Stem direction affects the fruit yield, plant growth, and physiological characteristics of a determinate-type processing tomato (Solanum lycopersicum L.)

Scientia Horticulturae 244 (2019) 102–108

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Stem direction affects the fruit yield, plant growth, and physiological characteristics of a determinate-type processing tomato (Solanum lycopersicum L.)

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Katsumi Ohta , Rintaro Makino Department of Agricultural and Forest Science, Shimane University, Matsue, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Auxin Cytokinin Mineral nutrient contents Photosynthetic rate Shoot growth

The aim of this study was to investigate whether vertical or horizontal training of the main stem of the determinate-type processing tomato ‘Shuho’ (Solanum lycopersicum L.) through physical stimulation affects its yield, growth, and physiological characteristics. There was no difference in the total fruit yield and fruit number per plant between the two training directions, but fruit yield, fruit number at 11 weeks after transplanting, and the marketable fruit percent were higher in vertically-trained plants. Furthermore, the stem length increased, the stem diameter decreased, and the lateral shoots at the sixth to seventh and first to fourth nodes were longer in vertically-trained plants due to a positive association between lateral shoot elongation and the concentrations of indole-3-acetic acid, isopentenyl adenine riboside, and isopentenyl adenine. Vertical training, if compared to horizontal one, also resulted in an increased leaf area and photosynthetic rate in the seventh true leaf, bleeding sap rate of the xylem, and dry weights and nitrogen, phosphorus, and potassium contents of the leaf, stem, upper lateral shoots, and roots. These results suggest that vertical training of the main stem is more efficient than horizontal training for increasing the marketable fruit ratio and initial fruit yield of processing tomatoes because it induces morphological changes and increased vegetative growth.

1. Introduction In determinate-type tomatoes that are grown for processing, the main stems are not usually trained during cultivation to reduce labor costs (Chen and Lal, 1999; Yanokuchi, 1997), resulting in plants growing horizontally along the ground. However, growers need to improve the efficiency of their cultural practices to maintain profitability. Recently, growers and consumers in many countries have shown an increasing interest in organic and low-input farm products to address health and environmental concerns in agriculture (Padel and Foster, 2005; Rembiałkowska, 2007), even if the high labor cost means that these products will be more expensive than other products that arise from more efficient production systems. The utilization of synthetic fertilizers, insecticides, and fungicides has been legally restricted in organic and low-input production systems, which has led to the introduction of integrated pest management, whereby net covering and natural enemies are used to deter pests (Noris et al., 2002; Minemura et al., 2003). However, the incidence of disease (i.e., phytophthora rot and gray mold), decay, and cracked fruit tends to be higher in organically cultivated determinate-type processing tomatoes that are grown

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in the open field in Japan because the leaves, shoots, and fruit are more likely to get wet in plants that are grown under unstaked cultivation with the high rainfall that occurs during the cultivation period (approximately 800–1200 mm from May to September). Consequently, there is concern among growers that the marketable fruit yield will decrease. Since tomatoes are herbaceous vegetable crops with an inherently creeping form (Yamaguchi, 1983), adventitious roots are extended from the stem when it comes in contact with the soil surface to absorb mineral nutrients and moisture when grown in the open field (Aoki, 2007). However, it has been found that the vegetative growth, yield, and marketable fruit percent of some tomato cultivars can be increased by staking or pruning the plants (Ahmad and Singh, 2005; Muhammad and Singh, 2007; Olasantan, 1985), while the plant height, leaf area per plant, total dry matter, and yield of tomato plants decreases under lodging as a result of heavy rainfall (Adelana, 1980). Therefore, it is possible that vertical training of the main stem will increase the plant growth and marketable fruit yield of unpruned processing tomatoes produced using organic cultivation techniques. However, little information is available on the effects of vertical and

Corresponding author. E-mail address: [email protected] (K. Ohta).

https://doi.org/10.1016/j.scienta.2018.09.008 Received 5 May 2018; Received in revised form 3 September 2018; Accepted 5 September 2018 0304-4238/ © 2018 Elsevier B.V. All rights reserved.

Scientia Horticulturae 244 (2019) 102–108

K. Ohta, R. Makino

horizontal training of the main stem on the physiological characteristics of determinate-type processing tomatoes. Therefore, the aim of this study was to analyze the effect of stem training direction on the morphological and physiological characteristics of tomato plants to clarify whether vertical training can improve fruit yield and percentage of the marketable fruit.

furrow irrigation twice per week and tensiometers (DM-8, Takemura Electrical Factory Co. Ltd., Tokyo, Japan). Tomato growing season in the open field was characterized by an average temperature of 23.4 °C (max = 35.7 °C, min = 10.3 °C), a cumulative solar radiation of 1610 MJ m–2 and by 326.0 mm of precipitation. No insecticides or fungicides were used.

2. Materials and methods

2.3. Measurements of stem and lateral shoot growth and mineral nutrient contents

2.1. Experimental site, plant materials, and cultivation The stem length from the soil surface to the shoot apex and the stem diameter were measured at 0–3 weeks after transplanting (WAT). The lengths of the lateral shoots that were generated from each node were measured at 1, 3, and 7 WAT. At 7 WAT, nine plants were sampled from each treatment and divided into stems, leaves on the main shoot, lateral shoots (which were separated into parts above the fifth node and below the fourth node), and roots, and then washed with deionized water. After being oven dried at 80 ± 5 °C for 72 h, the dried plants were ground to a fine powder in an electric mill (WB-1, AS ONE Corp., Osaka, Japan). The dry weights of leaf, stem, upper lateral shoot, lower lateral shoot, and root for analysis were used 13.0–18.0 g, 7.2–10.5 g, 2.2–3.1 g, 1.3–2.0 g, and 4.7–7.1 g, respectively. The total nitrogen (N) contents of the plants were analyzed about 20 mg of each organ using a CN coder (Sumigraph NC-90 A, Sumitomo Chemical Analysis Center Corp., Tokyo, Japan). After dry ashing, the phosphorus (P) contents were determined by vanadomolybdate absorption spectrometry, and the potassium (K), calcium (Ca), and magnesium (Mg) contents were determined using an atomic absorption spectrophotometer (AA-680, Shimadzu, Kyoto, Japan). 0.25 g of lower lateral shoot and 0.5 g of the other organs were used for these analysis. The mineral nutrient contents in each plant were calculated by dry weight multiplying its concentration.

The determinate processing tomato (Solanum lycopersicum L. cv. Shuho) (Nagano Chushin Agricultural Institute Experimental Station, Shiojiri, Japan) was used in the field and greenhouse experiments. Seeds were sown in plastic containers (34.5 cm × 27.0 cm × 7.5 cm) containing yellowish pumice (2–5 mm diameter) on April 7, 2015, and the containers were placed in a greenhouse at Shimane University, Matsue, Japan (35°49′ N, 133°07′ E). Tomato seedlings were then repotted in 0.37-L black plastic pots (12 cm diameter) containing potting medium consisting of 50% sandy loam and 50% bark compost on April 20, 2015. 2.2. Experimental design and treatments To determine plant growth, physiological characteristics, flowering, fruit yield, and the marketable fruit percent, tomato plants were transplanted into an experimental open field on May 15, 2015, at which time the sixth true leaf of each plant had fully expanded. The field was covered with black polyethylene mulch (1.35 m wide × 0.02 mm thick) and solid fertilizer (160 g kg–1 N, 39 g kg–1 P, 83 g kg–1 K) had been applied at 8.0 kg a–1 2 weeks before transplanting. The plants were arranged in a single 1.7 m wide row, with 0.85 m spacing between rows, 0.45 m spacing between plants, and a planting density of 1.31 plants m–2. The main stem of each tomato plant was trained in either a vertical direction with a stake (0.8 cm diameter × 75 cm long) using vinyl ties or in a horizontal direction with a stake and two or three Ushaped steel hooks (0.3 cm diameter × 20 cm long) (Fig. 1). A randomized complete block design was used with three replicates. In total, there were 15 plants per treatment, eight of which were used to measure plant growth, flowering, and yield, and the rest of which were used to analyze the bleeding sap rate of the xylem and mineral nutrient contents. Soil moisture was maintained between pF 1.5 and 2.3 using

2.4. Measurements of leaf area, leaf size, photosynthetic rate, and bleeding sap rate of the xylem At 7 WAT, the leaf lengths and widths of third and seventh nodes were measured for five plants per treatment, and the leaf areas were measured using a leaf area meter (LI-3100, LI−COR, Lincoln, Nebraska, USA). The photosynthetic rates were measured using a portable photosynthesis system (LC pro+, ADC, Hoddesdon, UK) at a photosynthetic photon flux density of 1656 μmol m−2 s−1, an air temperature of 30.2 ± 1.2 °C, and an ambient CO2 concentration of 410 ± 20 μmol mol−1. The bleeding sap rate of the xylem was then measured following Morita and Toyota (2000). The plant stem was cut at 5 cm above the soil surface and stuffed with quartz wool, the weight of which had been measured in advance. The quartz wool was then covered by a plastic bag, which was tied closed with rubber bands. After 6 h (10:00–16:00), the bag and quartz wool were removed, and the weight of the quartz wool was quickly measured. The difference between the final and initial weight of the quartz wool was then calculated to determine the bleeding sap rate of the xylem. 2.5. Flowering date, number of flowers, fruit yield, fruit number, and soluble solids content (SSC) The first flowering date of the main stem was recorded and the number of flowers on the first flower truss was counted for each plant in the experimental field. Mature red ripe fruit were harvested twice per week for 5 weeks, and the fruit number, fruit weight, and number of marketable fruit (i.e., those without insect pests, blossom-end rot, sunburn, decay, cracks, or insect pest damage) were recorded. The SSC values of 60 marketable fruit that were harvested at 12 and 13 WAT were evaluated using a digital refractometer (APAL-1, AS ONE Corp., Osaka, Japan) to measure the Brix values of fresh juice samples.

Fig. 1. Schematic diagrams of determinate-type processing tomato plants being vertically trained at 90° to the ground (a) or horizontally trained parallel to the ground (b). Dark and white leaves represent true leaves and cotyledons, respectively. O is the first terminal flower bud of the main stem (first flower cluster). 103

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plant−1, respectively) than in horizontally-trained plants (0.31 kg plant−1 and 2.9 fruit plant−1, respectively). Table 1 shows the marketable fruit percent, unmarketable fruit percent according to type of damage, and SSC of plants grown in each training direction. The marketable fruit percent was significantly higher in vertically-trained plants than in horizontally-trained plants. Furthermore, although there was no significant difference in the incidence of damage due to insect pests, blossom-end rot, and sunburn between the two training directions, there was a significantly lower incidence of decayed and cracked fruit in vertically-trained plants. There was no significant difference in SSC between the two training directions.

2.6. Auxin and cytokinin (CK) contents in the stems and lateral shoots To evaluate the indole-3-acetic acid (IAA), isopentenyl adenine riboside (iPR), isopentenyl adenine (iP), trans-zeatin riboside (tZR), trans-zeatin (tZ), dihydrozeatin riboside (DZR) and dihydrozeatin (DZ) contents in the stems and lateral shoots of plants, tomato seedlings were transplanted into 1.9-L (14.5 cm diameter × 17.2 cm deep) black plastic pots filled with 1.3 kg of potting medium consisting of 50% sandy loam and 50% bark compost in a greenhouse at Shimane University on the same day as the experimental field study began. The fertilizer and application rates were the same as used in the field experiment. The pots were placed on 0.2 m high benches with 0.45 m spacing between plants. Following transplantation, the main stems of the tomato plants were trained in either the vertical or horizontal direction with a stake using vinyl ties. A randomized complete block design was used with at least 20 plants per treatment. During the experimental period, the plants were watered every day. At 7 WAT, the stem and lateral shoot lengths of plants in the greenhouse exhibited the same differences between treatments as were observed in plants growing in the field. Therefore, the stems and lateral shoots of the third and seventh nodes of plants grown in each training direction were excised and immediately frozen in liquid N and stored at −80 °C until analysis. For analysis of the IAA and CK contents, the stems and lateral shoots were sampled using the same method as in our previous study (Ohta et al., 2018). Quantification of the IAA and CK contents was conducted for three independent technical replicates.

3.2. Effects of training direction on stem and lateral shoot growth and mineral nutrient contents Fig. 3 shows the stem length and diameter of plants grown in each training direction. Although there was no significant difference in stem length between the two training directions at 0 WAT, the stems of vertically-trained plants were significantly longer than those of horizontally-trained plants at 1 WAT (15.8 vs. 13.2 cm, respectively), and these differences continued through to 2 and 3 WAT. There was also no significant difference in the stem diameter at the third node between the two training directions at 0 WAT, whereas the diameters at the third and fifth nodes were significantly smaller in vertically-trained plants than in horizontally-trained plants at 1–3 WAT. The stem diameter at the seventh node was significantly smaller in vertically-trained plants than in horizontally-trained plants at 2 WAT, though this difference did not persist through to 3 WAT. Fig. 4 shows the lateral shoot length of plants grown in each training direction. There was no significant difference in the lateral shoot length at any of the nodes between the two training directions at 1 WAT. Furthermore, although the lateral shoots at the sixth and seventh nodes were significantly longer in vertically-trained plants than in horizontally-trained plants at 3 WAT, there was no difference in those at the other nodes between the two training directions. At 7 WAT, the lateral shoots at the sixth and seventh nodes were significantly longer in vertically-trained plants than in horizontally-trained plants, whereas the reverse was true for the first to fourth nodes. Table 2 shows the dry weight of plants grown in each training direction. The dry weights of the upper and total leaves, stems, total lateral shoots, upper lateral shoots, top parts, and roots were greater in vertically-trained plants than in horizontally-trained plants. However, the dry weight of the lower lateral shoots was significantly lower in vertically-trained plants than in horizontally-trained plants. There was no difference in the top/root (T/R) ratio between the two training directions. Table 3 shows the N, P, K, Ca, and Mg contents of plants grown in each training direction. The N, P, and K contents of the leaves and top parts of the plants, the N, P, K, and Mg contents of the stems, the N, P,

2.7. Statistical analysis Data were analyzed using analysis of variance (ANOVA) and Student’s t-test in SPSS ver. 19.0.0 (SPSS, Chicago, IL), and the differences among means were determined using Tukey’s test at P < 0.05. 3. Results 3.1. Effects of training direction on flowering date, number of flowers, fruit yield, fruit number, and soluble solids content (SSC) No difference was observed between the two training directions in the first flowering day or in the number of flowers on the first truss of the main stem, which were approximately 56 d and 5.9 flowers, respectively (data not shown). Fig. 2 shows the fruit yield and number in plants grown in each training direction. There was no significant difference in the total fruit yield or number between the two training directions, with values of approximately 5.7 kg plant−1 and 61.0 fruit plant−1, respectively, for both treatments. However, at the early harvest stage (11 WAT), the fruit yield and number were significantly higher in vertically-trained plants (0.50 kg plant−1 and 4.6 fruit

Fig. 2. Effects of stem direction on fruit yield (a) and fruit number (b) in the determinate-type processing tomato. The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). NS, not significant; *, P < 0.05; **, P < 0.01 (Student’s t-test). Values are the mean ± SE (n = 8). 104

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Table 1 Effect of stem direction on marketable fruit, insect pest, blossom-end rot, sunburned, decay, and cracked fruits percent of number, and soluble solid contents (SSC) in the processing tomato. Direction

Marketable fruit percent (%)

Insect pest fruit percent (%)

Blossom-end rot fruit percent (%)

Sunburn fruit percent (%)

Decay fruit percent (%)

Cracked fruit percent (%)

SSC (ºBrix)

Vertical Horizontal

91.7a 88.9b

2.4a 1.9a

1.2a 1.7a

2.8a 2.9a

1.0b 2.2a

0.9b 2.4a

5.0a 5.1a

The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). Different letters indicate significant difference between treatments (P < 0.05; Student’s t-test) (n = 60 in SSC; n = 8 all others).

Fig. 3. Effects of stem direction on stem length (a) and stem diameter (b) in the determinate-type processing tomato. The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). NS, not significant; *, P < 0.05; **, P < 0.01 (Student’s t-test). Values are the mean ± SE (n = 8).

horizontally-trained plants.

K, Ca, and Mg contents of the upper lateral shoots, and the N and P contents of the roots were higher in vertically-trained plants than in horizontally-trained plants. However, the P, K, Ca, and Mg contents of the lower lateral shoots were significantly lower in vertically-trained plants.

3.4. Effects of training direction on auxin and CK contents in the stems and lateral shoots At the third node, the IAA concentration in the stem were significantly higher in vertically-trained plants than in horizontallytrained plants, whereas the IAA concentration in the elongated lateral shoots was significantly lower in vertically-trained plants than in horizontally-trained plants (Fig. 5). Although there was no difference in any of the CK concentrations in the stem between the two training directions, the iPR, iP, and tZR concentrations in the elongated lateral shoots were also significantly lower in vertically-trained plants than in horizontally-trained plants. At the seventh node, there was no significant difference in the IAA concentration in the stem between the two training directions, whereas the IAA concentration in the elongated lateral shoots was significantly higher in vertically-trained plants than in horizontally-trained plants. Although there was no difference in any of the CK concentrations in the

3.3. Effects of training direction on leaf area, leaf size, photosynthetic rate, and bleeding sap rate of the xylem Table 4 shows the leaf area, length and width for the upper and total leaves, photosynthetic rate, and bleeding sap rate of the xylem in plants grown in each training direction. The leaf area was significantly 33.3% larger in vertically-trained plants than in horizontally-trained plant, as were the lengths of the third and seventh true leaves, and the width of the third true leaf. The photosynthetic rate of the third true leaf was significantly 25.0% lower in vertically-trained plants than in horizontally-trained plants, while that of the seventh true leaf to the reverse was significantly 15.7% higher. The bleeding sap rate of the xylem was significantly 33.3% higher in vertically-trained plants than in

Fig. 4. Effects of stem direction on lateral shoot length at 1 week after transplanting (WAT) (a), 3 WAT (b), and 7 WAT (c) in the determinate-type processing tomato. The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). NS, not significant; *, P < 0.05; **, P < 0.01 (Student’s t-test). Values are the mean ± SE (n = 8).

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Table 2 Effects of stem direction on dry weight, and Top/Root (T/R) ratio at 7 weeks after transplanting (WAT) in the processing tomato. Leaf (g plant−1)

Direction

Vertical Horizontal

Lateral shoot (g plant−1)

Stem −1

Upper

Lower

Total

(g plant

4.0a 2.8b

2.2a 1.7a

6.2a 4.5b

3.5a 2.4b

)

Top

Root −1

Upper

Lower

Total

(g plant

1.0a 0.7b

0.1b 0.3a

1.1a 1.0b

10.8a 7.8b

)

(g plant−1)

T/R ratio

2.4a 1.6b

4.7a 5.0a

The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). The upper and lower areas of the leaf and lateral shoot are defined as higher than the fifth node and lower than the fourth node, respectively. Different letters indicate significant difference between treatments (P < 0.05; Student’s t-test) (n = 5).

photosynthetic ability, it would be necessary to study it in the future. Furthermore, the percentage of marketable fruit was significantly higher for vertically-trained plants (Table 1), indicating that vertical training of the main stem reduces the number of decayed and cracked fruit because the fruit do not get as wet as in horizontally-trained plants. The dry weights of the leaves, stems, upper lateral shoots, top parts, and roots were higher in vertically-trained plants than in horizontallytrained plants, while the dry weights of the lower lateral shoots were lower (Table 2). The contents of many types of mineral nutrients in the leaves, stems, upper lateral shoots, and roots were also significantly higher in vertically-trained plants (Table 3), as were the leaf area and size, and the bleeding sap rate of the xylem (Table 4). The bleeding sap rate of the xylem can be used as an index of the nutrient content that is absorbed by the roots and transferred to the stem and leaves (Morita and Toyota, 2000; Nakano et al., 2002; Inada et al., 2016), and so it is useful for evaluating the amount of shoot growth. It is recognized that the mineral nutrient absorption in the shoots is increased because of the increased leaf area and xylem sap weight, as previously shown by Aoki (2007). There was no difference in the first flowering date and number of flowers between the two training directions. Therefore, it can be speculated that vertical training led to an increased initial yield and fruit number because of the increased vegetative growth. In contrast to these findings, in indeterminate-type tomatoes, the vegetative growth, fruit set, and fruit growth are higher in horizontallytrained plants than in vertically- or diagonally-trained plants because the water and mineral nutrient uptakes, and the vegetative and reproductive growth are reduced under greenhouse cultivation (Aoki, 1983). In addition, Aoki (2007) found that the sap rate of vertically- or diagonally-trained indeterminate-type tomatoes was lower than horizontally-trained plants. In the present study, however, it was found that the leaf area, bleeding sap rate of the xylem, and mineral nutrients contents were higher in vertically-trained determinate-type tomatoes than in horizontally-trained plants. It would be necessary to be examined about the differences between the two experimental results in the future. Although there was no difference in the flowering date of the first flower between vertically- and horizontally-trained plants, verticallytrained plants had a significantly higher photosynthetic rate of the true leaf at the seventh node. Light conditions the surroundings of the measured upper true leaves under vertical training were good because these ones were less shielded by the stems and leaves of the lateral shoots. In addition, it is presumed that the photosynthetic rate was high

Table 3 Effects of stem direction on the contents of N, P, K, Ca, and Mg at 7 weeks after transplanting (WAT) in the processing tomato. Organ

Direction

Content (mg plant−1)　 N

P　

K

Ca

Mg　

Leaf

Vertical Horizontal

106.7a 60.4b

3.3a 2.4b

13.8a 9.1b

14.3a 10.6a

4.5a 3.2a

Stem

Vertical Horizontal

27.3a 19.5b

2.2a 1.7b

8.0a 5.6b

2.2a 2.6a

2.6a 2.1b

Upper lateral shoot

Vertical Horizontal

46.3a 38.0b

1.5a 1.1b

4.5a 3.5b

1.2a 0.9b

1.1a 0.9b

Lower lateral shoot

Vertical Horizontal

8.7a 10.6a

0.3b 0.4a

1.0b 1.7a

0.4b 0.8a

0.3b 0.4a

Top

Vertical Horizontal

191.8a 125.8b

7.3b 5.7a

27.3b 19.9a

18.2a 14.9a

8.5a 6.5a

Root

Vertical Horizontal

28.1a 25.2b

0.8a 0.7b

2.9a 2.6a

0.6a 0.5a

1.5a 1.3a

The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). The upper and lower lateral shoots are defined as higher than the fifth node and lower than the fourth node, respectively. Different letters indicate significant difference between treatments (P < 0.05; Student’s t-test) (n = 3).

stem between the two training directions, the iPR and iP concentrations in the elongated lateral shoots were significantly higher in verticallytrained plants than in horizontally-trained plants. 4. Discussion Although there was no difference in the total yield and fruit number between the two training directions, vertically-trained plants had a higher fruit yield and number than horizontally-trained plants at the early harvest stage (11 WAT) (Fig. 2). Kaneko et al. (2015) found that, when the size of seedlings at transplanting is smaller, the leaf area per plant increases by 26–38%. However, in this study, since there was no difference in the light utilization efficiency between the two treatments, fruit yields were not different. Therefore, it is suggested that there was no difference in fruit yields between the two trainings for the same reason explained before. In this experiment, the light utilization efficiency and light distribution were not measured. However, since these parameters are related to physiological characteristics such as leaf

Table 4 Effect of stem direction on leaf area, length, width, photosynthetic rate, and bleeding sap rate of xylem at 7 weeks after transplanting (WAT) in the processing tomato. Direction

Vertical Horizontal

Leaf area (cm2)

995a 850b

Third true leaf (cm)

Seventh true leaf (cm)

Photosynthetic rate (μmol CO2 m−2 s−1)

Length

Width

Length

Width

Third true leaf

Seventh true leaf

25.9a 21.3b

21.3a 18.4b

32.6a 28.6b

24.7a 24.2a

6.3b 8.4a

12.5a 10.8b

The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). Different letters indicate significant difference between treatments (P < 0.05; Student’s t-test) (n = 5). 106

Bleeding sap rate of xylem (g h−1 plant−1)

0.8a 0.6b

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Fig. 5. Effects of stem direction on the concentrations of indole-3-acetic acid (IAA) (a and c), isopentenyl adenine riboside (iPR), isopentenyl adenine (iP), trans-zeatin riboside (tZR), trans-zeatin (tZ), dihydrozeatin riboside (DZR), and dihydrozeatin (DZ) (b and d) in the stem and lateral shoots of the third and seventh nodes at 7 weeks after transplanting (WAT) in the determinate-type processing tomato. The main stem was trained at 90° to the ground (vertical) or parallel to the ground (horizontal). NS, not significant; *, P < 0.05; **, P < 0.01 (Student’s t-test). Values are the mean ± SE (n = 3).

lateral shoot elongation may be related to changes in the photosynthetic rates in the true leaf at the nodes from which each lateral shoot was generated. Similarly, in the present study, the photosynthetic rate of the true leaf corresponded with the length of the lateral shoot that was generated from its node (Table 4). At the seventh node, the IAA, iPR, and iP concentrations in the lateral shoots were higher in vertically-trained plants than in horizontally-trained plants (Fig. 5a and b). Stem growth is induced by IAA, which is produced in the shoot apex and transported to the root. Since the IAA concentration in the stem at the third node was also higher in vertically-trained plants (Fig. 5c), it is possible that vertical training increases stem elongation. It is speculated that the lateral shoot length at the seventh node was higher in vertically-trained plants due to the increased concentrations of IAA and CKs in the lateral shoot, while the lateral shoot length at the third node was shorter in vertically-trained plants due to the low IAA and CK concentrations. Ito et al. (2001) previously showed that the amount of diffusible IAA and its transport were decreased by bending the shoots of Japanese pear. It has also been reported that the growth of the apical bud, and lateral shoots and their buds are reciprocally controlled by the concentrations of these endogenous plant growth regulators in apple (Bangerth et al., 2000; Watanabe et al., 2004; Zhang et al., 2015), Japanese morning glory (Hosokawa et al., 1990; Kitazawa et al., 2008), and other higher plants (Cline and Harrington, 2007; Ongaro and Leyser, 2008; Shimizu-Sato et al., 2009).

because high nitrogen contents in the leaves (Hirose and Werger, 1987). Consequently, it is thought that the harvesting period was brought forward and the initial yield was increased under vertical training due to the promotion of fruit growth after flowering. These results correspond with the findings of a previous study on grapevines (Hino et al., 1996). Furthermore, Kurahashi and Takahashi (1995) suggested that the fruit yield of apples was higher in a Y-trellis tree than in a central leader tree due to the higher photosynthetic rate and considered that this may have been due to improved light distribution characteristics. Similarly, in the present study, the higher photosynthetic rate that was observed in the true leaf in the upper position in vertically-trained plants will have resulted from reduced shading by the lateral shoots and will have caused more initial photosynthetic products to be produced compared with horizontally-trained plants, as shown in previous studies (Raviv and Blom, 2001; Kim and Lieth, 2004; Kim et al., 2004). The stem length was significantly greater and the stem diameter was significantly smaller in vertically-trained plants than in horizontallytrained plants at 1–3 WAT (Fig. 3a). It has previously been shown that horizontal training leads to higher levels of ethylene production than vertical training in cocklebur (Xanthium spp.) (Wheeler and Salisbury, 1980), tomato (Kurepin et al., 2010), apple (Sanyal and Bangerth, 1998), and Japanese pear (Ito et al., 2001). Furthermore, the increased 1-aminocyclopropane-l-carboxylic acid (ACC) content in a bending shoot is converted into ethylene, causing the internode length to decrease in bougainvillea (Liu and Chang, 2011) and the shoot diameter to increase in maize (Sarquis et al., 1991). Therefore, in this study, it was speculated that the stems of horizontally-trained plants were shorter and thicker than those of vertically-trained plants due to an increased level of ethylene production. Similarly, Harrison and Pickard (1989) found that horizontal training suppressed shoot elongation and increased shoot diameter due to shoot bending reducing the production of IAA in the shoot apices and the basipetal transport of IAA in the shoot. The lateral shoots at the sixth and seventh nodes were significantly longer in vertically-trained plants than in horizontally-trained plants at 7 WAT, whereas the reverse was true for those at the first to fourth nodes (Fig. 4c). A similar trend was observed by Oda et al. (2008), who suggested that the different effects of the two training directions on

5. Conclusion This study demonstrated that the main stem direction affects the productivity of the determinate-type processing tomato ‘Shuho’, with vertical training resulting in an increased initial fruit yield, fruit number, and percentage of marketable fruit compared with horizontal training. These results appeared to be due to the higher vegetative growth (leaf area and dry weight) as a result of improved light distribution characteristics by the photosynthetic rate and mineral nutrient absorption in vertically-trained plants. Especially, the increase of initial fruit yield would be caused by the enhanced growth of the stem and upper lateral shoots due to the higher auxin and CK concentrations. Moreover, it would be easier to find fruits during fruit harvesting 107

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periods in vertically-trained plants because the fruit can be found more easily. Thus, it can be concluded that vertical training of the main stem increases the harvesting efficiency, marketable fruit ratio, and initial fruit yield in this determinate-type processing tomato.

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