2013 Volume 82 Issue 4 Pages 317-321
Light conditions are poor around the lower trusses of tomato plants in a low truss number, high plant density production system. We determined the effects of leaf rearrangements above the fruit trusses on fruit maturation and quality in tomato plants pinched above the third truss and cultivated under a high-density growing system. Integrated solar radiation at first and second fruit trusses and surface temperature of fruits at second fruit truss were increased in plants treated with leaf rearrangements above the trusses compared with those of the control, and the maturation of fruits at the third truss treated with leaf rearrangement was 4.6 days earlier than that of the control. The concentration of ascorbic acid (AsA) in fruits of plants treated with leaf rearrangement was higher than that of control fruits. However, leaf rearrangement had no effect on yield and Brix of the fruit. These results indicated that higher solar radiation together with leaf rearrangement promoted fruit maturation and increased AsA content in the fruit of lower trusses of tomato plants cultivated under a low truss number, high plant density growing system.
Tomato plants show indeterminate growth and develop a truss every three leaves after the first truss. The fruits from several dozen trusses per plant can be harvested with careful cultivation management and environmental control. Tomato plants are trained with a high-wire system to obtain a high yield in a large-scale greenhouse (De Koning, 1993; Heuvelink, 1995), but high temperature injuries to tomato plants, such as during the summer in Japan, inhibit fruit production (Suzuki, 2006). Therefore, a novel cultivation system with a low truss number and high plant density has been developed to make tomato production in temperate areas economically feasible.
A tomato-growing system that employs high-density planting and pinching of the main stem to achieve a low node number, with fruit harvested from one to three trusses per plant, is designed for short-term cultivation and production throughout the year (Watanabe, 2006). This system could select the tomato cultivar and control the cultivation management of tomato fruit production throughout the year and decreased the risk of disease and pest incidence. A single-truss tomato production system, which is the simplest cultivation system utilizing a low truss number and high plant density, is utilized with supplemental lighting (Lu et al., 2012a, b; McAvoy et al., 1988, 1989), tomato production with high soluble solids (Araki et al., 2009; Sakamoto et al., 1999) and automated cultivation (Giacomelli et al., 1994; Okano et al., 2001). However, there are few reports of a three-truss tomato production system in which the yield per plant can be higher than that of a single-truss system.
The number of seedlings necessary for year-round production, the frequency of replanting and workload in a third-truss tomato production system can be less than those of a single-truss production system. However, the height of plants in a three-truss system is taller than in a single-truss system, so the light conditions around fruits in the lower trusses are poorer. The development and maturation of tomato fruits is dependent on temperature as the result of energy balance over the fruit. Effects of light and temperature environments on the cumulative crop yields of tomato are well understood (Cockshull et al., 1992). Moreover, the contents of sugar and ascorbic acid (AsA) in tomato fruit are correlated with solar radiation (Gautier et al., 2008), and the regulation of AsA content in tomato fruit is more highly dependent on irradiance of the fruit than on the leaf (Gautier et al., 2009). Solar radiation is also involved in the regulation of carotenoid biosynthesis in chloroplasts (Bramely, 2002). In the present study, we manipulated the leaf arrangement above the trusses to improve the light and thermal environment for fruit, and determined the effects of leaf rearrangement on fruit maturation and quality under a high-density tomato growing system with the main stem pinched above the third truss.
Seeds of tomato (Solanum lycopersicum L. ‘Reiyo’; Sakata Seed Co., Ltd., Kanagawa, Japan) were sown in a 128-cell tray filled with granulated rockwool (66R; Nitto Boseki Co., Ltd., Tokyo, Japan) on September 6, 2007. The seeds were germinated in the dark and grown in a temperature-controlled chamber equipped with fluorescent lamps (Nae Terrace; Mitsubishi Plastics Agri Dream Co., Ltd., Ibaraki, Japan) for 14 days after germination. The chamber was operated at 280 μmol·m−2·s−1 photosynthetic photon flux, 16 h photoperiod, 25/18°C day/night temperature and 1000 μmol·mol−1 CO2 concentration. The tray was sub-irrigated twice a day with a nutrient solution containing 4.3 N, 1.0 P, 2.0 K, 1.0 Ca, and 0.5 Mg (all mmol·L−1).
The experiments were carried out in a single span cabriolet greenhouse (3 × 9 × 5 m, NS oriented) covered with a polyethylene telephthalate film (Sixright; Mitsubishi Plastics Agri Dream Co., Ltd.) at Chiba University, Japan. The seedlings were potted into 10.5-cm plastic pots filled with granulated rockwool on 20 September, and 39 seedlings were transplanted into a NS-oriented channel in the greenhouse on 8 October. The intra- and inter-rows were 12.8 and 130 cm at a plant density of 6.01 plants·m−2. A nutrient solution containing 8.2 N, 2.0 P, 4.0 K, 1.5 Ca, and 1.0 Mg (all mmol·L−1) was automatically supplied using a drip irrigation system depending on solar radiation intensity. Plants were fed at planting, at anthesis of the first truss, and at anthesis of the third truss with 50, 75, and 100 mL nutrient solution per plant·MJ−1, respectively, and at the onset of harvesting of the second truss plants were fed with 100 mL rainwater per plant·MJ−1. Support poles were erected on opposite sides of tomato fruit trusses, and wires were strung between the support poles. From 23 October, the leaves were pulled onto the wire to expose the fruits of each truss (Fig. 1). Plants were pinched below the fourth truss on 3 December, and all lateral shoots were removed. The minimum temperature in the greenhouse was maintained at 10°C. The number of fruits per truss was four, and mature fruit was harvested. The fruit harvest was finished on 25 March, 2009.
Schematic illustration of training of leaves above the fruit trusses to improve sunlight radiation under a high-density tomato growing system with the main stem pinched above the third truss. (A) Control, (B) leaf rearrangement after training. The leaves above the fruit trusses were attached to the wire.
Total integrated solar radiation of second fruit at the third truss was measured with integrated solarimeter film (Optleaf R–2D; Taisei Chemical Co., Ltd., Tokyo, Japan) on 19–22 January. The percentage of dye feeding was measured at 470 nm with a photometer (THS-470; Taisei Chemical Co., Ltd.) before and after exposure. Integrated solar radiation was determined from the film fading ratio using a calibration curve provided by the film manufacturer (Taisei Chemical Co., Ltd). The surface temperature of fruits in the second and third trusses was measured with a copper-constantan thermocouple (0.32 mm diameter wire) attached to a data logger (GR-3000; Keyence Co. Ltd., Osaka, Japan) recorded at 2-min intervals on 11 March. The AsA and Brix of 8 fruits were measured immediately after harvesting. The AsA concentration in the fruit was determined with the Ascorbic Acid Test (Merck Co., Ltd., Darmstadt, Germany) using RQ Flex Plus (Merck Co., Ltd.). Brix of the squeezed fruit juice was determined with a refractometer (IPR-101; Atago Co., Ltd., Tokyo, Japan).
Each treatment (control and leaf rearrangement), consisting of 39 plants per bed, was replicated twice in the same greenhouse and finally data were recorded for 12 plants. Mean values were separated by t-test at the 0.05 significance level using XLSTAT software (Esmi Co., Tokyo, Japan).
Manipulation of the leaf rearrangement above the fruit trusses increased the integrated solar radiation of the fruit trusses by 3.5- and 2.6-fold compared with those of the control for the first and second trusses, respectively (Table 1). The surface temperature of fruits in the second truss with the leaf rearrangement was increased compared with that of the control during the day (Fig. 2), whereas the temperature of fruits in the third truss showed no difference between the control and with leaf rearrangement (data not shown).
| Integrated solar radiation at fruit trussz (MJ·m−2·day−1) | |||
|---|---|---|---|
| First | Second | Third | |
| Control | 1.5 | 2.3 | 4.4 |
| Leaf rearrangement | 5.2 | 5.9 | 5.1 |
| Significance | * | * | NS |
NS: Not significantly different at 0.05 level.
Effect of leaf rearrangement on the surface temperature of tomato fruit in the second fruit at third truss in a high-density growing system with the main stem pinched above the third truss. The value is the average temperature on 11 March, 2009.
Fruits of the first truss with the leaf rearrangement showed red pigmentation, whereas those of the control were light green (Fig. 3). In the control, the fruits were shaded by the leaves above the trusses and thus ripening was delayed (Fig. 3a); however, with the leaf rearrangement above the truss, fruits were exposed to sunlight and ripening was promoted (Fig. 3b). Therefore, the onset of harvesting with the leaf rearrangement was 8.6, 6.2, and 4.6 days earlier for the first, second and third truss, respectively, compared with those of the control (Fig. 4).
Effect of improved sunlight radiation on fruit trusses of tomato plants under a high-density growing system with the main stem pinched above the third truss. (A) Control, (B) plants in which leaves above the trusses were attached to the wire. Mature fruits in photograph B were on the first truss on 10 January, 2009.
Effect of leaf rearrangement on the onset of harvest of tomato fruit for each truss under a high-density growing system with the main stem pinched above the third truss. Error bars represent standard error (n = 39).
The AsA concentrations of the fruits in first and second trusses with leaf rearrangements were significantly higher than that of the control fruits (Table 2). Integrated solar radiation at the trusses was highly correlated with the AsA concentration of the fruit (R2 = 0.96) (Fig. 5). However, integrated solar radiation at the trusses had no effect on the fruit number and yield per plant. Brix of the fruit was not affected by integrated solar radiation or the positions of fruit trusses.
| Yield (kg/plant) | AsA concentration (mg·100 g−1 FW) | Brix (%) | |||
|---|---|---|---|---|---|
| First | Second | Third | |||
| Control | 1.76 | 13.4 | 14.5 | 16.0 | 6.2 |
| Leaf rearrangement | 1.83 | 15.4 | 16.6 | 17.0 | 6.4 |
| Significance | NS | * | * | NS | NS |
NS: Not significantly different at 0.05 level.
Relationship between total integrated solar radiation on tomato fruit and ascorbic acid concentration in fruit at each trusses under a high-density growing system with the main stem pinched above the third truss. Open and closed symbols indicate control and leaf arrangement. (○, ●) first truss, (□, ■) second truss, (△, ▲) third truss. Total integrated solar radiation was measured from flowering to harvest.
The surface temperature of tomato fruits with leaf rearrangements above the fruit trusses increased, and the harvest time for tomato fruits with rearrangement was earlier than that of the controls. The time of fruit maturation differs among seasons because of temperature fluctuations in the growing environment. Adams and Valdes (2002) reported that high temperatures before harvesting induced ripening as the fruit approached maturity. Indeed, the fruits of tomato plants grown at 14°C ripened 95 d after anthesis, whereas the fruits of plants grown at 26°C ripened 42 d after anthesis (Adams et al., 2001). Aikman (1996) reported that tomato fruits require a cumulative temperature of 840°C·d above a base temperature of 3.5°C for fruit maturation from anthesis to harvest. The effects of leaf rearrangement above the fruit trusses on the harvesting time were also attributable to the increase in surface temperature of the fruits.
At the ‘breaker’ stage of ripening in tomato fruits, the red coloration from lycopene begins to appear, the chlorophyll content decreases, and the organoleptic properties of the fruit change (Bramely, 2002). Tomato fruits in the first and second trusses of plants treated with leaf rearrangement ripened more rapidly than those of the control, and the onset of harvest of the first truss was 8.6 days earlier than that of control fruits, which might be because of the marked increase in total integrated solar radiation. Brief red-light treatment of harvested mature-green fruit increased lycopene accumulation by 2.3-fold compared with that under the dark condition, and red-light-induced lycopene accumulation was reversible by far-red light (Alba et al., 2000). Within the canopy, the visible light spectrum in sunlight is essentially depleted and far-red light is strongly represented (McNellis and Deng, 1995). These results suggested that, under a high-density growing system with the main stem pinched above the third truss, the ratio of red to far-red light at the lower trusses was lower than at higher trusses and lycopene accumulation in fruit in the lower truss was delayed.
The AsA concentration in the fruit at first and second trusses treated with leaf rearrangement was increased compared with that of control fruit. The AsA concentration in tomato fruit was more strongly affected by light intensity than temperature at tomato fruit (Gautier et al., 2009). The side of the fruit that is directly exposed to the light invariably has a higher AsA level than that on the shaded side (Brown, 1954; Venter, 1977). Under shaded conditions, the AsA content in tomato fruits decreases by 15–20% compared with fruit exposed to sunlight (Venter, 1977). Our results showed that integrated solar radiation on the fruit and AsA concentration in the fruit were positively correlated, which also supported the results of Brown (1954) and Venter (1977). However, the yield and sugar concentration of tomato fruits were not significantly affected by training the leaves, so yield and sugar concentration might be influenced by other factors.
The yield of tomato plants grown under the high wire training system in Japan (ca. 30 kg·m−2) is about half of that in the Netherlands (ca. 60 kg·m−2) (Higashide and Heuvelink, 2009). One of the reasons for this shortage is that high temperature in summer is too severe to produce tomato fruits. Therefore, a low truss, high density tomato production system has been studied to achieve continuous year-round production of greenhouse tomatoes in Japan. A low truss number is essential for a high-density growing system to obtain a high yield, but the mutual shading of each leaf in the plant canopy markedly decreases the light intensity that penetrates the plant canopy. In general, insufficient light will decrease the growth rate because of reduced photosynthesis, but the growth and fruit yields of tomato plants treated with or without leaf rearrangement were not significantly different in this study (data not shown). Given that leaf rearrangement did not affect the growth and fruit production of tomato plants under the high-density growing system employed in this study, supplemental lighting within the canopy was useful to obtain a high yield of fruits (Lu et al., 2012a, b).
In conclusion, manipulation of the leaf rearrangement above the fruit trusses in a high-density tomato growing system with the main stem pinched under the fourth truss increased integrated solar radiation at the fruit, as well as the surface temperature and AsA content of the fruit, and accelerated fruit maturation in the lower trusses. However, the yield and sugar content of the fruit were not significantly different irrespective of the position of the fruit truss. These results indicated that higher solar radiation with leaf rearrangement promoted the accumulation of antioxidants, e.g. AsA, in the fruit on lower trusses. In this study, physiological disorders with leaf rearrangement did not occur because of the low light intensity and temperature, but the degree of physiological disorder at high light intensity and temperature was not studied. Moreover, the operating time and cost of leaf rearrangement were also not clear. Further study is need to investigate the labor cost and seasonal effect of leaf rearrangement on AsA concentrations in tomato fruits produced under a high-density growing system with the main stem pinched below the fourth truss.
