INVITED REVIEWS
Analysis of Dry Matter Production and Improvement in High Soluble Solids Fruits Production in Tomato Grown Using Short-term, Low-truss Crop Management: A Review
2025 Volume 94 Issue 2 Pages 109-116
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2025 Volume 94 Issue 2 Pages 109-116
Short-term, low-truss (STLT) crop management is used in commercial greenhouses in Japan, usually for greenhouse tomatoes for year-round harvest and sale. When STLT crop management was first developed, it was used to produce high-Brix tomatoes by managing stress control through water and nutrient supply. It has recently been used by regular tomato growers. In STLS crop management, light use efficiency was significantly and positively correlated with CO2. Based on this relationship, dry matter (DM) production was predicted and the predicted DM was not significantly different from the observed DM when the average daytime CO2 concentration ranged mainly between about 400 and 650 μmol·mol−1. In STLT crop management, there was no significant difference in DM production between high Brix and regular tomatoes. Thus, in high Brix tomatoes, only a significant increase in fruit DM content induced a significant reduction in fruit fresh weight and increase in fruit Brix. In plants grown with salinity treatment, fruit Brix was significantly and highly correlated with the cumulative electrical conductivity (EC) of the drainage (cECd). Fruit Brix could be predicted and controlled based on cECd. Originating in Japan, high-Brix tomato production has reached Europe and other Asian countries, where demand is expected to increase. STLT crop management will support this increase. Models for yield and Brix could help growers.
Tomato (Solanum lycopersicum L.) is one of the most widely grown and eaten foods in the world. Among crops grown as vegetables, tomato production is the highest, with 186 million tones grown on 4.9 million ha worldwide in 2022 (FAOSTAT, 2022). In Japan, tomato ranks first in agricultural value and accounts for 10% of vegetable production (MAFF, 2023). Around the world, consumers consider taste, aroma, and health aspects as important qualities of tomato (Dorais et al., 2001). Japanese consumers and retailers all require a variety of sizes and year-round supply (Iwasaki et al., 2019; Nakano, 2020). Many companies in Japan have started to produce tomatoes in large greenhouses (Yamada, 2008), employing many workers year-round. To respond to consumers and retail demand, many cultivars are grown all over Japan in all seasons, using various crop management systems (Nakano, 2020). Short-term, low-truss (STLT) crop management and high soluble solids tomato management are now employed commercially (Fig. 1; Watanabe, 2006). In this review, we explain dry matter (DM) production by such plants and the use of models to predict fruit Brix.
Tomatoes grown using short-term, low-truss crop management.
STLT crop management is used in some commercial greenhouses in Japan, usually for greenhouse tomatoes. Tomato plants are grown at high density and are pinched above the 1st to the 3rd truss (Fig. 1; Itoh et al., 2020a; Watanabe, 2006). This practice is called low-node-order pinching, a high-density planting system (Ayarna et al., 2021; Iwasaki et al., 2019; Nakayama et al., 2021; Watanabe, 2006; Zhang et al., 2015), single-truss, high-density tomato production system (Johkan et al., 2014), or low-truss tomato production (Higashide, 2022).
STLT crop management is normally used for year-round harvest and sales (Watanabe, 2006). For year-round production, tomato plants are replaced several times a year in the same greenhouse and are grown in several greenhouses with different growing schedules (Watanabe, 2006). This brings advantages (Itoh et al., 2020a; Nakayama et al., 2021; Watanabe, 2006) because plants do not grow tall, so there is no need for a tall greenhouse, a high wiring system, or special trolleys. Because each crop is short lived, cultivars can be chosen according to the season; data on the relationship between the environment and plants for different seasons can be collected quickly and plant stress can be controlled to ensure high-Brix tomato production. There is no need to maintain the plants growing in upper trusses. Although it facilitates consistent year-round sales and employment, it requires many seeds or seedlings, a nursery system to supply seedlings year-round, complex planning, and a lot of labor to transplant and replacing the plants (Itoh et al., 2020a; Watanabe, 2006).
When STLT crop management was first developed, it was used to produce high-Brix tomatoes by facilitating stress control through water and nutrient supply (Watanabe, 2006). It was recently chosen by regular tomato growers for its additional advantages. A company in Saitama prefecture uses it to facilitate year-round selling and to reduce the risks associated with high wiring systems and special trolleys (Saitama Agricultural Technology Research Center (SATRC), 2020). Nakayama et al. (2021) used it in subtropical Japan to reduce yield losses caused by high temperatures. A low-volume-substrate cultivation system was developed and combined with it to reduce the costs of nutrients and substrate (Zhang et al., 2015), and was introduced in Ghana, reducing the overall costs while increasing yields of tropical tomato cultivars (Ayarna et al., 2021).
In STLT crop management, plants are grown repeatedly using hydroponics. However, root diseases can flourish in hydroponic systems (Vallance et al., 2011), especially in recirculating systems (Ehret et al., 2001). Heavy crop losses can result from fungi, oomycetes, nematodes, and viruses (Feng et al., 2018; Katan, 2017; SATRC, 2020).
To remove plant pathogens from irrigation water and hydroponic solutions, membrane filtration is used (Itoh and Iwasaki, 2018; Machado et al., 2013; Ohtani et al., 2000; Raudales et al., 2014; Stewart-Wade, 2011). Ralstonia solanacearum causes bacterial wilt disease in many crops, including tomato, in both soil and hydroponics (Caldwell et al., 2017; Fujiwara et al., 2012). A polyvinylidene fluoride ultra-filtration (PVDF UF) membrane (Minegishi and Matsuka, 2007) completely removed R. solanacearum (Itoh and Iwasaki, 2018), and was more effective than a previous filtration method (Raudales et al., 2014). This may have high potential for removing other pathogens and for disease control, since most pathogens are similar in size to, or larger than, R. solanacearum (Itoh and Iwasaki, 2018). However, there is a risk of recontamination of the recirculating solution after filtration: R. solanacearum increased more following filtered drainage than with unfiltered drainage (Itoh and Iwasaki, 2018), as it followed UV irradiation of the recirculating solution (Vallance et al., 2011). Rapid increases in pathogen numbers may be due to lack of competition from other microorganisms (Paulitz and Bélanger, 2001; Vallance et al., 2011). Accordingly, PVDF UF membrane filters should be installed after the drainage tank so that the filtered drainage can be applied directly to plants (Itoh and Iwasaki, 2018).
Higashide and Heuvelink (2009) defined a hierarchy of tomato yield components and growth characteristics based on DM production, in which a component in one level affects one in the next, and all of which can be measured or calculated. The hierarchy allows the effect of each component on DM production, and thus fresh yield, to be evaluated.
The hierarchy was used to show an improvement in light use efficiency (LUE) by breeding improved yield in Dutch tomato cultivars (Higashide and Heuvelink, 2009), and that both were improved by CO2 elevation and fogging in the greenhouse (Higashide et al., 2015) and by grafting on Dutch rootstock cultivars (Higashide et al., 2014). In cucumber, it was used to compare growth and yield of Japanese cultivars (Higashide et al., 2012; Maeda and Ahn, 2021), crop management including pinching and lowering (Higashide et al., 2012), and plant stages (Maeda et al., 2022). It showed that improvement in LUE may increase the yield of sweet peppers (Watabe et al., 2021), and that DM influenced fluctuations in the fruit set and yield of sweet peppers (Homma et al., 2022).
Using DM production models and non-destructive measurements, Saito et al. (2020a) predicted yields of greenhouse tomatoes. Saito et al. (2020b) controlled the temperature, CO2 concentration, and Leaf area index (LAI) and increased yields to > 50 kg·m−2·year−1. The predicted DM production was within the range of the observed standard deviation (SD).
These DM production models were used to predict yields of other fruit vegetables. Maeda and Ahn (2021) predicted the fresh yield of cucumber to within ± 5% at 200 days after transplanting (DAT). Watabe et al. (2022) predicted that of sweet pepper to within ± SD of the observed yield at 200 DAT. Homma et al. (2023) improved the accuracy of yield prediction in sweet pepper by adding fruit set models to the DM production models.
Kaneko et al. (2015) investigated the effects of planting stage and plant density on DM production, and Ohkubo et al. (2019) investigated the effects of the number of leaves per plant under STLT crop management. Light interception by plants appeared to determine DM production, depending on planting stage, plant density, and number of leaves.
Itoh et al. (2020a) grew tomato plants in six seasons under STLT crop management. The average daytime CO2 concentration differed significantly among the experiments, along with growing days, daytime air temperature, cumulative radiation, total yield, and total dry matter (TDM). LUE also differed significantly among the experiments and was significantly and positively correlated with CO2. Therefore, CO2 elevation in greenhouses may improve tomato yield under STLT crop management.
Using the regression of LUE against CO2 concentration, Itoh et al. (2020a) predicted TDM and compared it with the observed values at pinching and at the end of the experiments (Fig. 2). At both times, the predicted TDM was not significantly different from the observed TDM when the average daytime CO2 concentration mainly ranged between about 400 and 650 μmol·mol−1 (Fig. 2). However, since the regression was obtained at CO2 concentrations from 364 to 692 μmol·mol−1, predictions are inaccurate beyond that range. The accuracy of prediction decreases on account of human error in terms of management (Transplanted on 16 April). Because of irrigation failure at the beginning of the experiment, plant growth was restricted, so the observed TDM was significantly lower than predicted in Experiment 1 (Fig. 2).
(A, B) Mean daytime CO2 concentrations (A) before pinching and (B) for the whole experimental period. (C, D) Observed and predicted total aboveground DM production (TDM) (C) on the pinching date and (D) at the end of the experiment. In A and B, the bottom and top of each box represent the 25th and 75th percentiles, respectively. The heavy horizontal line represents the median. The whiskers above and below the boxes represent the minimum and maximum values, and the small circles outside this range represent outliers (values outside the range of the median ± 1.5 × the interquartile range). Boxes labeled with the same letter are not significantly different (P < 0.05) by the Kruskal–Wallis test followed by the Steel–Dwass test. In C and D, pairs of bars for observed and predicted TDM differ significantly as follows: NS, no significant difference; *P < 0.05, **P < 0.01, ***P < 0.001, by Student’s t-test (italics) and Welch’s t-test (roman type). Sample sizes on the pinching date: n = 48, 24, 16, 22, 24, and 16 in experiments Ex1 to Ex6, respectively; at the end of the experiment, n = 48, 24, 24, 16, 16, and 12, respectively. The Figure is reproduced from Itoh et al. (2020a) and partly changed with permission from the JSHS.
Soluble solids content is indicated as degrees Brix. High-Brix tomatoes, considered to have a Brix content of > 6%, are popular among Japanese consumers and fetch a higher price than regular tomatoes (Itoh et al., 2020b, 2022; Johkan et al., 2014). Growing high-Brix tomatoes does not require specific cultivars (Itoh et al., 2020b, 2022; Johkan et al., 2014), but since the response of Brix to salinity differs among cultivars (Dorais et al., 2001; Itoh et al., 2022; Schwarz and Kuchenbuch, 1997), growers select cultivars according to crop management or sales strategy.
Water or salinity stress can improve fruit quality (Chen et al., 2013; Dorais et al., 2001; Ehret and Ho, 1986; Lu et al., 2021; Mitchell et al., 1991; Tochigi and Kawasato, 1989). High-Brix tomatoes are grown by restricting irrigation or imposing salinity stress (Itoh et al., 2020b, 2022; Johkan et al., 2014). For this purpose, both soil and soilless culture systems have been developed to utilize water stress, including soil beds (Ban et al., 1994; Ito et al., 1994; Matsuura et al., 2002; Tochigi and Kawasato, 1989), substrate pots (Oishi et al., 2018), mixed compost substrate (Hosokawa et al., 2006), and hydrogel membrane systems (Mori, 2013). To provide salinity stress in commercial greenhouses, many hydroponic systems have been developed (Hohjo et al., 1996; Masuda et al., 1989; Ohkawa and Hayashi, 1996; Ohta et al., 1991; Oka et al., 2004; Sakamoto et al., 1999; Watanabe, 2006), along with a small-pot system (Oishi et al., 2018; Zhang et al., 2015), and a wet-sheet culture system (Sakamoto et al., 1999). For water stress, some devices were developed to test stress strength and were combined with an automatic irrigation system to produce optimal Brix tomatoes without inducing physiological disorders (Hikosaka, 2022). However, no systems to manipulate electrical conductivity (EC) or methods to diagnose stress automatically have been developed for salinity stress. In salinity stress, Johkan et al. (2014) and Itoh et al. (2022) adjusted nutrient solution EC to control and predict fruit Brix. Typically, however, growers increase the EC gradually by increasing the nutrient or salt content according to experience and intuition (Itoh et al., 2020b).
In stress conditions, the incidence of blossom-end rot increases and fruit number or yield decreases in the upper trusses of tomato plants (Dorais et al., 2001). Similarly, physiological disorders and fruit abortion on the upper trusses increase in high-Brix tomato production (Hohjo et al., 1996; Ito et al., 1994; Oishi et al., 1996). STLT crop management avoids these problems (Itoh et al., 2020a; Watanabe, 2006). It also reduces the range of Brix among trusses (Ito et al., 1994). Therefore, many commercial growers use STLT crop management to produce high-Brix tomatoes in Japan (Iwasaki et al., 2019; Watanabe, 2006). Since the yield is lower than that of regular tomatoes, increasing plant density can compensate for low yield (Itoh et al., 2020b; Iwasaki et al., 2019; Oishi and Moriya, 2008; Oishi et al., 1996; Oishi et al., 2018; Saito et al., 2006). STLT crop management with a density of 5.0 plants·m−2 in a commercial greenhouse (1730 m2) yielded 15.3 kg·m−2 per year of high-Brix tomatoes (Iwasaki et al., 2019).
In tomato, although salinity reduces fresh yield, the effect on dry weight differed among studies (Dorais et al., 2001). Dry weights of both plant and fruit decreased in some studies (De Pascale et al., 2015; Maggio et al., 2007; Romero-Aranda et al., 2001), but not in others (Ehret and Ho, 1986; Li and Stanghellini, 2001; Li et al., 2001; Schwarz and Kuchenbuch, 1997). Itoh et al. (2020b) compared DM production between salinized and non-salinized conditions in three seasons in soilless culture of high-Brix tomatoes with STLT crop management in a commercial greenhouse. In the salinized treatment, EC was increased gradually to ~7 dS·m−1 in the supplied nutrient solution and to > 10 dS·m−1 in the drainage solution at initial harvest. As a result, fruit Brix was significantly higher and fruit fresh weight was significantly lower than in the non-salinized treatment in all experiments. Also, at the end of the experiments, fruit DM contents were significantly higher and fresh weights were significantly lower in the salinized treatment. However, there were no significant differences between treatments in terms of dry weights, LAI or LUE (Fig. 3). Accordingly, there was no significant difference in DM production between treatments. Thus, in the hierarchy of yield components, only the significant difference in fruit DM content induced a significant difference in fruit fresh weight and fruit Brix between treatments (Fig. 3).
Effect of salinity treatment in the nutrient solution on the hierarchy of yield components and fruit characteristics. NS, not significant; ***significant difference at P < 0.001 or 95% confidence intervals between non-salinized nutrient solution and salinized nutrient solution in each experiment. White arrow: unaffected, black arrow: affected. The Figure is reproduced from Itoh et al. (2022) and partly changed with permission from the JSHS.
Although the increase in DM content in tomato fruit (Fig. 3) represents a concentration effect, some physiological studies have shown that the increase in sugars, organic acids, amino acids and antioxidant compounds exceeded the concentration effect (Saito et al., 2008; Yin et al., 2010; Zushi, 2010; Zushi et al., 2005). These effects were induced as a response to salinity stress on plants (Zushi, 2010). Therefore, physiological methods should be combined with a calculation of DM production to better understand the mechanisms required to produce high Brix tomatoes.
The imposition of water stress first limits water uptake, and increasing EC reduces DM production (Dorais et al., 2001; Heuvelink et al., 2003; Li and Stanghellini, 2001; Maas and Hoffman, 1977). De Pascale et al. (2015) reported a linear reduction in LUE at ECs from about 2 to 8 dS·m−1. Although drainage EC was > 10 dS·m−1 in the report by Itoh et al. (2020b), it may be possible to reduce water uptake without reducing DM production. Gradually increasing EC or pinching above the third truss may mitigate the effect of high EC and thus avoid any reductions in DM production.
Fruit Brix content is positively correlated with EC (Cornish, 1992; Dorais et al., 2001; Li et al., 2001; Schwarz and Kuchenbuch, 1997). In these reports, the EC of the supplied nutrient solution was kept constant, unlike in the methods used in high-Brix tomato production in Japan. As studies on the effect of changes in the EC of the nutrient solution on fruit Brix did not quantify the relationship between them (Hohjo et al., 1996; Johkan et al., 2014; Saito et al., 2006; Sakamoto et al., 1999), Itoh et al. (2022) investigated this relationship under STLT crop management in a commercial greenhouse in Japan and reported that the fruit Brix was significantly and highly correlated with the cumulative EC of the drainage (cECd) when plants were grown under salinity treatment (Fig. 4). They defined cECd as the sum of the daily EC from anthesis to the harvest date of each fruit, and used a model to predict Brix from cECd. Aiming at a Brix of ≥ 6%, they used the model as an indicator to manipulate the EC of the nutrient solution, and were able to control and predict fruit Brix; salinization increased observed Brix as predicted. At a cECd greater than the target, > 86.9% of fruits had a Brix content of > 6% (Fig. 5), and the marketable yield was > 88.2%. Root-mean-square errors between the observed and predicted Brix were 0.60–1.25 (Fig. 5). This model is promising for producing high-Brix tomatoes.
Fruit Brix as a function of the cumulative electrical conductivity (EC) in the drainage solution in the period from anthesis to harvest in a salinized treatment in three experiments (n = 222). The Figure is reproduced from Itoh et al. (2022) with permission from the JSHS.
Observed fruit Brix as a function of predicted fruit Brix in salinized treatment and salinized-reuse treatment, RMSE and ME in four experiments with transplantion on 2 February 2015 (A), 16 March 2015 (B), November 24 2015 (C), and 19 January 2016 (D). Dotted line; 1:1 relationship. My: Momotaro York, Mh: Momotaro Haruka, My-reuse: Momotaro York with drainage reuse cultivation. The Figure is reproduced from Itoh et al. (2022) with permission from the JSHS.
Water uptake by plants and accumulation by fruit were reduced under highly salinized conditions without a reduction in DM accumulation (Ehret and Ho, 1986; Itoh et al., 2020b; Li et al., 2001; Sakamoto et al., 1999). Thus, cECd may be more closely related to water uptake or accumulation in fruit than to DM production. Further studies are required to clarify this relationship and its mechanism.
To survive, greenhouse horticulture must adapt to climate change, population growth, environmental degradation, the UN’s Sustainable Development Goals, the aging of growers, and diversification of diets. STLT crop management and high-Brix tomato production offer one solution to growers. Originating in Japan, high-Brix tomato production has reached Europe and other Asian countries (Iwasaki, 2021), where demand is expected to increase. STLT crop management will support this increase. Models of yield and Brix could help growers to achieve sustainable crops.
