The Impact of Insect-Proof Screen on Microclimate, Reference Evapotranspiration and Growth of Chinese Flowering Cabbage in Arid and Semi-Arid Region

Abstract

Despite the steadily increasing area under protected agriculture there is a current lack of knowledge about the effects of the insect-proof screen (IPS) on microclimate and crop water requirements in arid and semi-arid regions. Field experiments were conducted in two crop cycles in Ningxia of Northwest China to study the impact of IPS on microclimate, reference evapotranspiration (ET₀) and growth of Chinese Flowering Cabbage (CFC). The results showed that IPS could appreciably improve the microclimate of the CFC field in the two crop cycles. During the first crop cycle (C1), compared with no insect-proof screen (NIPS) treatment, the total solar radiation and daily wind speed under the IPS treatment were reduced by 5.73% and 88.73%. IPS increased the daily average air humidity, air, and soil temperature during C1 by 11.84%, 15.11% and 10.37%, respectively. Furthermore, the total solar radiation and daily wind speed under the IPS treatment during the second crop cycle (C2) were markedly decreased by 20.45% and 95.73%, respectively. During C2, the daily average air temperature and air humidity under the IPS treatment were increased slightly, whereas the daily average soil temperature was decreased by 4.84%. Compared with NIPS treatment, the ET₀ under the IPS treatment during C1 and C2 was decreased by 6.52% and 21.20%, respectively, suggesting it had great water-saving potential when using IPS. The plant height, leaf number and leaf circumference of CFC under the IPS treatment were higher than those under the NIPS treatment. The yield under the IPS treatment was significantly increased by 36.00% and 108.92% in C1 and C2, respectively. Moreover, irrigation water use efficiency (IWUE) was significantly improved under the IPS treatment in the two crop cycles. Therefore, it is concluded that IPS can improve microclimate, reduce ET₀, and increase crop yield and IWUE in arid and semi-arid areas of Northwest China.

1. Introduction

Drought has long been the primary factor limiting crop production due to the shortage and uneven distribution of water resources in arid and semi-arid regions of China [1,2]. More than 90% of Ningxia lies in arid or semi-arid zones with annual precipitation of less than 400 mm. The temporal distribution of precipitation is uneven, with 70% of the annual precipitation mainly concentrated from July to September, and there is a high rate of evaporation [3]. With long radiation time, and a large temperature difference between day and night, it has become an important base for inland leafy vegetables in Hong Kong and Macau. The area of vegetables in the region is approximately 131,000 ha, and the area is increasing year by year [4]. There are a series of problems, such as low utilization efficiency of water resources, and unreasonable structure (i.e., agricultural water use accounts for about 90% of the total) [5]. The shortage of irrigation water restricts the improvement of agricultural productivity and economic development in this area [6,7]. Therefore, improving water use efficiency (IWUE) through reasonable measures is an important guarantee for the sustainable development of local agriculture.

Cultivation technology covered by an insect-proof screen (IPS) is one of the important measures for the production of pesticide-free vegetables, which is of great significance to exclude the penetration of insects, such as sweet potato whitefly (Bemisia tabaci) (Insecta: Hemiptera: Homoptera: Sternorrhyncha: Aleyrodoidea: Aleyrodidae: Aleyrodinae) or leaf miner [8,9,10]. The cultivation under the screens allows significant reductions in pesticide application (if high-mesh screens are used) and increases marketable yield by reducing the radiation and wind damage to the fruit [11,12]. However, it may have an important impact on microclimate and crop water requirements [13]. Screens impede the exchanges of radiation, mass, heat and momentum between the crop and the atmosphere, thus modifying crop microclimate and reducing water requirements [14]. Experimental evidence has been presented that suggests that significant reductions in water use can be achieved when crops are grown under protective screens and nets [15,16]. Low evaporation demand under shading can increase the stomatal conductance of plants in a way that may reduce irrigation demand, resulting in water saving, and promoting vegetable growth, edible yield, nutritional quality, and pest control [17,18]. Measurements have shown a reduction of about 50–70% in ventilation rate, and about 50% reduction in crop water use, as compared to estimated values for an open pepper field [9]. At present, studies on crop water requirements with IPS are mainly concentrated in the Mediterranean area, which is characterized by hot and dry summers and mild and rainy winters. The Ningxia area belongs to the arid and semi-arid temperate continental monsoon climate, which has four distinct seasons, characterized by a dry climate, strong evaporation, and concentrated precipitation. The climate of Ningxia is quite different from the Mediterranean climate, but there are few studies on the effect of IPS on crop water demand in a temperate continental monsoon climate. Therefore, it is of great significance to study the influence of IPS on water demand for improving irrigation management.

In addition, the studies of IPS were mainly focused on crops such as tomato [19,20], sweet pepper [15], cucumber [18], and banana [17]. Compared with other vegetables, leafy vegetables have fast growth, short growth periods, multiple cropping, are rich in minerals, and are vulnerable to pests [21]. Furthermore, leafy vegetables require high-frequency irrigation because of shallow roots and the large demand for water and fertilizer. Consequently, IPS technology confirmed to be beneficial to improving crop production [8,11], has been introduced and considered a potential agriculture practice for improving the IWUE of leafy vegetables in Northwest China. The Chinese Flowering Cabbage (CFC) is one of the most popular and frequently consumed leafy vegetables and has a large planting scale in this and many other areas [22]. Moreover, CFC has a large planting scale in many areas at home and abroad. At present, there was no study on the CFC under IPS in the arid and semi-arid regions.

This study was motivated by the demands for reducing ineffective transpiration evaporation and improving IWUE in the arid and semi-arid areas of Northwest China. The objectives of this research were to (i) determine the effects of IPS on microclimate parameters and ET₀ in the arid and semi-arid region, and (ii) quantify the effects of IPS on the growth, yield, and IWUE of CFC.

2. Materials and Methods

2.1. Experimental Field

The experiments were conducted at Wuzhong National Agricultural Science and Technology Park Management Committee (37°57′ N, 106°6′ E, 1130 m above sea level), Wuzhong city, Ningxia province in northwest China. The area belongs to the arid and semi-arid temperate continental monsoon climate with annual rainfall of 184.6–273.5 mm and annual evaporation of 1000–4000 mm. With sufficient sunshine, strong evaporation, sparse precipitation, and large temperature difference between day and night, the annual average sunshine duration of the site was 3000 h. The mean annual temperature was 6 °C, and the frost-free period was 176 days. In 2016, the maximum, minimum and average air temperatures were 34.7, 10.2 and 21.7 °C, respectively. The soil texture of the experiment site was sandy loam, which is well drained.

On the day before planting, undisturbed soil samples at three locations of the field were taken at 0–40 cm soil for determination of the basic physical and chemical properties of soil. The bulk density was measured by 100 cm³ rings and field capacity was followed by the method by Veihmeyer and Hendrickson [23]. The masses of organic matter, total nitrogen, available potassium and available phosphorus were analyzed in accordance with the protocol described by Lu [24]. The bulk density and field capacity of soil were 1.65 g cm⁻³ and 23.7 cm³ cm⁻³, respectively. The masses of organic matter and total nitrogen and available potassium (K₂O) and available phosphorus (P₂O₅) of soil were 1.57, 0.20, 130.2, and 12.9 mg kg⁻¹, respectively.

2.2. Experimental Treatments

Field experiments were conducted from May to August (in two growth cycles) in 2016. It had two treatments (insect-proof screen and no insect-proof screen). The experiment of insect-proof screen (IPS) treatment was carried out in an insect-proof (60 meshes, white net) screenhouse that stretch out from east to west with a length of 100 m, a span of 10 m, a ridge height of 1.3 m, a side wall shoulder height of 2.2 m (Figure 1). The experimental field of no insect-proof screen (NIPS) treatment was arranged adjacent to the field of IPS treatment, and the size was consistent with the IPS treatment.

In the experiment, Chinese Flowering Cabbage (Brassica parachinensis L. ssp. Chinensis var. utilis Tsen et Lee, Youlv 702) was seeded using a hand-push type planter on 13 May and 18 July in the first crop cycle (C1) and second crop cycle (C2), respectively. The row spacing was 15 cm. Thinning took place at growing stage of two leaves and one heart and the average plant spacing after thinning was 10 cm. According to the customary dosage of local vegetable farmers, organic fertilizer application rate of 110 t hm⁻² was evenly scattered on the soil surface of study area before the first crop sowing, and then plowed into a 0–20 cm soil layer with a micro-tiller (LKNZ Farm Machinery, Jining, China). Finally, in order to make it fully mixed, turn up the soil three times. Furthermore, unified pest and weed control management for all treatments followed conventional practices in the region. The CFC was harvested on 23 June and 18 August in C1 and C2, respectively.

2.3. Irrigation Schedule

The irrigation method was drip irrigation and the emitter spacing was 10 cm, and the nominal flow rate for each emitter (IrriGreen, Beijing, China) was 2.8 L h⁻¹ at 0.1 MPa pressure. In order to ensure that water was irrigated evenly, a probe pipe (AZ-Trime, Beijng, China) with a depth of 1 m was installed between the two treatments. The water content in the 0–60 cm soil layer was measured every 2–3 days with a TDR meter (PICO-BT, ANDRES Industries AG, Etlingen, Baden-Württemberg, Germany). The measurement frequency was increased after irrigation or rainfall. CFC plants were irrigated to 90% of field capacity (θ_f) when the soil water content of the experiment was depleted to 75% of the θ_f. During the experimental season, there was no precipitation recorded at the experimental site. The irrigation amount of each treatment is consistent, and the water meter for each irrigation amount is accurately measured. Irrigation water amounts (Ir, mm) were determined as follows [25]:

Ir = 100 × (θ_f − θ_i) × γ × H × p

(1)

where θ_f is the soil field capacity (cm³ cm⁻³), θ_i is the average value of the actual mass water content measured by TDR (cm³ cm⁻³); γ is the soil bulk density (g cm⁻³); H is the planned wet depth of soil (m), and H is 0.3 m in this experiment; p is drip irrigation water use efficiency 0.95 [26].

2.4. Measurements

The microclimate parameters such as solar radiation (R_s), air humidity (RH), wind speed (u), air temperature (T_a), and soil temperature (T_s) at the 10 cm layer were monitored by the weather station (HOBO U30 station, Bourne, MA, USA) which had been installed at 2 m above ground in the middle of the IPS and NIPS treatments and had been well calibrated before installation. The microclimate parameters were measured every second and average values were recorded every 30 min on the HOBO data logger (HOBOware Pro, Bourne, MA, USA).

To examine the potential effect of the IPS on crop water use, reference crop evapotranspiration (ET₀) was estimated using the FAO Penman–Monteith method [27], using the IPS and NIPS measured meteorological data such as T_a, R_s, and u measured. Assuming that the daily soil heat flux density is zero, the ET₀ is calculated from:

$$E{T}_0=\frac{0.408\Delta R_n+\gamma \frac{900}{T_a+273}u(e_s-e_a)}{\Delta +\gamma (1+0.34u)}$$

(2)

where $\Delta$ is the slope pressure curve (kPa C⁻¹), $\gamma$ is the psychrometric constant (kPa C⁻¹), $R_n$ is the net radiation at the crop surface, (e_s − e_a) is the vapour pressure deficit of the air (kPa). The use of Equation (2) implicitly implies that bulk surface resistance was 70 s m⁻¹, aerodynamic resistance (s m⁻¹) was equal to 208/u, and crop albedo was fixed at 0.23.

During each growing season, three representative CFC plants per treatment were selected for plant height, leaf number, and leaf circumference area determination. The growth parameters were measured every seven days. The plant height was measured from the vegetable sprout base to the tip using a ruler with accuracy of 0.01 mm. Leaf number was determined by the number of actual green leaves per plant. In the measurement, the longest canopy length and width of the marked plants were recorded and leaf circumference area was calculated by multiplying the longest canopy length and width for each plant.

The yield of CFC for each treatment was determined from two rows of three equally distributed locations in each field. At each location, 1.0 m² of CFC plants were manually harvested and the samples were cut from the third green leaf at the base of the plant with a flat cut. The weight of plants was measured using an electronic balance (Leqi battery balance, Suzhou, China) with precision of 0.01 g. The average yield of the three locations samples for each treatment was used to represent the value of treatment.

Irrigation water use efficiency (IWUE, kg m⁻³) was calculated based on the following equation:

$$IWUE=\frac{100\times GY}{I}$$

(3)

where GY is the yield (t ha⁻¹) and I is the irrigation amount (mm).

2.5. Statistical Analysis

One-way analysis of variance (ANOVA) with three replications was used to test whether the IPS had a significant effect on plant growth, yield, and IWUE at the probability levels of 0.05 or 0.01. These statistical tests were performed by using the SPSS version 18.0 software (version 18.0, SPSS, Chicago, IL, USA).

3. Results

3.1. Daily Average Solar Radiation, Air Humidity and Wind Speed

IPS had a great influence on daily solar radiation (R_s), air humidity (RH) and wind speed during C1 and C2 (Figure 2). The R_s under the IPS and NIPS treatments during C1 varied from 41.31 to 275.25 W m⁻² and 53.23 to 287.75 W m⁻², respectively. The average daily R_s under the IPS and NIPS treatments were 198.27 W m⁻² and 210.33 W m⁻² during C1, respectively. The total R_s under the IPS treatment was 8327.2 W m⁻², which was 5.73% lower than that under the NIPS treatment during C1 (Figure 2a). Moreover, the range of R_s under the IPS and NIPS treatments during C2 varied from 45.38 to 275.30 W m⁻² and 59.73 to 327.93 W m⁻², respectively. The average R_s under the IPS and NIPS treatments were 198.47 W m⁻² and 249.49 W m⁻² during C2, respectively. The total R_s under the IPS treatment during C2 was 7740.33 W m⁻², which was 20.45% lower than that under the NIPS treatment (Figure 2d). It can be concluded that IPS can markedly reduce R_s, especially during C2.

During C1, the daily average RH under the IPS and NIPS treatments were 26.38–89.27% (mean = 47.22%) and 21.38–84.27% (mean = 42.22%), respectively (Figure 2b). The daily average RH under the IPS and NIPS treatments during C1 showed a gradual downward trend, which may be due to the gradual increase in T_a. The daily average RH under the IPS treatment was increased by 11.84% during C1 (Figure 2b). During C2, the daily average RH under the IPS and NIPS treatments were 35.48–90.83% (mean = 60.58%) and 32.33–90.51% (mean = 59.35%), respectively (Figure 2e). The daily average RH during C2 showed a trend of first decreasing and then gradually increasing. This may be due to the gradual increase in T_a and then the gradual decrease, resulting in changes in daily average RH. The daily average RH under the IPS treatment was 2.08% higher than that under the NIPS treatment during C2 (Figure 2e). It can be seen that IPS can markedly increase the daily average RH, especially in C1.

The daily average wind speed during C1 under the IPS and NIPS treatments were 0.14 (0–1.16 m s⁻¹) and 1.28 (0–2.83 m s⁻¹) m s⁻¹, respectively (Figure 2c). During C2, the average wind speed under the IPS and NIPS treatments were 0.06 (0–0.56 m s⁻¹) and 1.41 (0.42–3.65 m s⁻¹) m s⁻¹, respectively (Figure 2f). Compared with NIPS treatment, the daily average wind speed under the IPS treatment was decreased by 88.73% and 95.73% during C1 and C2, respectively. It can be found that the wind speed under IPS treatment was greatly reduced and IPS gave wind efficient protection.

3.2. Air and Soil Temperature

The Earth’s surface is the interface between the soil and the atmosphere for heat exchange, and its temperature is directly affected by changes in air temperature (T_a). There is a positive correlation between T_a and R_s. During C1 and C2, the IPS had large effects on the T_a and soil temperature (T_s) (Figure 3 and Figure 4). The trends of change in daily T_a and T_s were similar during C1 and C2. The daily average T_a (T_a-avg) and T_s (T_s-avg) were markedly correlated (R² = 0.992 under the IPS treatment and R² = 0.989 under the NIPS treatment during C1, R² = 0.996 under the IPS treatment, R² = 0.998 under the NIPS treatment during C2). The T_a-avg under the IPS and NIPS treatments were well correlated with corresponding maximum T_a (T_a-max) during C1 and C2, with R² ≥ 0.85.

During C1, daily T_a and T_s showed a gradually increasing trend. The variation range of daily T_a-max and minimum T_a (T_a-min) were 14.34–37.56 °C and 3.01–19.25 °C under the IPS treatment, and 11.84–35.06 °C and 0.51–16.75 °C under the NIPS treatment, respectively (Figure 3a,b). The daily maximum T_s (T_s-max) and minimum T_s (T_s-min) were 17.01–34.47 °C and 7.95–22.71 °C under the IPS treatment, and 15.01–32.47 °C and 5.95–20.71 °C under the NIPS treatment, respectively (Figure 3d,e). The daily T_a-avg and T_s-avg were 10.63–26.83 °C (mean = 20.32 °C) and 14.59–26.83 °C (mean = 21.83 °C) under the IPS treatment, and 8.13–24.33 °C (mean = 17.82 °C) and 12.59–24.83 °C (mean = 19.83 °C) under the NIPS treatment, respectively (Figure 3c,f). It can be found that the variation range of daily T_a-max, T_a-min, and T_a-avg under the NIPS and IPS treatments were larger than the daily T_s-max, T_s-min, and T_s-avg, while the daily T_s-max, T_s-min, and T_s-avg at 10 cm layer were relatively stable. Compared with NIPS treatment, the daily T_a-max, T_a-min, and T_a-avg under the IPS treatment were increased by 7.13–21.12% (mean = 10.09%), 14.93–57.29% (mean = 27.81%), and 10.28–30.75% (mean = 15.11%), respectively (Figure 3a–c). The daily T_s-max, T_s-min, and T_s-avg under the IPS treatment were increased by 6.16–13.32% (mean = 7.73%), 9.66–33.64% (mean = 14.72%), and 8.06–15.89% (mean = 10.37%), respectively (Figure 3d–f). Therefore, IPS greatly increased the daily T_a and T_s during C1, especially the daily T_a-min and T_s-min.

During C2, daily T_a and T_s showed a trend of gradual increased first and then decreased with the development of the Chinese Flowering Cabbage (CFC). The daily T_a-max and T_a-min were 21.82–41.24 °C and 14.05–22.42 °C under the IPS treatment, and 21.22–37.18 °C and 14.89–23.33 °C under the NIPS treatment, respectively (Figure 4a,b). The daily T_s-max and T_s-min were 23.50–33.63 °C and 20.32–26.97 °C under the IPS treatment, and 23.50–40.40 °C and 17.87–26.13 °C under the NIPS treatment, respectively (Figure 4d,e). The daily T_a-avg and T_s-avg were 19.10–29.56 °C (mean = 25.60 °C) and 22.94–29.69 °C (mean = 26.91 °C) under the IPS treatment, and 19.28–29.02 °C (mean = 25.11 °C) and 22.52–31.94 °C (mean = 28.38 °C) under the NIPS treatment, respectively (Figure 4c,f). There was no obvious difference in the daily T_a-max, T_a-min, and T_a-avg between IPS and NIPS treatments. However, compared with the NIPS treatment, the daily T_s-max and T_s-avg under the IPS treatment were decreased by 12.30% and 4.84%, respectively (Figure 4d,f). While the daily T_s-max and T_s-avg under the NIPS treatment were generally higher than that under the IPS treatment, this decline was very similar to what happened to the IPS treatment during C2. The high similarity hinted at the strong impact that external T_a had on conditions inside the screenhouse. However, the daily T_s-min under the IPS treatment was increased by 4.92% (Figure 4e).

3.3. Diurnal Variation of Microclimate

Figure 5 and Figure 6 show the diurnal courses of the average values of T_a, T_s, RH and variation amplitude of these three parameters in C1 and C2, respectively. Each set of internal conditions represents the average of data from 8 days. The data presented in Figure 5 and Figure 6 were collected during 8 d (1–8 June in C1, 6–13 August in C2). It can be seen that the diurnal variation of T_a, T_s and RH under the IPS treatment was consistent with those under the NIPS treatment. The diurnal changes of T_a and T_s under the NIPS and IPS treatments in C1 and C2 were approximately sine functions.

In C1, from AM 0:00 to PM 24:00, the T_a, T_s, and RH under the IPS treatment were markedly higher than those under the NIPS treatment (Figure 5). The highest T_a and T_s under the IPS and NIPS treatments approximately appeared at PM 15:30 and PM 16:00, respectively. The lowest T_a and T_s appeared at AM 5:00 and AM 6:30, respectively. The results showed that the change of T_s lagged behind T_a, and the average lag time was one hour. The highest T_a (27.23 °C) and lowest T_a (14.56 °C) during all hours of the day under the IPS treatment were increased by 10.11% and 20.73%, respectively, compared with NIPS treatment (Figure 5a). The highest T_s (27.33 °C) and lowest T_s (17.01 °C) during all hours of the day under the IPS treatment were increased by 7.89% and 13.32%, respectively, compared with NIPS treatment (Figure 5b). Obviously, the effect of increasing the lowest T_a and T_s during all hours of the day in C1 was greater than the effect on the highest T_a and T_s, which would benefit crop growth especially in early spring. The diurnal variation of RH under the IPS and NIPS treatments was opposite to the T_a. The RH during all hours of the day under the IPS treatment was always higher than that under the NIPS treatment. The average RH under the IPS treatment over a day was 61.61%, which was 8.83% higher than that under the NIPS treatment (Figure 5c). The amplitude variation of T_a change, T_s change and RH change (measured by the ratios of the difference value of IPS and NIPS treatments to the value of NIPS treatment) over a day was just opposite to the variation trend of T_a, T_s, and RH over the day in C1 (Figure 5d). These values of T_a change (10.11–20.73%), T_s change (7.89–13.33%), and RH change (6.64–13.75%) during all hours of the day in C1 were positive. It can be seen that IPS could increase T_a, T_s, and RH during all hours of the day in C1.

In C2, from AM 8:30 to PM 18:30, the T_a under the IPS treatment was higher than that under the NIPS treatment and they were slightly lower than that under the NIPS treatment at other times (Figure 6a). From AM 2:30 to AM 8:00, the T_s under the IPS treatment was higher than that under the NIPS treatment and they were lower than that under the NIPS treatment at other times (Figure 6b). Compared with NIPS treatment, the highest T_s (32.78 °C) under the IPS treatment over a day was decreased by 13.70%, whereas the lowest T_s (26.34 °C) over a day was increased by 1.86%. Obviously, the IPS treatment had a greater effect on reducing the T_s-max in C2 than the T_s-min. The results showed that the IPS treatment had a certain buffer effect on high temperatures. From AM 8:00 to PM 17:00, the RH under the IPS treatment was slightly lower than that under the NIPS treatment and they were obviously higher than that under the NIPS treatment at other times (Figure 6c). The average RH during all hours of the day under the IPS treatment (54.63%) was 2.57% higher than that under the NIPS treatment (53.26%). From AM 8:00 to PM 18:00, the values of amplitude variation of T_a change were positive; nevertheless, the values were negative at other times (Figure 6d). From AM 8:30 to midnight (AM 0:00), the values of T_s change were negative. The trend of amplitude variation of RH change was opposite to that of T_a change. Hence, IPS could reduce T_s when the T_a was high during the daytime, and increase the T_s and RH when the T_a was low at night in C2.

3.4. Crop Evapotranspiration

Figure 7 shows the change process of the reference crop evapotranspiration (ET₀) under the IPS and NIPS treatments during the two growth periods. It can be seen that the daily ET₀ under the NIPS treatment was higher than that under the IPS treatment during C1. The maximum daily ET₀ under the IPS treatment in C1 was 7.32 mm, which was 11.02% lower than the maximum daily ET₀ under the NIPS treatment (8.23 mm). In C1, the accumulation of ET₀ under the IPS treatment (214.51 mm) was decreased by 6.52%, compared with that under the NIPS treatment (229.46 mm) (Figure 7a).

The trend of change in daily ET₀ in C2 under the IPS treatment was more consistent with that under the NIPS treatment, but the difference in daily ET₀ between the two treatments was more obvious compared with the C1. The maximum daily ET₀ under the IPS treatment was 7.40 mm, which was 24.94% lower than the maximum daily ET₀ under the NIPS treatment (9.86 mm) (Figure 7b). The accumulation of ET₀ under the IPS treatment during the growth period in C2 was 205.88 mm, which decreased by 21.20% compared with that under the NIPS treatment (261.26 mm). It can be seen that the IPS treatment can appreciably reduce the ET₀.

Figure 8 shows the relationship between ET₀ under the NIPS and IPS treatments. It can be seen that the ET₀ under the NIPS and IPS treatments was linearly correlated (shown with a solid line in Figure 8). The ET₀ under the IPS treatment was 91.22% of that under the NIPS treatment in C1, and the ET₀ under the IPS treatment was 78.09% of that under the NIPS treatment in C2. It can be seen that under the condition of selecting the same crop coefficient, the crop water requirement under the IPS treatment in C1 and C2 will be reduced by about 8.78% and 21.91%, respectively, compared with NIPS treatment. This may be due to IPS reduced incoming R_s and wind speed. Therefore, the IPS treatment had an important effect on ET₀ and crop water requirements.

3.5. The Growth of CFC

The changes in plant height, leaf number, and leaf circumference area of CFC under the NIPS and IPS treatments are shown in Figure 9. It can be seen that the plant height, leaf number, and leaf circumference area of CFC under the IPS treatment were higher than that under the NIPS treatment. As the growth period progressed, the difference between the two treatments was gradually significant. In C1, the plant heights of CFC under the IPS treatment on 31 May, 8 June, 15 June, and 21 June were 3.29%, 15.01%, 31.11%, and 8.33% higher than that under the NIPS treatment, respectively (Figure 9a). In C2, the plant heights of CFC under the IPS treatment on 27 July, 3 August, 10 August, and 18 August were 5.66, 7.38, 10.60, and 21.85 cm, respectively, which were increased by 23.6%, 8.49%, 40.71%, and 44.22% compared with that under the NIPS treatment (Figure 9d). The results of variance analysis showed that the IPS had a significant impact on the plant heights on June 15 and 21 June in C1, and the plant height on 18 August in C2. The result showed that IPS could significantly increase the plant height of CFC (

Table 1. Effect of IPS on growth and development of CFC.

Stubble Number	Date		Plant Height	Leaf Number	Leaf Circumference Area
C1	DAS18	31 May	NS (p = 0.740)	NS (p = 0.332)	NS (p = 0.184)
	DAS26	8 June	NS (p = 0.230)	NS (p = 0.161)	NS (p = 0.423)
	DAS33	15 June	** (p = 0.003)	NS (p = 0.085)	* (p = 0.027)
	DAS39	21 June	** (p = 0.035)	NS (p = 0.335)	* (p = 0.044)
C2	DAS12	27 July	NS (p = 0.249)	** (p = 0.007)	** (p = 0.001)
	DAS19	3 August	NS (p = 0.339)	* (p = 0.036)	* (p = 0.027)
	DAS26	10 August	NS (p = 0.056)	* (p = 0.036)	* (p = 0.021)
	DAS34	18 August	** (p = 0.004)	** (p = 0.000)	** (p = 0.006)

The values followed by the same letter in the column are not significantly different at the level of 0.05. NS = not significant at the 0.05 level. * = significant at the 0.05 level; ** = very significant at the 0.01 level.

).

It can be seen that the leaf number of CFCs under the IPS treatment was higher than that under the NIPS treatment. For example, the leaf number of CFC on 21 June under the IPS treatment (9.5) was 8.57% higher than that under the NIPS treatment (8.75) in C1, and the leaf number of CFC under the IPS treatment (9.7) was 49.23% higher than that under the NIPS treatment (6.5) on 18 August in C2 (Figure 9b,e). The variance results showed that the difference in leaf number between the two treatments in C1 at different stages was not significant. There were significant differences in leaf number between the two treatments at different growth stages in C2. These results showed that IPS could promote the increase in the number of leaves in the later growth period of CFC, especially during C2 (Table 1).

The changes in the leaf circumference area of the CFC under the NIPS and IPS treatments are shown in Figure 9c,f, respectively. It can be seen that the differences in leaf circumference area of CFC between the two treatments gradually increased with the growth period in C1. On 21 June, the leaf circumference area of CFC under the IPS treatment was significantly higher than that under the NIPS treatment. On 31 May, 8 June, 15 June and 21 June, the leaf circumference area of the CFC under the IPS treatment were 13.93, 145.25, 316.33, and 401.00 cm², which were 12.68%, 17.04%, 44.03%, and 77.96% higher than that under the NIPS treatment, respectively. The leaf circumference area of the CFC under the IPS treatment was significantly higher than that under the NIPS treatment in C2, and it was more obvious in the early growth stage of CFC. For example, on 27 July, 3 August, 10 August, and 18 August, the leaf circumference area of the CFC under the IPS treatment were 87.78, 215.27, 274.16, and 425.14 cm², which were 339%, 120%, 69.15%, and 98.36% higher than that under the NIPS treatment, respectively. The results of variance analysis showed that IPS had a significant impact on the leaf circumference area during C1 and C2. Therefore, IPS can significantly promote the increase in leaf circumference area, and it may be related to a certain degree of insect damage to the CFC under the NIPS treatment, especially in C2.

3.6. Yield and Irrigation Water Use Efficiency

Table 2. Effect of IPS on yield and IWUE of CFC.

Treatment	C1		C2
	Yield (kg ha−1)	IWUE (kg m−3)	Yield (kg ha−1)	IWUE (kg m−3)
IPS	10031.68 + 310.15	2.33 + 0.07	13707.59 + 1536.59	3.01 + 0.34
NIPS	7376.13 + 40.25	1.71 + 0.01	6561.06 + 290.32	1.44 + 0.06
ANOVA
n	** (p = 0.002)	** (p = 0.002)	* (p = 0.017)	* (p = 0.017)

* = significant at the 0.05 level; ** = very significant at the 0.01 level.

shows the effect of IPS on yield and irrigation water use efficiency (IWUE) of the CFC. The result showed that IPS had significant effects on the yield and IWUE in C1 and C2. The yield of CFC under the IPS treatment (10,031.68 kg ha⁻¹) was 36.00% higher than that under the NIPS treatment (7376.13 kg ha⁻¹) in C1. The IWUE of IPS and NIPS treatments were 2.33 and 1.71 kg m⁻¹ in C1, respectively. The yield of CFC under the IPS treatment (13,707.59 kg ha⁻¹) was 108.92% higher than that under the NIPS treatment (6561.06 kg ha⁻¹) in C2. The IWUE of IPS and NIPS treatments were 3.01 and 1.44 kg m⁻¹ in C2, respectively. The yield and IWUE of NIPS treatment in C2 both were the lowest, which may be due to the higher T_a and R_s. The result showed that the IPS treatment could save water and increase vegetable yield under the same irrigation conditions. The yield and IWUE of CFC were greatly increased under the IPS treatment, especially in C2.

4. Discussion

Although screenhouses have been used for many years, their popularity is mainly due to the desire to reduce the use of pesticides and requires an increased understanding of their climate. Previous studies were mostly under the conditions of the Mediterranean climate, with hot, dry summers and mild, wet winters. There have been few previous studies on the climate and water use of screenhouses in the arid and semi-arid temperate continental monsoon climate, and pressure on water resources makes this information timely. Early research on screen climate reported general characteristics, but did not combine crop growth, yield, and water consumption. The current study was conducted in the arid and semi-arid region of Northwest China to study the effects of IPS on the main microclimate, ET₀, crop growth, and IWUE of CFC.

4.1. Microclimate

In recent years, many researchers have found that R_s in the screen house decreased [28,29], the wind speed decreased [30,31], and RH increased, compared with those outside. The results of this study showed that IPS can appreciably reduce the daily average R_s. The daily average R_s under the IPS treatment during C1 and C2 were reduced by 4.34–22.39% and 8.42–30.57% compared with NIPS treatment, respectively (Figure 2). Ombódi et al. [32] observed that IPS decreased incoming radiation by 23–39%, while cultivated pepper in walk-in plastic tunnels (2.3 height, 5 m width, and 40 m length) was covered by external photo-selective shade nets in Hungary. The IPS obstructed the air flow and reduced the evaporation of moisture in the screen, which caused the RH in the screen higher than the outside [14,28]. During C1 and C2, the daily average RH under the IPS treatment was 11.84% and 2.08% higher than that under the NIPS treatment, respectively. Xing et al. [33] found that IPS can increase daily RH by 1.9–7.6% compared with that in the open field. Xu et al. [34] carried out a field experiment in Nanjing characterized by a subtropical monsoon climate and the area of each plot was 150 m². The daily RH in the screenhouse was 1.81% and 1.96% higher than that in the open field in 2014 and 2015, respectively. In our study, the average wind speed under the IPS treatment during C1 and C2 was decreased by 57.75–100% and 80.72–100%, respectively, compared with NIPS treatment. Previous studies had shown that IPS increased air resistance and reduced air flow, thereby reducing the wind speed [14,18]. A reduction in wind speed observed in a study conducted in the central of Israel was lower than 78–86% reported for a screenhouse made of a 50-mesh insect-proof screen [35].

Although the effect of the screen is always to reduce radiation and air velocity, its effect on temperature is much more complex [36]. The T_a is an integrated outcome of several simultaneous energy transfer processes, which include radiation exchange, convection (ventilation), and evapotranspiration. In this study, the daily average T_a and T_s under the IPS treatment during C1 increased by 10.28–30.75% and 8.06–15.89% compared with NIPS treatment, respectively (Figure 3). During C1, IPS can effectively increase the T_a and T_s, especially the T_s-min. However, the daily T_a-avg and T_s-avg under the IPS treatment during C2 were increased by −0.94–4.31% and −8.95–5.16% compared with NIPS treatment, respectively. Furthermore, IPS can distinctly reduce the T_s-max by 0.00–17.55% and increase the T_s-min by 1.22–14.11% during C2, respectively (Figure 4). The IPS treatment had different effects on the T_a and T_s of the two trials because the study area was relatively cold during C1 (in spring) and hot during C2 (in summer and autumn).

By analyzing the diurnal changes of the microclimate, it was found that the T_a, T_s, and RH under the IPS treatment were higher than those outside from AM 0:00 to PM 24:00 within a day during C1 (Figure 5). However, the T_s under the IPS treatment was 0.09–2.52% higher than that under the NIPS treatment from AM 2:30 to AM 8:00, and it was 0.25–17.23% lower than that under the NIPS treatment at other times during C2 (Figure 6). From AM 8:00 to PM 18:30, the T_a under the IPS treatment was 1.11–8.53% higher than that under the NIPS treatment, and it was 0.59–3.15% lower than that under the NIPS treatment at other times during C2 (Figure 6). The RH was opposite to the trend of the T_a. Obviously, the IPS can improve the T_a, T_s, and RH during all hours of the day in C1, especially for T_a and T_s at night. The IPS can reduce T_s during the daytime in C2, and increase T_s and RH at night. These results indicated that the IPS had a certain buffering effect on high temperatures.

4.2. Crop Evapotranspiration

Atmospheric water requirement combines the effects of R_s, wind speed, T_a and RH on crop microclimate, and has the greatest influence on crop water requirement. Therefore, the corrected microclimate, especially the reduced R_s and wind speed, may reduce crop evapotranspiration (ET) [37,38]. The cumulative amounts of ET₀ in C1 and C2 were reduced by 6.52% and 21.20% compared with that under the NIPS treatment (Figure 7). It can be seen that IPS can effectively reduce ET₀. It was of greater significance to reduce the amount of irrigation, especially in summer and autumn. In the past two decades, there have been studies on the effects of IPS on crop microclimate and water use. In many regions, such as the Mediterranean, screens are normally fitted to all vents in intensive greenhouse production [15,19,20,39]. It reduced by about 30% the airflow through the insect-proof house and consequently the air exchange rate in southern Israel [20]. ET₀ under a shading screen was 38% lower than the estimated value for the open atmosphere in Northern Negev, Israel. The main reason for the observed decrease in ET₀ was the distinctly reduced R_s, while lower wind speeds inside the screenhouse have contributed to a smaller extent [13,40]. Similarly, Pirkner et al. [41] reported a 34% reduction in evapotranspiration for a table-grape vineyard cultivated in a 10%-shading screenhouse, as compared with estimations in open field vineyard in Israel. Möller et al. [13,15,16] showed that the ET of sweet pepper grown in a 50-mesh insect-proof screen was reduced by 30–50% compared to that in an open pepper field. In another study, ET₀ of sweet pepper was investigated by using three different screens, they found that the mean relative reduction in ET₀ with respect to outside ET₀ in pearl insect-proof screen, white insect-proof screen and green shade screen were 17.4%, 41.3%, and 42.6%, respectively [42].

4.3. Crop Growth, Yield and IWUE

The IPS can improve the microclimate of the farmland and reduce crop evapotranspiration, thereby promoting crop growth and increasing yield. The plant height of CFC under the IPS treatment at harvest of C1 and C2 were 8.33% and 44.22% higher than those under the NIPS treatment (Figure 9a,d). In our study, the IPS can significantly increase the growth and development of CFC, especially in C2. Ilić et al. [43] showed that shade application of nets to tomato plants was effective in substantially improving vegetative growth parameters, (i.e., leaf area index and leaf pigments) under excessive R_s during the summer period.

The yields of CFC under the IPS treatment in C1 and C2 were 36.00% and 108.92% higher than those under the NIPS treatment, respectively, and IPS can significantly improve IWUE in the two crop cycles (Table 2). In general, a properly higher temperature can positively affect crop growth. Sufficient radiation is needed for photosynthesis; however, under supra-optimal radiation plants may be under stress, close their stomata and reduce production. Vegetables exposed to high levels of direct radiation may suffer sunburn which significantly reduces the marketable yield [14]. In this study, there was no obvious difference in the average R_s under the IPS treatment in C1 (198.27 W m⁻²) and C2 (198.47 W m⁻²) (Figure 2). The daily T_a-avg (25.60 °C) under the IPS treatment in C2 was 25.98% higher than that in C1 (Figure 3 and Figure 4). It can be seen that the growth and yield of CFC under the IPS treatment in C2 were higher than that in C1 because of the increase in temperature. The IPS could appreciably reduce R_s, and reducing R_s may be favorable for improving the thermal climate and avoiding sunburn in hot climates. The total R_s under the IPS treatment in C1 and C2 were 20.45% and 5.73% lower than that under the NIPS treatment, respectively. For the NIPS treatment, the average R_s (249.49 W m⁻²) in C2 was 18.62% higher than that in C1 (Figure 2). The CFC is a short-season crop and 70% of the root system distribute in the 10–30 cm soil layer [44]. To ensure that irrigation water can be absorbed by crops, high-frequency irrigation is required to keep the water in the main distribution area of the crop root system. The accumulation of ET₀ under the NIPS treatment during C2 was 261.26 mm, which was 13.86% higher than that in C1 (Figure 7). The yield of CFC under the NIPS treatment in C2 was lowest in all treatments for the two trials which could be due to the extremely high R_s and ET₀ under the NIPS treatment. IPS had the potential for water saving and retention, and the yield of CFC had also been greatly improved, which had a more obvious impact on summer and autumn crops. Earlier studies showed that IPS cultivation in semi-arid areas with strong radiation can reduce the physiological stress caused by strong radiation on plants by reducing R_s [45], prevent the decline of crop photosynthesis [46,47], and improve crop yield and quality [18,48]. Möller et al. [13] found that the IWUE of sweet pepper planted in the screenhouse was significantly higher than that in the open field in Northern Negev, Israel.

5. Conclusions

Field experiments with two growing crops of CFC were conducted in the arid and semi-arid areas of Ningxia to evaluate the effect of IPS on field microclimate, ET₀, vegetable growth, yield, and IWUE. The following conclusions were supported by our study:

Compared with NIPS treatment, IPS could appreciably decrease solar radiation and wind speed during C1 and C2, and protect CFC from sunburn and wind damage. Air temperature, soil temperature, and air humidity were obviously increased during C1, whereas soil temperature was distinctly decreased during C2. These effects have the potential of reducing crop evapotranspiration under IPN treatment, as illustrated by the estimates of ET₀. IPS could create a better air, soil water, and thermal environment, significantly promote the growth and development of CFC, and thereby significantly improve IWUE compared with NIPS treatment in the two crop cycles. The application of IPN was very important for improving the microclimate, ET₀, vegetable growth, yield, and IWUE of CFC.

IPS was recommended for leafy vegetable planting to both increase crop yield and improve water use efficiency in the arid and semi-arid regions of China.