Effects of Humidification Based on Vapor Pressure Deficit (VPD) on Plant Growth, Fruit Yield, and Fruit Quality Traits in June-bearing Strawberry

Abstract

To develop novel humidification technology for strawberry production to achieve higher yields and improve fruit quality, four Japanese June-bearing strawberry cultivars were grown in a greenhouse with or without humidification treatment (HT) based on vapor pressure deficit (VPD). HT reduced VPD from transplanting to March, but did not affect the VPD condition from April to May. Soon after transplanting, HT enhanced plant growth and the daily leaf-emergence rate, and significantly advanced flower-bud emergence and first-fruit harvest for the first inflorescence for ‘Koiminori’, ‘Kaorino’, and ‘Saga i9’. However, HT significantly delayed flower-bud differentiation and first-fruit harvest of the second inflorescence of ‘Kaorino’ and ‘Yumenoka’. HT significantly increased the total weight of marketable fruit for ‘Koiminori’, ‘Kaorino’, and ‘Saga i9’. In addition, HT significantly increased the total fresh weight of marketable fruit harvested until December for ‘Koiminori’ and ‘Saga i9’. HT did not significantly affect the firmness of fruit skin (FFS), soluble-solid content (SSC), fruit acidity (FA), or SSC/acidity of ‘Koiminori’, ‘Kaorino’, and ‘Yumenoka’.

Introduction

Demand for Japanese strawberry fruit by consumers has increased in recent years (MAFF, 2021a, b). However, the area dedicated to strawberry production has gradually decreased (MAFF, 2021c). To satisfy demand for Japanese strawberry fruit, environmental control technologies have been evaluated to improve the efficiency of domestic strawberry production. Environmental control technologies to regulate the day length (Kimura and Fujimoto, 1971; Kimura et al., 1968), temperature (Okimura, 2009; Sato and Hiraoka, 1971; Tagawa et al., 2021, 2022), CO₂ concentration (Hidaka et al., 2022; Itani et al., 1998, 1999; Kawashima, 1991; Oda, 1997; Yoshioka, 1994), and relative humidity (Choi and Jeong, 2020; Kato et al., 2015; Lieten, 2002) have been developed in Japan since the 1970s. However, humidity control based on relative humidity and vapor pressure deficit (VPD) is currently considered to be suboptimal compared with other environmental control technologies.

High humidity can prevent excessive transpiration and low water potential within a plant body, and promote cell elongation and organ volume expansion. As a result, it improves the growth and fruit yield of strawberry plants (Choi and Jeong, 2020; Johnson et al., 1992; Kato et al., 2015; Lieten, 2002; Ninomiya et al., 2012). Especially, the combination of high humidity control and CO₂ supply can strongly promote plant growth by increasing the CO₂ gas exchange rate through the stomata in cucumber (Suzuki et al., 2014) and tomato (Iwasaki et al., 2017). However, excessive humidity can diminish fruit set by retaining pollen within the anthers, making it challenging for insects to facilitate pollen transport in strawberry plants (Lieten, 2002). Additionally, extremely high humidity conditions decrease transpiration and transport of calcium to the apex and expanding leaves, leading to an increase in tip burn and a decrease in strawberry plant growth (Ehret and Ho, 1986; Islam et al., 2004; Lieten, 2002). Excessive humidity can also increase gray mold disease (Shpialter et al., 2009; Yunis et al., 1990). To consistently improve the fruit yield of strawberry, optimization of humidification technology suitable for strawberry production is highly desirable.

Recently, Yamanaka et al. (2022a) developed a new humidification technology to prevent shrinkage of the fruit-stalk diameter in strawberry, which was termed the differential VPD control system (D-VPDCS). In the D-VPDCS, it was suggested that the rate of VPD change (the first-time derivatives of VPD (VPD’ (hPa·min⁻¹)) (Eq. 1) and the rate of VPD’ change (the second-time derivatives of VPD (VPD” (hPa·min⁻²)) (Eq. 2) strongly affected the change in fruit-stalk diameter. Increases in VPD, VPD’, and VPD” may disrupt the balance between transpiration and water uptake from the roots and reduce water potential within a plant body after excessive transpiration, resulting in shrinkage of fruit-stalk diameter in strawberry plants (Grossiord et al., 2020; Johnson et al., 1992; Murai-Hatano et al., 2009; Ninomiya et al., 2012; Yamanaka et al., 2020).

  

$${\mathit{VPD\prime }}_t= \left({\mathit{VPD}}_t-{\mathit{VPD}}_{t-1}\right)÷\left(t-\left(t-1\right)\right)$$(Equation 1)

 t: arbitrary time (min)

 VPD_t: VPD at arbitrary time (hPa)

 VPD′_t: Rate of VPD at arbitrary time (hPa·min⁻¹)

  

$${\mathit{VPD″}}_t=\left({\mathit{VPD\prime }}_t-{\mathit{VPD\prime }}_{t-1}\right)÷\left(t-\left(t-1\right)\right)$$(Equation 2)

 VPD″_t: Rate of VPD′ at arbitrary time (hPa·min⁻²)

Because the change rates in fruit-stalk diameter are associated with VPD’ and VPD”, the D-VPDCS humidifies based on VPD, VPD’, and VPD” (Yamanaka et al., 2022a). When VPD, VPD’, and VPD” simultaneously exceed 7 hPa, 0.1 hPa·min⁻¹, and −0.04 hPa·min⁻² during the daytime, respectively, the D-VPDCS indicates that VPD rapidly increases over a short period time and strawberry plants experience water stress (Yamanaka et al., 2022b). In the present study, we investigated the plant growth, fruit yield, and fruit quality of four Japanese strawberry cultivars grown under the D-VPDCS to evaluate its effectiveness for strawberry production.

Materials and Methods

Plant materials and growth environments

The June-bearing strawberry cultivars ‘Koiminori’, ‘Kaorino’, ‘Yumenoka’, and ‘Saga i9’ were used. On June 30 and July 1, 2021, runner tips were transplanted into 9-cm-diameter polyethylene pots filled with a mixed substrate (pH-adjusted peat moss:perlite, 6:1 [v/v]). On July 14, a solid piece of fertilizer (Bigguwan L; JCAM AGRI. Co., Ltd., Tokyo, Japan) was supplied to each nursery plant. From August 17 to September 14, a 1/1,000- or 1/2,000-strength nutrient solution (OK-F-1; OAT Agrio Co., Ltd., Tokyo, Japan) was supplied to each nursery plant. Three nursery plants were transplanted into a bowl-shaped plastic pot (diameter: 30 cm, capacity: 6 L) filled with pH-adjusted peat moss and placed on the high bench of two greenhouses on September 18 (‘Kaorino’), September 23 (‘Koiminori’ and ‘Saga i9’), and September 26 (‘Yumenoka’) at the Western Region Agricultural Research Center, NARO (Zentsuji, Kagawa, Japan). A nutrient solution (OAT Agrio A Solution; OAT Agrio Co., Ltd.) with electrical conductivity (EC) ranging from 0.6 to 1.4 dS·m⁻¹ was supplied to the transplanted plants. The two greenhouses were similarly ventilated, heated, and CO₂-treated using an environmental controller (YoshiMax; Sanki Keiso Co., Ltd., Tokyo, Japan) in accordance with Yoshida and Yasuba (2020). Excess flowers in the inflorescences were removed to standardize the number of fruit that developed as follows: 10 fruits in the primary inflorescences; 10 fruits/three or more leaves, six fruits/two leaves, three fruits/one leaf, and zero fruits/zero leaves in the second and subsequent inflorescences until February; and three fruit in all inflorescences from March to May. During cultivation, the number of buds per crown was limited to three or fewer.

Humidification treatment

The D-VPDCS was installed only in one greenhouse (humidification treatment, HT); the other greenhouse was untreated as a control (non-humidification treatment, NT). An original system for the HT consisted of mist nozzles (item no. 63100-052000; NETAFIMJAPAN, Tokyo, Japan), a programmable datalogger (CR1000; Campbell Scientific, Inc., Logan, UT, USA), and a thermo-hygrometer (CVS-HMP155; Climatec, Inc., Tokyo, Japan) covered with a shelter (CO-RS1; Climatec, Inc.) with a DC fan (C4010H12BPLB-W-7; Misumi Group, Inc., Tokyo, Japan) (WARC/NARO, 2021). The mist nozzles were installed between the ground and the top of the high bench (Fig. 1). The mist was sprayed for 1 min from 06:00 to 17:00 from September 29, 2021, to May 31, 2022, when the VPD condition simultaneously met the following thresholds: VPD > 7 hPa, VPD’ > 0.1 hPa·min⁻¹, and VPD” > −0.04 hPa·min⁻². The mist was dispersed with a fan installed on the ground. The interval between each mist-spray emission was 2 min. The scan interval of the program was 1 second.

Fig. 1

Schematic diagram of a greenhouse with the humidification treatment setup used in this study.

Plant growth and flowering traits

The petiole length, leaf length, and width of the central leaflet, in the third young leaf, were measured with a ruler each month from October to May. The number of leaves was counted each month from October to May. The daily leaf-emergence rates were calculated by dividing the number of leaves by the number of days between counts. The number of leaves on the lateral second branch was determined. The dates of flower bud emergence and first-fruit harvest were recorded for the first and second inflorescences.

For these assessments, the sample number was 24 for ‘Koiminori’ and ‘Yumenoka’, and 12 for ‘Kaorino’ and ‘Saga i9’.

Fruit yield traits

The number of achenes of the first, second, third, and fourth ranks of fruit in the first inflorescence were counted for ‘Koiminori’, ‘Kaorino’, and ‘Yumenoka’. Six to 11 fruit were sampled to determine the number of achenes. Fully ripe fruit were harvested twice a week. Fruit of weight > 7 g resulting from successful fertilization, of uniform shape with no deformities, and free of pests and diseases were classified as marketable fruit. The total number and fresh weight of marketable fruit were averaged for each pot (three plants). Fourteen pots for ‘Koiminori’ and ‘Yumenoka’, and six pots for ‘Kaorino’ and ‘Saga i9’ were sampled for assessment of fruit yield traits.

Fruit quality traits

For ‘Koiminori’, ‘Kaorino’, and ‘Yumenoka’, the firmness of the fruit skin (FFS), soluble-solid content (SSC), and acidity of the fruit (FA) were recorded twice a month from December to May. The equatorial plane on the solar side of the fruit was penetrated vertically with a force tester (MCT-2150; A&D Co., Ltd., Tokyo, Japan) equipped with a cylindrical plunger with a tip diameter of 15 mm and load cell of 500 N. The measurement was conducted at the equator of each fruit and the loading speed was set to 100 mm·min⁻¹. The maximum break strength was recorded as the FFS. Three fruits were squeezed using a hand juicer and the liquid was filtered through a composite-fiber bag. The SSC and acidity were measured using a saccharin-acidity meter (PAL-BX|ACID4; ATAGO Co., Ltd., Tokyo, Japan). The data for fruit quality traits are presented for each of the following periods: from transplanting to December 31, 2021 (Period I); from January 1 to February 28, 2022 (Period II); and from March 1 to May 31, 2022 (Period III). The number of fruits sampled to assess the fruit quality traits was nine for ‘Yumenoka’ in Period I, 18 for ‘Koiminori’ and ‘Kaorino’ in Period I, 36 in Period II, and 54 in Period III.

Measurement of environmental variables

The air temperature and relative humidity for each growth environment were recorded at 10-min intervals using a thermo-hygrometer (RTR-576-S; T&D Corp., Nagano, Japan) covered with a shelter (CO-RS1; Climatec, Inc.) and a DC fan (C4010H12BPLB-W-7; Misumi Group, Inc., Tokyo, Japan) (WARC/NARO, 2021).

In this study, VPD was calculated from the air temperature and relative humidity (Buck, 1981).

Statistical analysis

All statistical analyses were performed using Statcel-the Useful Addin Forms on Excel, 4th ed. (OMS Publish, Tokyo, Japan). Data for plants that suffered severe damage from pests and diseases were excluded from the statistical analysis. In addition, outliers for petiole length, leaf length, leaflet width, achene number, total number, and fresh weight of marketable fruit were excluded in accordance with the Smirnov–Grubbs outlier test (Grubbs, 1969).

Results

Growth environments

The daily- and daytime-average (06:00–18:00) air temperatures were almost identical under HT and NT from transplanting to January (Fig. 2A, B). However, the air temperatures were lower under HT than under NT from February to May. From transplanting to May, the daily- and daytime-average VPDs were lower under HT than under NT (Fig. 2C, D).

Fig. 2

Daily-average (A) and daytime-average (B) air temperature, and daily-average (C) and daytime-average (D) vapor pressure deficit (VPD) during the experimental period.

The frequency of recordings with the specified VPD condition between 06:00 and 17:00 is shown in Table 1. VPD was recorded at 10-minute intervals between 6:00 and 17:00, and VPD’ and VPD” were calculated from these VPD recordings. Each day, is these VPD, VPD’, and VPD” values simultaneously exceeded their respective thresholds, it was counted as one instance of a recording with the specified VPD condition. This his was repeated at 10-minute intervals. The total count of such occurrences was accumulated. In HT, the frequency of recordings with ‘VPD ≥ 7 hPa’ decreased compared to NT by 12, 28, 35, 32, 33, 20, 10, and 6% in October, November, December, January, February, March, April, and May, respectively (Table 1). HT reduced the frequency of recordings with ‘VPD ≥ 7 hPa, and VPD’ > 0.1 hPa·min⁻¹, and VPD” > −0.04 hPa·min⁻²’ by 23, 46, 58, 54, 59, 33, 14, and 16% in October, November, December, January, February, March, April, and May, respectively, compared to NT (Table 1).

Table 1

Frequency of recordings with ‘VPD ≥ 7 hPa’ and with ‘VPD ≥ 7 hPa, and VPD’ > 0.1 hPa·min⁻¹, and VPD” > −0.04 hPa·min⁻²’ between 06:00 and 17:00.

Plant growth and flowering traits

The petiole length was significantly longer under HT than under NT from October to December for all cultivars, ‘Koiminori’ and ‘Kaorino’ in January, for ‘Koiminori’, Kaorino’, and ‘Saga i9’ in February, and for ‘Kaorino’ and ‘Saga i9’ in March, respectively (Table 2). The central leaflet length was significantly longer under HT than under NT for all cultivars in October and December, and for ‘Kaorino’ and ‘Saga i9’ in November, respectively. The central leaflet width was significantly wider under HT than under NT for all cultivars in October and December, for ‘Saga i9’ in November, and for ‘Kaorino’ and ‘Saga i9’ in May, respectively. The daily leaf-emergence rates were significantly higher in HT than in NT for every cultivar from transplanting to October 14, 2021 (Fig. 3). Additionally, the daily leaf-emergence rates were also significantly higher in HT than in NT for ‘Kaorino’ from October 14 to November 19, 2021, and for ‘Yumenoka’ from November 19 to December 15. However, the daily leaf-emergence rates were lower in HT than in NT for ‘Yumenoka’ from April 20 to May 18, 2021, and for ‘Saga i9’ from March 23 to April 20, 2021.

Table 2

Petiole length, central leaflet length, and central leaflet width for four strawberry cultivars grown under non-humidification treatment (NT) and humidification treatment (HT).

Fig. 3

Daily leaf-emergence rate for four strawberry cultivars grown under humidification treatment (HT). ***, **, *, and NS indicate significant difference at P < 0.001, 0.01, 0.05, and no significant difference at P = 0.05 by t-test, respectively.

No significant differences were observed between NT and HT in the number of leaves that emerged from transplanting to the emergence of the first inflorescence (Table 3). However, the number of leaves that emerged between the first and second inflorescences significantly increased under HT compared with that under NT for ‘Kaorino’ and ‘Yumenoka’. For the first inflorescence, the emergence of the inflorescence and first-fruit harvest were earlier under HT than under NT for all cultivars and for ‘Koiminori’, ‘Kaorino’, and ‘Saga i9’, respectively (Table 4). There were no significant differences between NT and HT in emergence of the second inflorescence for all cultivars. HT significantly delayed the first-fruit harvest in the second inflorescence for ‘Kaorino’, and ‘Yumenoka’, compared to NT.

Table 3

Number of leaves that emerged from transplanting to emergence of the first inflorescence and between the first and second inflorescence of four strawberry cultivars grown under non-humidification treatment (NT) and humidification treatment (HT).

Table 4

Days from transplanting to emergence of flower buds and from transplanting to first-fruit harvest for the first inflorescences and second inflorescences for strawberry plants grown under non-humidification treatment (NT) and humidification treatment (HT).

Fruit yield traits

The achene number for the first inflorescence was significantly higher under HT than under NT for ‘Koiminori’ and ‘Kaorino’ in the first and second rank, and for ‘Kaorino’ in the third rank, respectively (Table 5).

Table 5

Number of achenes for the first inflorescence for three strawberry cultivars grown under non-humidification treatment (NT) and humidification treatment (HT).

In ‘Koiminori’, the number of marketable fruits significantly increased under HT compared with those under NT in December, March, and May, but significantly decreased under HT compared with those under NT in January (Table 6). HT significantly increased the fresh weight of marketable fruits in December, February, March, and May, but significantly decreased that in January, compared to NT. As a result, the total number and fresh weight of marketable fruits were significantly higher in HT than NT for ‘Koiminori’.

Table 6

Number and fresh weight of marketable fruit for four strawberry cultivars grown under non-humidification treatment (NT) and humidification treatment (HT).

In ‘Kaorino’, HT had a significantly larger number of marketable fruits in November, February, and March, but had a significantly smaller number in December, April, and May, compared to NT (Table 6). There was a significantly greater fresh weight of marketable fruits in HT in November, January, February, and March, but a significantly lower fresh weight of December and April, compared with NT. Consequently, HT significantly increased only total fresh weight of marketable fruits in ‘Kaorino’.

In ‘Yumenoka’, the number of marketable fruits was significantly fewer in HT than in NT (Table 6). The fresh weight of marketable fruits increased under HT compared with under NT in March and April. However, there were no significant differences between HT and NT in the total number and fresh weight of marketable fruits in ‘Yumenoka’.

In ‘Saga i9’, HT significantly increased the fruit number in December and March, but decreased that in January and February, compared to NT (Table 6). The fresh weight of marketable fruits was significantly greater in HT than in NT in December and March, but significantly lower in HT than in NT in January. Accordingly, the total number and fresh weight of marketable fruits were greater in HT than in NT for ‘Saga i9’.

Fruit quality traits

The average fresh weight of marketable fruits was significantly decreased under HT compared with NT for ‘Koiminori’, ‘Yumenoka’, and ‘Saga i9’ during Period I, but was significantly increased under HT in comparison with NT for ‘Koiminori’, ‘Kaorino’, and ‘Saga i9’ during Period II, and for ‘Koiminori’ and ‘Kaorino’ during Period III (Table 7). The FFS was significantly higher in HT than in NT for ‘Yumenoka’ in December, and during Period II, but significantly lower in HT than NT for ‘Kaorino’ during Period II. HT significantly decreased the SSC for ‘Kaorino’ during Period II compared to NT. There were no significant differences in the FA for ‘Koiminori’, ‘Kaorino’, and ‘Yumenoka’. Throughout the cultivation period, the SSC/acidity ratio exceeded 19 for ‘Koiminori’, ‘Kaorino’, and ‘Yumenoka’. The seasonal fluctuation in SSC/acidity tended to be smaller under HT than NT for ‘Kaorino’, but tended to be larger under HT than NT for ‘Yumenoka’.

Table 7

Average fresh weight of marketable fruit (AFW) for four strawberry cultivars, and the firmness of the fruit skin (FFS), soluble-solid content (SSC), and fruit acidity (FA) for three strawberry cultivars grown under non-humidification treatment (NT) and humidification treatment (HT).

Discussion

To develop a new humidification technology to improve fruit yield and quality in strawberry, we determined the effects of a novel humidification system (D-VPDCS) on plant growth, fruit yield, and fruit quality traits.

Throughout the cultivation period, HT reduced the frequency of recordings of ‘VPD ≥ 7 hPa, and VPD’ > 0.1 hPa·min⁻¹, and VPD” > −0.04 hPa·min⁻²’ more than that of recordings of ‘VPD ≥ 7 hPa’ (Table 1). The VPD condition satisfying ‘VPD ≥ 7 hPa, and VPD’ > 0.1 hPa ·min⁻¹, and VPD” > −0.04 hPa·min⁻²’ may be considered indicative of a rapid VPD increase. Therefore, it was concluded that the D-VPDCS effectively mitigated the rapid increase in VPD compared to a humidification method only using VPD as the threshold for humidifying. However, there were small differences between NT and HT in the VPD condition and plant growth from April to May (Table 2; Fig. 3). In these periods, the D-VPDCS could not effectively humidify due to seasonal conditions such as high temperature, low relative humidity, and full-opened side ventilation. Further consideration will be needed in terms of the operation method of D-VPDCS such as the type of mist nozzle and humidification ON/OFF interval to make the VPD condition effective from April to May.

From transplanting to March, HT promoted vegetative plant growth (Table 2; Fig. 3). Especially from transplanting to mid-October, the leaf-emergence rate significantly increased under HT (Fig. 3). It was considered that the D-VPDCS promoted photosynthesis and increased the leaf area from transplanting to March (Choi and Jeong, 2020; Kato et al., 2015; Lieten, 2002).

As a result of the higher leaf-emergence rate of HT soon after transplanting, HT accelerated the emergence of the first inflorescence and the first-fruit harvest for ‘Koiminori’, ‘Kaorino’, and ‘Saga i9’ (Table 4). Therefore, HT increased the total number and fresh weight of marketable fruits for ‘Koiminori’ and ‘Saga i9’ during Period I (Table 6). It was thought that the increase in the total number of marketable fruits under HT significantly reduced the AFW during Period I compared to NT (Table 7) (Khanizadeh et al., 1993). However, the number and fresh weight of marketable fruits in January was significantly lower in HT than in NT for ‘Koiminori’ and ‘Saga i9’ due to a shift in the fruit harvest peak (Table 6).

In ‘Kaorino’, the number and fresh weight of marketable fruits in November were much higher in HT than in NT, but those in December were much lower in HT than in NT (Table 6). As a result, almost all first inflorescence fruits were harvested completely both under NT and HT during Period I. Strawberry fruit weight tended to be greater under low temperature conditions (Kumakura and Shishido, 1994). It was thought the AFW during Period I was lower in HT than NT for ‘Kaorino’ despite the increase in the achene number for the first inflorescence because of the temperature during the ripening period. However, the average date of starting fruit harvest on the second inflorescence for ‘Kaorino’ was December 27, 2021, under NT, and January 8, 2022, under HT, respectively. ‘Kaorino’ exhibits extremely early flowering (Kitamura et al., 2015). For ‘Kaorino’, it was considered that HT decreased the total number and fresh weight of marketable fruits during Period I due to a delay in flower-bud differentiation of the second inflorescence.

HT delayed flower-bud differentiation of the second inflorescence not only for ‘Kaorino’, but also for ‘Yumenoka’ (Table 3). HT resulted in a later date for the first-fruit harvest in the second inflorescence for ‘Kaorino’ and ‘Yumenoka’ (Tables 3 and 4). HT may increase assimilation of nitrogen and promote vegetative growth, resulting in delayed flower-bud differentiation of the second inflorescence (Breen and Martin, 1981). Therefore, HT significantly decreased the fruit number of marketable fruits in February for ‘Yumenoka’ compared to NT. In ‘Saga i9’, the decrease in fruit number of marketable fruits in February in HT was also speculated to be due to delayed flower-bud differentiation of the second inflorescence, although no significant difference was observed in this factor. However, HT barely influenced flower-bud differentiation of the second inflorescence and did not decrease the fruit number of marketable fruits in February for ‘Koiminori’. This was considered to be due to cultivar traits of ‘Koiminori’, which have no leaves or only one leaf between the first and second inflorescences (Morishita et al., 2016).

HT tended to increase the achene number for the first inflorescence (Table 5). Therefore, it was thought that the D-VPDCS promoted development of the first inflorescence flower buds in parallel with enhanced vegetative growth (Forney and Breen, 1985).

In the current experiment, HT barely influenced the fruit yield of ‘Yumenoka’ (Table 6). This may be because the number of ‘Yumenoka’ fruits removed per inflorescence was 2–5 more than removed under the conventional growth method or because artificial lighting for extension of day length ceased approximately two weeks before that for the conventional method (Nagasaki-ken Ichigo Bukai, JA ZEN-NOH Nagasaki, 2016).

In this study, there were no significant effects of HT on fruit quality traits such as the FFS, SSC, FA, and SSC/acidity for ‘Koiminori’, ‘Kaorino’, and ‘Yumeoka’ (Table 7; Fig. 4). Because the SSC/acidity ratio exceeds 12, which is the borderline of good taste (Sato and Kitajima, 2007; Sone et al., 2000) for ‘Koiminori’, ‘Kaorino’, and ‘Yumenoka’, the fruit taste might satisfy the minimum level for acceptability both in NT and HT. Because sugar composition, flavor, and texture must also be considered for strawberry fruit (Ogiwara et al., 1998), we will further study the effects of VPD on taste-related traits.

Fig. 4

Soluble-solid content (SSC)/acidity ratio for three strawberry cultivars grown under non-humidification treatment (NT) and humidification treatment (HT). Error bars indicate the standard error (n = 6 for ‘Koiminori’ and ‘Kaorino’ in December, n = 3 for ‘Yumenoka’ in December, n = 12 for three cultivars in Period II, and n = 18 for three cultivars in Period III). NS indicates no significant difference at P = 0.05 by t-test.

Based on the present results, from transplanting to March, the D-VPDCS effectively improved the VPD condition and promoted plant growth. The D-VPDCS increased fruit yield within the transplanting year for ‘Koiminori’ and ‘Saga i9’, and during the cultivation period for ‘Koiminori’, ‘Kaorino’, and ‘Saga i9’. However, the D-VPDCS could also delay flower-bud differentiation for the second inflorescence of ‘Kaorino’, ‘Yumenoka’, and ‘Saga i9’. To improve the D-VPDCS, we will investigate the mechanism of VPD’s effects on plant growth, flower-bud differentiation, fruit yield, and fruit quality in strawberry in more detail.

Acknowledgements

The authors acknowledge the members of group 3 of Western Region Operation Unit 2, Technical Support Center of Western Regions, NARO, Dr. Yoneda, Dr. Matsuda, Mrs. Shikata, as well as Mrs. Ono for assistance with cultivation, research, and making valuable suggestions. We thank Robert McKenzie, PhD, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

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