MAT should be considered when optimizing RZT.

Physiological activities such as stomatal and internal structures are related to different RZTs (Dodd et al., 2000; Zhang et al., 1997). Growth and nutrient uptake improved with root-zone cooling when grown in high air temperatures, were promoted by root-zone heating at low air temperatures, and have a low production cost. Hence, combining the effect of MAT with RZT is a growth stress regulator (Yasushi et al., 2013, 2014).

In our study we confirmed the cooperative effects of MAT and RZT by comparing the morphological characteristics of spinach during cultivation at different root and ambient temperatures. Throughout the year, growth is influenced mainly by MAT. Although the RZT treatment promoted the growth of spinach to some extent, the effect of a controlled RZT was limited and much less than that of MAT. For instance, by controlling RZT, spinach production can be improved, but the improvement does not reach the optimal production level in autumn.

Effect of RZT on the growth of hydroponic spinach in different seasons.

Previous studies have highlighted the significant effects of manipulating RZT on plant growth. For instance, a relatively lower temperature is more favorable for flavonoid accumulation (Lin et al., 2017). We also found that controlling RZT could be a useful approach for improving at sub- or supraoptimal temperatures. In addition, our study compared the growth of spinach in summer, autumn, and winter with RZTs for optimal production. The effect of RZT was dependent on MAT variations (Fig. 11). In general, when MAT is sub- or supraoptimal, RZT may regulate the growth of spinach. However, there were no significant differences in dry weight, leaf area, fresh weight, and leaf length in RZT-15, RZT-20, and RZT-25 during optimal MAT conditions in autumn. These observations confirm that RZT did not affect growth in an optimal environment significantly. In sub- and supraoptimal MAT, RZT affected dry weight, leaf area, and leaf length significantly. In summary, the regulatory effect of RZT on the growth of hydroponic spinach is not always effective. The seasonal climate should be considered when controlling RZT.

Fig. 11.
Fig. 11.

Effect of root-zone temperature (RZT) on the growth speed of spinach in year-round cultivation environments. The solid lines represent the common performance of the plant without RZT; the dashed lines represent the performance of the plant by regulating the RZT confirmed in the experiment. Arrows indicate the variation of growth speed under different temperature conditions.

Citation: HortScience 57, 4; 10.21273/HORTSCI16499-22

Perspectives and implications of RZT control for future applications.

The dry-to-fresh weight ratio is an important plant parameter because it determines the level of photosynthesis production during cultivation, and reflects the quality of spinach directly. In sub- and supraoptimal MAT conditions, the control of RZT improved the dry-to-fresh weight ratio, contributing to an increased quality of marketable product. The balance between photosynthesis and respiration is regulated by RZT, resulting in an increase in the dry-to-fresh weight ratio (Gent, 2016; Ryan, 1991). When compared with other control methods, it is a feasible approach to improve field production in temperatures outside the optimal range.

Studies have confirmed that leafy vegetables grown in high temperatures can result in poor production quality (Koevoets et al., 2016). In our study, a low RZT reduced the negative effects of high temperatures in summer cultivation, allowing photosynthesis to transfer energy into biomass. In addition to the positive effect on stomal and photosynthesis, we demonstrated that LUE was associated with MAT. LUE was affected by the background temperature (season); it was significantly greater in summer and autumn than in winter. In summer and autumn, the average LUE was more than 0.5 g⋅MJ–1, whereas in winter it was ≈0.2 g⋅MJ–1. The greatest LUE (0.8 g⋅MJ–1) occurred in the RZT-20 treatment with optimal MAT. The seasonal variation in dry mass production in our trials can be explained by reduced rates of photosynthesis.

In addition, unlike MAT, the high RZT treatment (RZT-25) did not improve LUE in the suboptimal MAT of winter. Conversely, a large amount of biomass was in the root section, with a decreased accumulation in the aerial sections. Therefore, MAT should be considered before controlling RZT in field production systems.

Conclusion

The cooperation of RZT and MAT influences growth regulation, biomass production, and dry matter distribution. We compared spinach growth at RZT-15, RZT-20, and RZT-25 in different seasons in a year-round cultivation system to evaluate the effect of RZT. The high RZT (RZT-25) did not contribute to an increase in LUE, or root or dry matter fraction of the aerial sections in any season. Furthermore, MAT was more influential than RZT, and the regulation effect of RZT was effective only when MAT was outside the optimal growth range. Our results highlight that the accumulation of biomass is a complex system and that controlling RZT should be combined with seasonal variations in MAT. Therefore, we suggest RZT be maintained at less 20 °C during year-round greenhouse production. The results of this study provide a starting point for further optimizing controlled environments in plant production systems, and also widen our understanding of the basic regulatory effects on plant growth.

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  • Yasushi, K., Matsuo, S., Kanayama, Y. & Kanahama, K. 2014 Effect of root-zone heating on root growth and activity, nutrient uptake, and fruit yield of tomato at low air temperatures J. Jpn. Soc. Hort. Sci. 83 4 295 301 https://doi.org/10.2503/jjshs1.MI-001

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  • Yasushi, K., Matsuo, S., Suzuki, K., Kanayama, Y. & Kanahama, K. 2013 Root-zone cooling at high air temperatures enhances physiological activities and internal structures of roots in young tomato plants J Jpn. Soc. Hort. Sci. 82 4 322 327 https://doi.org/10.2503/jjshs1.82.322

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  • Ye, Z., Huang, L., Bell, R.W. & Dell, B. 2003 Low root zone temperature favours shoot B partitioning into young leaves of oilseed rape (Brassica napus) Physiol. Plant. 118 213 220 https://doi.org/10.1034/j.1399-3054.2003.00085.x

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  • Zhang, F., Dashti, N., Hynes, R.K. & Smith, D.L. 1997 Plant growth-promoting rhizobacteria and soybean [Glycine max (L.) Merr.] growth and physiology at suboptimal root zone temperatures Ann. Bot. 79 243 249 https://doi.org/10.1006/anbo.1996.0332

    • Search Google Scholar
    • Export Citation
  • Zhang, Y.P., Qiao, Y.X., Zhang, Y.L., Zhou, Y.H. & Yu, J.Q. 2008 Effects of root temperature on leaf gas exchange and xylem sap abscisic acid concentrations in six Cucurbitaceae species Photosynthetica 46 3 356 362 https://doi.org/10.1007/s11099-008-0065-1

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Contributor Notes

This work was supported by the Ministry of Agriculture, Forestry and Fisheries (MAFF) commissioned project study on future agricultural production using artificial intelligence (grant no. JP18064796).

We thank Takano Nobuo for help in preparing the experiment materials used in this work.

R.W. is the corresponding author. E-mail: wanr309@affrc.go.jp.

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    Schematic illustration of the effects of root-zone temperature on spinach growth. Blue represents the effects of environmental factors aboveground on the growth of plants that have been confirmed in other experiments; orange indicates the promotion effect of root-zone temperature and mean air temperature defined in this experiment. CO2 = carbon dioxide.

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    Growth of spinach in the production by the nutrient film technique system. Seedlings were cultured in a growth chamber with a day/night air temperature of 22/19 °C. The seedlings were transplanted and grown in an experimental greenhouse.

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    Schematic representation of the experimental setup. (A) In the winter and autumn growing cycles, root-zone temperature (RZT) was achieved by using water heaters in each nutrient tank. (B) In the summer growing cycle, the RZT was achieved by mixing the water in the cooling tanks with groundwater.

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    Temperature variations inside the experimental greenhouse in the winter, autumn, and summer growing cycles.

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    Temperature differences between root-zone temperature and mean air temperature during the seasonal experiments using the nutrient film technique system.

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    Changes in total plant dry weight, fresh weight, leaf area, and leaf length in different seasons. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each season.

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    Winter variations in total plant dry weight, fresh weight, leaf area, and leaf length. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each root-zone temperature treatment (15, 20, and 25 °C).

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    Autumn variations in total plant dry weight, fresh weight, leaf area, and leaf length. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each root-zone temperature treatment (15, 20, and 25 °C).

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    Summer variations in the total plant dry weight, leaf area, fresh weight, and leaf length. Lines are significant (P < 0.05) linear regressions fitted in OriginPro (2021b; OriginLab Corp., Northampton, MA). Linear regression lines of each season’s color are indicated. Each data point shows the sampling data in each root-zone temperature treatment (15, 20, and 25 °C).

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    Variations in (A) dry matter ratio, (B) specific leaf area, and (C) light use efficiency in each root-zone temperature treatment (15, 20, and 25 °C) under seasonal climates summer, autumn, and winter). The data in the upper row are mean ± se of the means (n = 6). Results of the least significant difference multiple range test are presented in the bottom row. Significant at *P ≤ 0.05, **P ≤ 0.01, or ***P ≤ 0.001.

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    Effect of root-zone temperature (RZT) on the growth speed of spinach in year-round cultivation environments. The solid lines represent the common performance of the plant without RZT; the dashed lines represent the performance of the plant by regulating the RZT confirmed in the experiment. Arrows indicate the variation of growth speed under different temperature conditions.

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  • Yasushi, K., Matsuo, S., Kanayama, Y. & Kanahama, K. 2014 Effect of root-zone heating on root growth and activity, nutrient uptake, and fruit yield of tomato at low air temperatures J. Jpn. Soc. Hort. Sci. 83 4 295 301 https://doi.org/10.2503/jjshs1.MI-001

    • Search Google Scholar
    • Export Citation
  • Yasushi, K., Matsuo, S., Suzuki, K., Kanayama, Y. & Kanahama, K. 2013 Root-zone cooling at high air temperatures enhances physiological activities and internal structures of roots in young tomato plants J Jpn. Soc. Hort. Sci. 82 4 322 327 https://doi.org/10.2503/jjshs1.82.322

    • Search Google Scholar
    • Export Citation
  • Ye, Z., Huang, L., Bell, R.W. & Dell, B. 2003 Low root zone temperature favours shoot B partitioning into young leaves of oilseed rape (Brassica napus) Physiol. Plant. 118 213 220 https://doi.org/10.1034/j.1399-3054.2003.00085.x

    • Search Google Scholar
    • Export Citation
  • Zhang, F., Dashti, N., Hynes, R.K. & Smith, D.L. 1997 Plant growth-promoting rhizobacteria and soybean [Glycine max (L.) Merr.] growth and physiology at suboptimal root zone temperatures Ann. Bot. 79 243 249 https://doi.org/10.1006/anbo.1996.0332

    • Search Google Scholar
    • Export Citation
  • Zhang, Y.P., Qiao, Y.X., Zhang, Y.L., Zhou, Y.H. & Yu, J.Q. 2008 Effects of root temperature on leaf gas exchange and xylem sap abscisic acid concentrations in six Cucurbitaceae species Photosynthetica 46 3 356 362 https://doi.org/10.1007/s11099-008-0065-1

    • Search Google Scholar
    • Export Citation
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