Effects of Irrigation Interval and Irrigation Level on Growth, Photosynthesis, Fruit Yield, Quality, and Water-Nitrogen Use Efficiency of Drip-Fertigated Greenhouse Tomatoes (Solanum lycopersicum L.)

Abstract

The inefficient irrigation strategy is an important factor affecting the yield and water productivity of tomatoes in greenhouses, seriously hindering the development of the cultivation industry. While the impact of irrigation level on tomato growth and yield has been extensively studied, irrigation interval, another crucial component of irrigation schedule, as well as their interaction, remain poorly explored. There were four irrigation levels (W1: 125% ET_c, W2: 100% ET_c, W3: 75% ET_c, and W4: 50% ET_c; ET_c represented crop evapotranspiration) and three irrigation intervals (D1: 4-day interval, D2: 7-day interval, and D3: 10-day interval), aiming to explore the effects of different irrigation intervals and levels on the performance of tomatoes. Here, we showed that the moderate increases in irrigation level and interval promoted root growth, improved nitrogen uptake and distribution, and enhanced plant height, stem diameter, leaf area index, and aboveground biomass, thereby promoting the net photosynthetic rate of plants and fruit yield. The fruit quality indicators of total soluble solids, vitamin C, and soluble sugar decreased with increasing irrigation level but increased with decreasing irrigation interval. Higher irrigation levels increased tomato water consumption and resulted in lower water-nitrogen use efficiency. Overall, compared with W2D2 and W2D3, the yield of W2D1 increased by 8.0% and 26.1%, respectively, and the water productivity increased by 5.7% and 19.3%, respectively, and the soluble sugar increased by 7.1% and 17.5%, respectively. In addition, nitrogen uptake in tomato organs increased and then decreased with the increase of irrigation level, while it consistently increased with decreasing irrigation interval. At the harvest period, the nitrogen uptake in plant organs followed the order of fruit > leaf > stem. Taken together, W2D1 (100% ET_c and 4-day interval) is the recommended irrigation strategy for this experiment, which can provide a theoretical basis and technical support for the sustainable production strategy of greenhouse drip irrigation tomatoes.

1. Introduction

Tomatoes (Solanum lycopersicum L.) are one of the most economically significant vegetable crops globally and have gained considerable popularity in recent years. Tomatoes are rich in antioxidants, such as vitamin C, lycopene, and phenolic compounds, which are linked to a reduced risk of certain cancers and cardiovascular diseases [1,2]. Consequently, the rising demand for tomatoes has significantly stimulated the development of the tomato cultivation industry. Currently, growing tomatoes in greenhouses has certain benefits that promote year-round vegetable supply and allow for optimal control of agronomic and environmental factors (mainly external rainfall) [3,4]. However, tomatoes have high water requirements, necessitating sufficient irrigation to meet their growth needs [5]. In greenhouses, crops are shielded by plastic film for extended periods, preventing the use of natural rainfall and relying solely on irrigation as the water source. Thus, in arid and semi-arid regions where agricultural water resources are limited, developing efficient irrigation strategies and implementing water-saving irrigation technologies are crucial for the sustainable use of water resources and enhancing the productivity of greenhouse tomato.

Compared with traditional irrigation methods, drip irrigation is generally considered to be a more efficient water-saving technology. It can uniformly and continuously transport water and nutrients to the root system, reduce or eliminate the evaporation loss of surface soil caused by irrigation [6], thereby improving water and nutrient use efficiency [7]. Furthermore, drip irrigation is more likely to achieve an optimal balance between yield and quality, showing significant potential in improving water productivity [8,9]. Consequently, it has been widely adopted in greenhouse crop production.

Irrigation level is a critical factor in greenhouse crop production. Local farmers often lack scientific irrigation management, resulting in significant variations in irrigation level and leading to common occurrences of inefficient irrigation practices. Many studies have demonstrated that excessive irrigation does not proportionally increase yield and may also cause water waste and imbalance between water supply and demand, ultimately reducing tomato yield and quality [10,11] and even increasing the risk of soil nutrient loss. Conversely, insufficient irrigation can cause premature crop aging, smaller fruit size, and increased susceptibility to disease, negatively impacting crop growth, photosynthesis, and productivity [12,13]. Improper irrigation scheduling can also lead to secondary soil salinization and reduced water productivity [14]. Therefore, ensuring the appropriate amount of irrigation is essential for the effective utilization of resources and sustained crop yields.

Irrigation interval is another important factor in drip irrigation management, which can optimize the allocation of limited irrigation water throughout the growth period, thereby determining the optimal irrigation cycle for crops. It is inconclusive how crops are affected by irrigation interval, even when the same level of irrigation is applied at different irrigation intervals. Some studies have found that the productivity of some greenhouse crops responds positively to small irrigation intervals, such as cotton, pineapple, potato, bell pepper, etc. [15,16,17,18]. On the contrary, other studies have indicated that irrigation interval has either no effect or even adverse effects on the yields of sesame and wheat [19,20]. These differences may reflect different responses to irrigation interval across crops, climatic conditions, soil texture, wetting patterns, and growing seasons. Additionally, low-water, small irrigation intervals could lead to greater infiltration losses compared to high-water, large irrigation intervals, and the interaction was not significant [21,22]. Jia et al. [23] further pointed out that different drip irrigation intervals can affect the soil moisture state, thereby changing indicators such as water absorption in the root zone and temperature, influencing water productivity and crop growth. Other studies have demonstrated that appropriate irrigation interval modifies soil moisture profiles, accelerates leaf growth and dry matter accumulation, and also regulates nitrogen uptake and nutrient distribution ratios to a certain extent [24,25].

At present, although the effects of single variables such as irrigation level and interval on tomato cultivation has been extensively studied, there is still a deficiency in comprehensively evaluating key indicators such as plant growth dynamics, yield formation, water and nitrogen synergistic utilization efficiency, nutrient distribution of various organs, and photosynthetic characteristics based on local water resource conditions to determine the optimal combination of irrigation level and interval for tomatoes under the film drip irrigation in solar greenhouses, which needs to be improved. Therefore, the primary objectives of this study were to: (1) investigate the interaction of irrigation level and irrigation interval on the growth, physiology, nitrogen uptake and distribution of drip-irrigated greenhouse tomato in the Northwest region, (2) explore the effects of irrigation level and irrigation interval on tomato yield, quality, WP, and NUE, and (3) obtain the best irrigation strategy for efficient production of greenhouse tomato under drip irrigation in Northwest China. It provides novel insights into optimizing the tomato irrigation system and elucidates the synergistic effects of irrigation level and irrigation interval. The results are of great significance for guiding regional water-saving irrigation technologies, solving the problem of agricultural water shortage in arid areas, and ensuring the sustainable development of agriculture in Northwest China.

2. Materials and Methods

2.1. Experimental Site Description

The greenhouse experiment was conducted over two consecutive seasons (March–July 2023 and August–December 2023) at the Key Laboratory of Soil and Water Engineering in Arid Areas (34°20′ N, 108°04′ E, 521 m above sea level), Northwest A&F University, Yangling, Shaanxi, China. The experimental area is characterized by a semi-humid and drought-prone climate, with an annual rainfall of 590 mm, an average temperature of 13.67 °C, potential evaporation of 1500 mm, average annual sunshine of 2163.8 h, and a frost-free period of 211 days. The experimental greenhouse, oriented east-west, was 75 m by 7.5 m. The soil type was loam, with a bulk density of 1.43 g/cm³, field capacity of 24% (mass water content), pH of 8.1, and nutrient concentrations of 33.2 mg/kg available nitrogen (Kjeldahl method), 23.5 mg/kg available phosphorus (Olsen-P method), and 88.8 mg/kg available potassium (NH4OAc extraction). A small weather station (HOBO U30 logger, Onset Computer Corporation, Bourne, MA, USA) was installed in the greenhouse to record climate variables (Figure 1).

2.2. Experimental Design

The tomato variety utilized in the experiment was ‘Jinpeng 101’, transplanted on 31 March 2023 (Spring 2023) and 25 August 2023 (Autumn 2023) and harvested on 22 July 2023 (Spring 2023) and 31 December 2023 (Autumn 2023). To ensure the survival of tomato seedlings, each plot received 30 mm of irrigation water after transplanting (based on the existing soil water content). The irrigation was stopped approximately 10 days before harvest. Each experimental plot was 7.1 m in length and 0.9 m in width. A buffer zone measuring 0.3 m in width was established between the two adjacent plots. Two parallel drip irrigation lines were installed in each plot (the spacing is 0.4 m, the pipe diameter is 16 mm, and the flow rate is 2 L/h), with 14 tomato plants arranged along each drip line, maintaining a row spacing of 40 cm (Figure 2). The study employed a completely randomized block design, comprising 12 treatments with 3 replicates. There were three irrigation intervals (the irrigation interval is set according to the suggestions of local farmers and some pre-experiments): D1 (4-day interval), D2 (7-day interval), and D3 (10-day interval), and four irrigation levels: W1 (125% ET_c), W2 (100% ET_c), W3 (75% ET_c), and W4 (50% ET_c), where ET_c represents the crop evapotranspiration, calculated as follows:

$$E{T}_c=K_c\times E{T}_0$$

(1)

where ET₀ represents the reference crop evapotranspiration, calculated by the FAO Penman-Monteith model [26]; K_c represents the crop coefficient, tomato with a value of 0.6 at the seedling stage, 1.1 at the flowering and fruiting stage and the fruit expansion stage, and 0.8 at the harvest stage [27].

The application rates of nitrogen fertilizer (urea, N ≥ 46%), phosphorus fertilizer (superphosphate, P₂O₅ ≥ 16%), and potassium fertilizer (potassium chloride, K₂O ≥ 60%) during the two seasons of tomato were 130 kg ha⁻¹, 130 kg ha⁻¹, and 250 kg ha⁻¹, respectively. The application rates at the seedling stage, flowering and fruiting stage, first fruit expansion stage, second fruit expansion stage, and maturity stage were 12.5%, 12.5%, 25%, 25%, and 25% of the total fertilizer rates, respectively. The fertilizer was completely dissolved in water and applied with irrigation water through a fully automatic irrigation fertilizer machine (NETAFIM, Hazerim, Israel). The specific irrigation timing and cumulative irrigation level of the tomato experiments are shown in Figure 3.

2.3. Sampling and Measurements

2.3.1. Growth Indexes

Six plants were marked for each treatment, and the plant height and stem diameter were measured with a steel tape measure and a vernier caliper, respectively. The leaf area index (LAI) was calculated by the ratio of total leaf area to land area, and the tomato leaf area was measured by the punching method [28]. Destructive sampling was required for aboveground biomass. During the five growth periods of tomato, three plant samples were randomly collected from each treatment. After the stems, leaves, and fruits were separated, they were immediately placed in an oven, dried at 105 °C for 30 min to inactivate the enzyme, then dried at 75 °C to constant weight, and finally weighed with an electronic balance.

During the maturity stage, the tomato roots were sampled by the soil coring method (10 cm in diameter and 10 cm in height) between the two tomato plants and near the tomato plants. A root scanner (EPSON Perfection V700, Epson, Tokyo, Japan) was used to scan the image at 600 dpi pixels, and then image analysis software (Win RHIZO, Version 2016a) was used to analyze and determine the root length. Finally, the analyzed root samples were placed in an oven at 75 °C, dried, and weighed.

2.3.2. Plant Nitrogen Uptake and Distribution

Six plants were selected for each treatment to determine the plant nitrogen uptake at the end of maturity. The plants were divided into three parts: stem, leaf, and fruit, and placed in an oven at 105 °C for 0.5 h, then dried at 75 °C to constant weight. The dried samples were crushed with a small grinder, extracted with H₂SO₄-H₂O₂, and finally the nitrogen content of each organ of the sample was determined using a continuous flow analyzer (AutoAnalyzer 3, Bran + Lubbe GmbH, Hamburg, Germany). The calculation was as follows [29]:

$$N uptake \left({\mathrm{kg} \mathrm{ha}}^{-1}\right)=N content (\%)\times Aboveground biomass \left({\mathrm{kg} \mathrm{ha}}^{-1}\right)$$

(2)

2.3.3. Photosynthetic Indicators

The third fully expanded leaf of six representative and randomly selected tomato plants from each plot were sampled. The net photosynthetic rate (Pn, µmol m⁻² s⁻¹), transpiration rate (Tr, mmol m⁻² s⁻¹), and stomatal conductance (Gs, mol m⁻² s⁻¹) of each leaf were measured on sunny days between 10 a.m. and 12 a.m., using the LI6800 portable photosynthesis measurement system (LI-COR, Inc., Lincoln, NE, USA), with airflow rate of 0.5 mmol s⁻¹ and CO₂ concentration of 400 μmol mol⁻¹.

2.3.4. Fruit Yield and Its Components

At the maturity stage, 15 samples were randomly selected from each treatment for measurement. The fruits were picked manually in batches every 5–7 days, and the number of fruits per plant and the weight of mature fruits per plant were recorded for each treatment. Finally, the total yield was estimated by multiplying the average yield per plant by the planting density.

2.3.5. Crop Evapotranspiration, Water Productivity, and Nitrogen Use Efficiency

The soil water balance method was used to estimate water consumption during the entire growth period [30], and the formula is as follows:

$$ET=I+P+U-D-R-\Delta W$$

(3)

where ET represents crop evapotranspiration, I represents irrigation level (mm), P represents effective rainfall (mm), U represents groundwater recharge (mm), D represents deep seepage (mm), R represents runoff (mm), and ΔW represents the change in soil moisture in the 0–100 cm profile during the test (mm). No rainfall occurred in the greenhouse, so P = 0. This experiment used drip irrigation, and the level of irrigation each time was small, so D could be ignored; because the surface was flat, R could negligible; the groundwater level was deep (>50 m), so U was negligible. The above formula was thus simplified as:

$$ET=I-\Delta W$$

(4)

The calculation methods of water productivity (WP, kg/m³), irrigation water productivity (IWP, kg/m³), and nitrogen use efficiency (NUE, kg/kg) were as follows:

$$WP={\displaystyle \frac{100Y}{ET}}$$

(5)

where Y is the total tomato yield. The same below.

$$IWP={\displaystyle \frac{100Y}{I}}$$

(6)

$$NUE={\displaystyle \frac{1000Y}{NU}}$$

(7)

where NU is the total nitrogen uptake of tomato plants.

2.3.6. Tomato Quality

At maturity, 15 uniformly sized and similarly colored fruits were randomly selected from each treatment. The fruits were rinsed with distilled water and allowed to dry naturally before assessing tomato quality indicators. Soluble solids (TSS) were measured using a handheld refractometer (UV2300, Techcom Co., Ltd., Shanghai, China). The concentrations of soluble sugar (SS) and vitamin C were determined by anthrone colorimetric and molybdenum blue colorimetric methods [31], respectively. Organic acid (OA) content was quantified by titration with 0.1 mol/L NaOH [32], and the sugar-acid ratio (SAR = SS/OA) was subsequently calculated.

2.4. Statistical Analysis

Data analysis was performed using the SPSS Statistics 27 statistical software (SPSS Inc., Chicago, IL, USA). The Origin 2025 software (OriginLab, Northampton, MA, USA) was used to draw charts and perform correlation analysis. The principal component analysis (PCA) was used to evaluate the comprehensive score of greenhouse tomato. The analysis of variance was used to evaluate the interaction effect of irrigation interval and irrigation level. Statistical analyses were performed using Duncan’s multiple range test at p < 0.05 to determine significant differences among treatments.

3. Results

3.1. Growth Indexes

The effects of irrigation level (W) and irrigation interval (D) on plant height, stem diameter, leaf area index (LAI), and aboveground biomass of tomato were highly significant in the two seasons (p < 0.01), while the interaction between the two factors (W × D) was not significant. The plant height and stem diameter of tomato plants increased with decreasing irrigation intervals. From W4 to W2, both plant height and stem diameter increased with the increase in irrigation level, while from W2 to W1, a decreasing trend was observed (Figure 4). The LAI and aboveground biomass increased rapidly as the growth period progressed, stabilizing eventually (Figure 5 and Figure 6). Except for the seedling stage and flowering fruit stage, the LAI initially increased and then decreased with increasing irrigation level, reaching the maximum value in the W2 treatment. At the mature stage, the LAI of the D1 treatment was 4.2% and 11.3% higher than that of the D2 and D3 treatments, respectively, peaking at 5.4 in spring and 3.0 in autumn in the W2D1 treatment. The changes in aboveground biomass were consistent in both growing seasons, with differences between treatments being most pronounced at the maturity stage. For the mature stage, the aboveground biomass increased first and then decreased with the increase of irrigation level, reaching the peak at W2. Compared to that of W1, W3, and W4, the aboveground biomass of W2 increased by 3.5%, 14.4%, and 27.2%, respectively. Furthermore, the aboveground biomass increased as the irrigation interval decreased, with D1 and D2 treatments showing increases by 13.3% and 9.4% compared to D3, respectively.

The irrigation level (W) and irrigation interval (D) significantly influenced the total root length and dry weight of tomato in the two growing seasons (p < 0.01). No interaction was observed between W and D on root dry weight, whereas a significant interaction regarding root length was noted only in spring 2023 (p < 0.05). The patterns of total root length and dry weight in response to irrigation level were consistent, following the order: W2 > W1 > W3 > W4 (Figure 7). In comparison to the W4 treatment, W1, W2, and W3 enhanced total root length by 35.1%, 46.6%, and 17.2%, respectively, while increasing dry weight by 22.5%, 34.4%, and 9.8%. Compared to D2 and D3 treatments, D1 resulted in increases of total root length by 12.3% and 37.9%, as well as an increase in root dry weight by 6.1% and 16.6%, respectively.

3.2. Plant Nitrogen Uptake and Distribution

Nitrogen uptake in the stems, leaves, and fruits of tomato plants increased with the decrease in irrigation interval but exhibited a trend of initial increase followed by a decrease with increasing irrigation level, peaking at W2 (Figure 8). No significant differences in nitrogen uptake by leaves and fruits were observed between W1 and W2 treatments. For W2 treatment, the average nitrogen uptake by fruits increased by 3.7%, 23.6%, and 40.2%, respectively, compared to W1, W3, and W4. Compared with W4, the average nitrogen uptake of stems under W1, W2, and W3 treatments increased by 40.3%, 51.8%, and 20.4%, respectively. Within the same irrigation volume treatment, fruit nitrogen uptake was highest in D1, followed by D2 and D3, with D1 showing increases of 5.9% and 27.5%, respectively. Compared to W4, total plant nitrogen increased by 30.8%, 36.7%, and 15.0% in spring, and by 40.2%, 47.8%, and 16.9% in autumn for W1, W2, and W3, respectively. Compared with D1, the nitrogen uptake of D2 and D3 plants was reduced by 3.6% and 18.3% in the two growing seasons. During the fruit maturity stage, nitrogen absorbed by the tomato plants was predominantly allocated to the fruits, with fruit nitrogen comprising approximately 50% of total plant nitrogen (52.6–57.8% in spring 2023 and 46.4–53.4% in autumn 2023). The nitrogen uptake in the tomato organs followed the order of fruit > leaf > stem, with 22.8–26.5% nitrogen in leaves and 17.5–21.7% in stems.

3.3. Photosynthetic Indicators

The photosynthetic parameters at various stages of growth (flowering and fruit setting, fruit expansion, and maturity) in the two growing seasons were presented in

Table 1. Effects of different irrigation strategies on leaf photosynthesis of tomatoes in spring 2023.

Treatment	Pn (μmol m−2 s−1)			Tr (mmol m−2 s−1)			Gs (mol m−2 s−1)
	Flowering	Expansion	Maturity	Flowering	Expansion	Maturity	Flowering	Expansion	Maturity
W1D1	20.14 ± 1.12 a	23.32 ± 1.27 b	12.33 ± 0.36 b	10.29 ± 0.84 a	11.29 ± 0.30 b	6.92 ± 0.40 bc	0.49 ± 0.02 a	0.52 ± 0.01 bc	0.30 ± 0.01 b
W2D1	19.09 ± 0.54 ab	25.58 ± 0.64 a	13.30 ± 0.28 a	9.57 ± 0.67 abc	12.24 ± 0.24 a	7.69 ± 0.18 a	0.47 ± 0.02 a	0.55 ± 0.01 a	0.34 ± 0.01 a
W3D1	18.42 ± 0.67 bc	20.68 ± 0.44 cd	10.89 ± 0.35 de	8.84 ± 0.28 cd	10.30 ± 0.12 cd	6.24 ± 0.28 de	0.43 ± 0.02 b	0.46 ± 0.01 d	0.27 ± 0.01 cd
W4D1	16.28 ± 0.75 de	17.29 ± 0.75 e	9.63 ± 0.58 g	7.83 ± 0.32 ef	9.67 ± 0.22 ef	5.68 ± 0.12 fg	0.38 ± 0.03 de	0.41 ± 0.01 f	0.24 ± 0.01 ef
W1D2	19.73 ± 1.06 ab	21.76 ± 0.86 c	11.52 ± 0.56 cd	9.70 ± 0.66 ab	10.73 ± 0.39 c	6.46 ± 0.19 d	0.47 ± 0.01 a	0.50 ± 0.03 c	0.28 ± 0.01 bc
W2D2	18.50 ± 0.86 bc	23.26 ± 0.84 b	12.59 ± 0.2 b	9.25 ± 0.30 bc	11.63 ± 0.50 b	7.18 ± 0.21 b	0.43 ± 0.03 b	0.53 ± 0.02 ab	0.32 ± 0.03 a
W3D2	17.55 ± 0.98 cd	19.27 ± 0.92 d	10.45 ± 0.48 e	8.28 ± 0.18 de	10.07 ± 0.10 de	5.82 ± 0.21 f	0.40 ± 0.02 bcd	0.44 ± 0.02 de	0.26 ± 0.01 de
W4D2	15.55 ± 0.67 e	15.67 ± 0.89 f	8.78 ± 0.26 h	7.49 ± 0.35 fg	9.22 ± 0.42 f	5.35 ± 0.11 gh	0.35 ± 0.04 ef	0.38 ± 0.01 gh	0.23 ± 0.01 fg
W1D3	16.68 ± 0.69 de	19.26 ± 1.06 d	10.37 ± 0.58 ef	8.90 ± 0.17 cd	10.05 ± 0.44 de	6.06 ± 0.12 ef	0.41 ± 0.02 bc	0.42 ± 0.02 ef	0.25 ± 0.01 de
W2D3	15.70 ± 1.09 e	20.20 ± 1.63 d	11.60 ± 0.27 c	8.40 ± 0.15 de	10.62 ± 0.28 c	6.58 ± 0.16 cd	0.39 ± 0.01 cd	0.47 ± 0.01 d	0.29 ± 0.02 bc
W3D3	14.19 ± 0.29 f	17.77 ± 0.79 e	9.70 ± 0.33 fg	8.08 ± 0.19 ef	9.47 ± 0.44 f	5.71 ± 0.31 fg	0.37 ± 0.01 de	0.40 ± 0.03 fg	0.22 ± 0.01 f
W4D3	13.65 ± 1.04 f	15.02 ± 0.75 f	8.37 ± 0.84 h	6.77 ± 0.73 g	8.67 ± 0.38 g	5.17 ± 0.05 h	0.33 ± 0.02 f	0.37 ± 0.02 h	0.20 ± 0.01 g
Source of variation
W	**	**	**	**	**	**	**	**	**
D	**	**	**	**	**	**	**	**	**
W × D	ns	ns	ns	ns	ns	*	ns	*	ns

The leaf photosynthesis of tomato in spring 2023. Different letters indicate significant difference at p < 0.05 for treatments. The same below. *, p < 0.05, **, p < 0.01; ns, non–significant. W: irrigation level, D: irrigation interval.

and

Table 2. Effects of different irrigation strategies on leaf photosynthesis of tomatoes in autumn 2023.

Treatment	Pn (μmol m−2 s−1)			Tr (mmol m−2 s−1)			Gs (mol m−2 s−1)
	Flowering	Expansion	Maturity	Flowering	Expansion	Maturity	Flowering	Expansion	Maturity
W1D1	15.98 ± 0.50 a	18.36 ± 0.47 b	9.96 ± 0.39 b	8.99 ± 0.69 a	10.41 ± 0.35 ab	5.85 ± 0.10 bc	0.44 ± 0.02 a	0.48 ± 0.03 ab	0.27 ± 0.01 bc
W2D1	15.16 ± 1.18 ab	19.99 ± 0.55 a	11.01 ± 0.36 a	8.27 ± 0.23 bc	10.92 ± 0.24 a	6.49 ± 0.18 a	0.42 ± 0.01 a	0.52 ± 0.02 a	0.30 ± 0.01 a
W3D1	14.35 ± 0.59 bc	16.03 ± 0.55 c	9.06 ± 0.11 de	8.03 ± 0.24 bcd	9.34 ± 0.34 c	5.31 ± 0.17 d	0.41 ± 0.01 ab	0.40 ± 0.01 cd	0.24 ± 0.01 de
W4D1	11.99 ± 0.80 ef	12.71 ± 1.12 ef	7.94 ± 0.40 fg	6.31 ± 0.54 f	8.04 ± 0.31 ef	4.78 ± 0.20 ef	0.37 ± 0.01 cd	0.36 ± 0.02 ef	0.22 ± 0.01 fg
W1D2	15.31 ± 0.36 ab	17.76 ± 0.68 b	9.69 ± 0.32 bc	8.65 ± 0.56 ab	10.00 ± 0.29 b	5.73 ± 0.28 c	0.43 ± 0.01 a	0.47 ± 0.02 b	0.26 ± 0.02 cd
W2D2	14.44 ± 0.62 bc	19.32 ± 1.04 ab	10.70 ± 0.33 a	8.02 ± 0.6 bcd	10.44 ± 0.45 ab	6.21 ± 0.14 ab	0.41 ± 0.01 ab	0.5 ± 0.03 ab	0.30 ± 0.02 ab
W3D2	13.80 ± 1.12 cd	14.92 ± 0.84 cd	8.66 ± 0.07 e	7.4 ± 0.52 de	8.79 ± 0.68 cd	5.09 ± 0.11 de	0.39 ± 0.01 bc	0.37 ± 0.01 de	0.23 ± 0.01 ef
W4D2	11.34 ± 0.23 ef	12.06 ± 0.74 f	7.69 ± 0.10 g	5.93 ± 0.35 f	7.52 ± 0.37 fg	4.55 ± 0.36 fg	0.34 ± 0.02 de	0.32 ± 0.01 fg	0.21 ± 0.01 fg
W1D3	13.50 ± 0.44 cd	14.46 ± 1.33 cd	8.86 ± 0.09 de	7.65 ± 0.52 cde	8.42 ± 0.17 de	5.09 ± 0.29 de	0.38 ± 0.02 bc	0.4 ± 0.04 cd	0.23 ± 0.02 ef
W2D3	13.24 ± 0.19 cd	15.15 ± 1.31 cd	9.31 ± 0.09 cd	7.34 ± 0.10 de	8.74 ± 0.53 cd	5.32 ± 0.04 d	0.37 ± 0.01 cd	0.42 ± 0.03 c	0.24 ± 0.02 de
W3D3	12.55 ± 0.39 de	13.74 ± 1.04 de	8.16 ± 0.15 f	7.05 ± 0.27 e	8.14 ± 0.12 def	4.62 ± 0.23 f	0.37 ± 0.02 cd	0.36 ± 0.01 de	0.19 ± 0.02 gh
W4D3	10.83 ± 0.91 f	11.50 ± 0.40 f	7.24 ± 0.31 h	5.75 ± 0.18 f	7.09 ± 0.25 g	4.23 ± 0.30 g	0.33 ± 0.03 e	0.31 ± 0.01 g	0.18 ± 0.01 h
Source of variation
W	**	**	**	**	**	**	**	**	**
D	**	**	**	**	**	**	**	**	**
W × D	ns	*	ns	ns	ns	ns	ns	*	ns

The leaf photosynthesis of tomato in autumn 2023. *, p < 0.05, **, p < 0.01; ns, non–significant. W: irrigation level, D: irrigation interval.

. The net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) exhibited an overall trend of initial increase and then decrease as the growth period advanced, peaking during the expansion period. Both irrigation level and irrigation interval significantly affected Pn (p < 0.01). Notably, during the fruit expansion period in the autumn of 2023, the two factors interacted with each other (Table 1 and Table 2). Pn increased with the irrigation level, reaching the peak at W2. By the end of the maturity period, Pn at W2 treatment was 9.2%, 20.3%, and 37.8% higher than W1, W3, and W4, respectively. There was a negative correlation between Pn and irrigation interval; the Pn at D1 increased by 4.9% and 14.2% compared to D2 and D3, respectively. Irrigation level and irrigation interval also significantly affected Tr (p < 0.01), with interaction observed only at the end of the spring maturity period (p < 0.05). The highest Tr was observed under W2D1 treatment. Compared to W1, W3, and W4 during the maturity stage, the Tr of W2 increased by 10.4%, 20.7%, and 32.5% in spring, and by 8.0%, 20.1%, and 32.9% in autumn, respectively. Similarly, irrigation level and irrigation interval have extremely significant effects on Gs (p < 0.01), with significant interaction observed during the expansion period (p < 0.05) but not in other growth periods (ns). A moderate increase in irrigation level enhanced leaf stomatal conductance, with Gs under W2 conditions being 11.8–40.0% higher than other treatments. Additionally, decreasing the irrigation interval positively influenced Gs, with D1 and D2 treatments showing 21.5% and 16.5% higher than D3.

3.4. Yield and Its Components

Irrigation interval and level significantly impacted tomato yield (p < 0.01), though the interaction had no significant impact on total yield (

Table 3. Comparison of total yield and yield components between different treatments in two growing seasons.

Treatment	Spring 2023			Autumn 2023
	Mean Fruit Weight (g)	Fruit Number (n)	Total Yield (t ha−1)	Mean Fruit Weight (g)	Fruit Number (n)	Total Yield (t ha−1)
W1D1	167.41 ± 0.82 a	17.98 ± 0.22 ab	131.85 ± 1.06 ab	96.32 ± 4.88 ab	11.10 ± 0.60 a	47.20 ± 2.26 ab
W2D1	169.72 ± 2.25 a	18.36 ± 0.12 a	136.78 ± 3.06 a	103.06 ± 8.48 a	11.15 ± 0.51 a	50.10 ± 3.92 a
W3D1	163.40 ± 1.10 b	16.57 ± 0.28 cd	118.22 ± 1.22 d	88.58 ± 4.21 cd	10.01 ± 0.12 bc	40.12 ± 3.28 cd
W4D1	155.23 ± 1.11 de	14.90 ± 0.17 f	101.22 ± 0.93 f	76.28 ± 1.43 ef	9.33 ± 0.15 def	31.61 ± 1.07 fg
W1D2	160.93 ± 1.53 bc	17.30 ± 1.13 bc	122.42 ± 6.96 cd	90.81 ± 5.76 bc	10.60 ± 0.33 ab	43.12 ± 3.61 bc
W2D2	163.43 ± 1.89 b	17.91 ± 0.29 ab	128.14 ± 1.01 bc	96.51 ± 3.16 ab	10.81 ± 0.73 a	45.85 ± 4.37 ab
W3D2	156.86 ± 3.08 d	16.08 ± 0.15 de	110.25 ± 1.47 e	83.30 ± 1.84 d	9.81 ± 0.51 cd	36.19 ± 2.47 def
W4D2	146.68 ± 2.14 f	14.06 ± 0.53 g	89.92 ± 4.98 g	74.12 ± 1.63 fg	8.91 ± 0.52 ef	29.35 ± 2.85 g
W1D3	152.23 ± 1.70 e	16.15 ± 0.98 de	106.76 ± 5.95 ef	82.95 ± 3.48 de	9.46 ± 0.22 cde	34.51 ± 1.71 ef
W2D3	157.27 ± 5.63 cd	17.04 ± 0.41 c	116.82 ± 1.29 d	86.52 ± 7.03 cd	9.67 ± 0.44 cd	37.06 ± 2.69 de
W3D3	151.69 ± 2.12 e	15.68 ± 0.28 ef	103.83 ± 3.04 f	76.39 ± 1.16 ef	8.67 ± 0.19 f	29.53 ± 0.66 g
W4D3	141.34 ± 3.29 g	13.95 ± 0.69 g	85.93 ± 2.31 g	69.28 ± 2.51 g	7.74 ± 0.15 g	23.34 ± 0.89 h
Source of variation
W	**	**	**	**	**	**
D	**	**	**	**	**	**
W × D	ns	ns	ns	ns	ns	ns

The total yield and yield components in spring 2023 and autumn 2023. **, p < 0.01; ns, non–significant. W: irrigation level, D: irrigation interval.

). At the same irrigation level, decreasing the irrigation interval enhanced yields. Compared with D3, the total spring yield of D1 and D2 treatments significantly increased by 18.1% and 9.1%, while in autumn, the difference between treatments D1 and D2 was not significant. Increasing the irrigation level from W4 to W2 resulted in continuous yield increases, but further increasing to W1 caused a yield decline (Table 3). Under the same irrigation interval, increasing the irrigation level can also significantly enhance the tomato yield. The average yield of W2 was 6.2%, 20.3%, and 47.8% higher than W1, W3, and W4, respectively, with no significant difference between W1 and W2. The response patterns of average single fruit weight and number of fruits per plant to irrigation level and irrigation interval were basically consistent with the total yield. Compared to W4, the single fruit weight of W1, W2, and W3 increased by 7.7–9.7%, 9.3–11.4%, and 5.3–7.3% (spring 2023), and by 19.7–26.3%, 24.9–35.1%, and 10.3–16.1% (autumn 2023) under D1, D2, and D3, respectively. In both seasons, the number of fruits per plant was highest in W2D1 (18.4 and 11.2) and lowest in W4D3 (13.9 and 7.7). Overall, the total yield of tomatoes in autumn was 66.87% lower than that in spring.

3.5. Crop Evapotranspiration, Water Productivity, and Nitrogen Use Efficiency

Irrigation level and irrigation interval had significant effects on crop evapotranspiration, water productivity, and irrigation water productivity (p < 0.05), while W and D had no interaction (Figure 9). Crop evapotranspiration (ET) increased with increasing irrigation level. Compared with W4, the average crop evapotranspiration of W1, W2, and W3 increased by 108.7%, 76.2%, and 39.1%, respectively. Total water consumption in autumn was 36.5% lower than that in spring. Both water productivity (WP) and irrigation water productivity (IWP) increased as irrigation level decreased, reaching the highest values at W4. With D1 treatment, the IWP of W2, W3, and W4 increased by 29.7%, 49.4%, and 91.9%, respectively, compared with W1. Under D2 conditions, the average IWP of W2, W3, and W4 increased by 30.8%, 50.1%, and 83.7%, respectively, compared with W1. Irrigation interval is negatively correlated with WP. Compared to D3, the average WP of D1 and D2 increased by 20.6% and 12.7%, respectively. The average WP of W1, W2, W3, and W4 in spring were 32.7 kg/m³, 44.6 kg/m³, 47.6 kg/m³, and 54.5 kg/m³, and in autumn were 19.3 kg/m³, 22.5 kg/m³, 23.4 kg/m³, and 26.3 kg/m³, respectively. Overall, the average WP in autumn 2023 decreased by 49.1% compared to that in spring.

Irrigation level and irrigation interval had significant effects on the nitrogen use efficiency (NUE) of tomato in spring (p < 0.05), while irrigation interval had no significant effect on the NUE of autumn tomatoes. There was no interaction between W and D in the two growing seasons (Figure 9d). In general, no significant differences were observed among the treatments. NUE initially increased and then decreased, reaching its maximum value at W3. Compared with that of W4, the NUE of W1, W2, and W3 increased by 2.8%, 4.1%, and 6.5%, respectively. In addition, decreasing the irrigation interval was beneficial for the improvement of NUE. The NUE of D2 and D3 decreased by 1.7% and 3.3%, respectively, compared to D1.

3.6. Fruit Quality

The irrigation level significantly affected the TSS, SS, VC, OA, and SAR of tomatoes (p < 0.01), while the irrigation interval significantly influenced the SAR of the tomatoes (p < 0.05) and had a highly significant impact on other quality indicators (p < 0.01). The interaction effect between two factors on quality indicators was not significant (ns). The quality parameters exhibited similar trends across both growing seasons, increasing as the irrigation level decreased (

Table 4. Effects of different irrigation strategies on tomato quality in spring 2023.

Treatment	Total Soluble Solids Content (%)	Vitamin C (mg/kg)	Soluble Sugar (%)	Organic Acid (%)	Sugar-Acid Ratio
W1D1	5.56 ± 0.10 fg	188.2 ± 5.62 d	2.76 ± 0.11 fg	0.27 ± 0.01 ef	10.30 ± 0.22 abc
W2D1	5.95 ± 0.09 bcd	206.93 ± 6.49 bc	3.19 ± 0.07 bc	0.30 ± 0.01 bc	10.67 ± 0.21 a
W3D1	6.14 ± 0.16 b	214.42 ± 6.49 ab	3.27 ± 0.08 ab	0.31 ± 0.01 ab	10.71 ± 0.26 a
W4D1	6.36 ± 0.10 a	223.78 ± 8.58 a	3.39 ± 0.17 a	0.32 ± 0.02 a	10.75 ± 0.37 a
W1D2	5.36 ± 0.09 gh	175.09 ± 6.49 ef	2.67 ± 0.08 g	0.26 ± 0.01 fg	10.14 ± 0.24 bc
W2D2	5.67 ± 0.23 ef	197.57 ± 6.49 cd	2.92 ± 0.07 de	0.28 ± 0.01 de	10.38 ± 0.18 abc
W3D2	5.89 ± 0.08 cd	205.06 ± 5.62 bc	3.04 ± 0.11 cd	0.29 ± 0.01 cd	10.51 ± 0.32 ab
W4D2	6.06 ± 0.11 bc	214.42 ± 3.24 ab	3.19 ± 0.13 bc	0.30 ± 0.01 bc	10.69 ± 0.32 a
W1D3	5.20 ± 0.15 h	165.73 ± 11.24 f	2.49 ± 0.11 h	0.25 ± 0.01 g	9.95 ± 0.19 c
W2D3	5.44 ± 0.06 g	184.46 ± 8.58 de	2.69 ± 0.05 fg	0.26 ± 0.01 fg	10.19 ± 0.17 bc
W3D3	5.79 ± 0.08 de	190.07 ± 8.58 d	2.84 ± 0.08 ef	0.27 ± 0.01 ef	10.41 ± 0.16 ab
W4D3	5.96 ± 0.09 bcd	206.93 ± 6.49 bc	3.14 ± 0.1 bc	0.29 ± 0.01 bcd	10.67 ± 0.19 a
Source of variation
W	**	**	**	**	**
D	**	**	**	**	*
W × D	ns	ns	ns	ns	ns

The tomato quality in spring 2023. *, p < 0.05, **, p < 0.01; ns, non–significant. W: irrigation level, D: irrigation interval.

and

Table 5. Effects of different irrigation strategies on tomato quality in autumn 2023.

Treatment	Total Soluble Solids Content (%)	Vitamin C (mg/kg)	Soluble Sugar (%)	Organic Acid (%)	Sugar-Acid Ratio
W1D1	4.97 ± 0.15 de	150.75 ± 3.24 cd	2.54 ± 0.17 ef	0.31 ± 0.02 de	8.29 ± 0.32 cd
W2D1	5.30 ± 0.26 bc	165.73 ± 5.62 b	2.95 ± 0.06 bc	0.34 ± 0.01 b	8.70 ± 0.11 ab
W3D1	5.53 ± 0.06 ab	169.48 ± 6.49 ab	3.10 ± 0.17 ab	0.35 ± 0.02 ab	8.81 ± 0.29 ab
W4D1	5.73 ± 0.06 a	178.84 ± 12.97 a	3.29 ± 0.10 a	0.37 ± 0.02 a	9.00 ± 0.29 a
W1D2	4.80 ± 0.17 ef	139.51 ± 6.49 de	2.32 ± 0.08 fg	0.29 ± 0.01 ef	8.04 ± 0.15 d
W2D2	5.10 ± 0.17 cd	152.62 ± 3.24 c	2.81 ± 0.10 cd	0.33 ± 0.01 bc	8.49 ± 0.13 bc
W3D2	5.30 ± 0.10 bc	160.11 ± 5.62 bc	2.92 ± 0.10 bc	0.33 ± 0.01 bc	8.71 ± 0.11 ab
W4D2	5.60 ± 0.10 a	169.48 ± 3.24 ab	3.07 ± 0.15 ab	0.35 ± 0.03 ab	8.78 ± 0.23 ab
W1D3	4.37 ± 0.06 g	132.02 ± 9.73 e	2.23 ± 0.18 g	0.28 ± 0.02 f	8.07 ± 0.26 d
W2D3	4.70 ± 0.10 f	139.51 ± 3.24 de	2.53 ± 0.11 ef	0.30 ± 0.01 de	8.30 ± 0.27 cd
W3D3	5.00 ± 0.17 de	150.75 ± 3.24 cd	2.68 ± 0.07 de	0.31 ± 0.01 cd	8.53 ± 0.10 bc
W4D3	5.23 ± 0.21 cd	167.60 ± 8.58 ab	3.03 ± 0.19 bc	0.34 ± 0.01 ab	8.77 ± 0.34 ab
Source of variation
W	**	**	**	**	**
D	**	**	**	**	*
W×D	ns	ns	ns	ns	ns

The tomato quality in spring 2023. *, p < 0.05, **, p < 0.01; ns, non–significant. W: irrigation level, D: irrigation interval.

). Compared to W4 treatment, the average TSS, VC, SS, OA, and SAR reduced by 13.5%, 18.1%, 21.5%, 15.9%, and 6.8% at W1, while these decreased by 8.1%, 9.9%, 10.6%, 7.6%, and 3.4% at W2, respectively. For W3 treatment, they were 3.8%, 6.2%, 6.7%, 5.1%, and 1.7%. Aside from SAR, decreasing the irrigation interval enhanced the accumulation of tomato quality indicators. TSS, VC, SS, and OA under D1 treatment were 4.0%, 6.1%, 6.7%, and 4.6% higher than those under D2 treatment, respectively.

3.7. Principal Component Analysis and Correlation Matrix

To comprehensively compare the correlations between fruit yield, quality, water productivity, leaf area index, net photosynthetic rate, nitrogen uptake, and other indicators with irrigation level and irrigation interval, a correlation matrix and PCA diagram were constructed. Each indicator was downscaled for the spring and autumn of 2023 to obtain two principal components based on the principle that the cumulative contribution exceeds 85% and the eigenvalue is greater than 1. The PCA diagram (Figure 10) showed that Yield, Pn, LAI, MFW, RL, NUE, and Fruit in both growing seasons were positively correlated with PC1, whereas SS and WP were negatively correlated with PC1. Additionally, except for Fruit in spring, all other indicators were positively correlated with PC2; while Yield, RL, MFW, Fruit, SS, NUE, and WP in autumn were positively correlated with PC2, and LAI in autumn was negatively correlated with PC2. The principal component scores were calculated to determine the optimal combination of irrigation level and irrigation interval, revealing that the comprehensive scores of W2D1 in both growing seasons were highest (

Table 6. Comprehensive principal component scores and rankings among treatments.

Spring 2023			Autumn 2023
Treatment	Score	Rank	Treatment	Score	Rank
W1D1	0.74	2	W1D1	0.68	3
W2D1	1.42	1	W2D1	1.37	1
W3D1	0.64	4	W3D1	0.56	4
W4D1	−0.04	7	W4D1	−0.07	7
W1D2	0.08	6	W1D2	0.14	5
W2D2	0.71	3	W2D2	0.85	2
W3D2	0.08	5	W3D2	0.02	6
W4D2	−0.83	10	W4D2	−0.47	9
W1D3	−0.88	11	W1D3	−0.81	11
W2D3	−0.14	8	W2D3	−0.25	8
W3D3	−0.58	9	W3D3	−0.78	10
W4D3	−1.20	12	W4D3	−1.26	12

). The correlation matrix of yield, net photosynthetic rate, quality, water productivity, plant nitrogen uptake, and other indicators is shown in Figure 11. The tomato yield in both growing seasons was highly significantly positively correlated with MFW, FN, PH, SD, AB, LAI, ET, Stem, Leaf, Fruit, Pn, Tr, and Gs (p < 0.01), and negatively correlated with WP, IWP, and SS.

4. Discussion

4.1. Effects of Different Irrigation Levels and Intervals on Crop Growth, Nitrogen Uptake, and Distribution

Tomatoes are highly sensitive to water stress [33]. Compared with full irrigation, deficit irrigation leads to reductions in the tomato plant height, stem diameter, root length, and aboveground biomass [9,34], while moderate irrigation levels benefit aboveground biomass in the entire growth period [35], which is consistent with our findings. Adequate irrigation provides the necessary water for the growth of tomato [35,36], while water deficit leads to water stress in crops, which makes the root water uptake rate lower than the transpiration rate and hinders the growth and proliferation of cells in the growth stage [37]. Furthermore, the irrigation interval affects tomato growth and development. Zhang et al. [38] observed significant effects of different irrigation intervals on tomato height, LAI, and aboveground biomass (p < 0.01), indicating that smaller and moderate irrigation intervals enhance tomato growth. In the present study, D1 increased tomato LAI at maturity by 4.2% and 11.3% compared to D2 and D3, likely due to optimal irrigation interval balancing soil moisture and oxygen in the root zone, promoting root growth [39]. A larger root system enhances nutrient absorption and increases leaf area, supporting greater aboveground biomass through enhanced solar radiation interception by larger leaves. In addition, spring indicators such as stem diameter, root length, and dry matter were greater than in autumn, which may be caused by environmental variations in temperature and humidity [40]. According to Mendonça et al. [41], higher solar irradiance and air temperature in spring lead to this situation.

Roots are the main fertilizer absorption organ [42,43]. Similarly, nutrient absorption in tomato across seasons demonstrated a trend of initial increase followed by decrease with irrigation level, which may be due to the fact that optimal moisture conditions promoted root length and surface area, thereby improving the distribution of roots in soil and promoting the ability to absorb nitrogen fertilizer [44]. Deficit irrigation not only leads to a general decline in total nitrogen but also reduces the nitrogen content in various plant organs. Some studies have shown that low soil moisture limits nitrogen uptake and assimilation by leaves and roots [45], which may be due to the fact that insufficient irrigation inhibits plant growth, thereby affecting nitrogen uptake in vegetative and reproductive organs. Wang et al. [46] found that nitrogen uptake was closely related to total root length. A smaller irrigation interval can reduce the fluctuation of soil moisture in the upper soil [47], maintain optimal soil moisture suitable for plant root growth, and enhance root development to increase nitrogen availability and uptake.

In the present study, D1 increased nitrogen uptake compared to the other irrigation intervals, probably because a smaller irrigation interval can maintain soil moisture levels and reduce water stress and nutrient leaching. Additionally, excessively high soil temperatures can accelerate crop leaf senescence [48]. Smaller irrigation interval helps maintain more suitable root zone temperatures, potentially delaying leaf senescence and improving nitrogen uptake in tomato plants. Nitrogen absorbed by tomato plants is primarily allocated to fruits, followed by leaves and stems, consistent with findings by Wang et al. [25]. In summary, different irrigation strategies effectively regulate nutrient distribution and transfer within tomato, maintaining a balance between nutritional and growth.

4.2. Effects of Irrigation Level and Interval on Leaf Photosynthesis

Photosynthesis is highly sensitive to drought stress and is a crucial source of energy for crop reproduction. The reduction of photosynthesis parameters often leads directly to decreased crop growth and yield. In this study, both excessive and insufficient irrigation adversely affected leaf gas exchange parameters, leading to decreases in Pn, Tr, and Gs, aligning with previous studies [49,50]. Both excessive and insufficient irrigation can exacerbate growth inhibition, potentially due to hypoxia [51] or stomatal closure [52]. Stomatal closure is one of the responses of the tomato plant to water stress and can reduce its water loss [53]. It is primarily regulated by guard cells, and the imbalances of plant water status, water transport, and turgor pressure may directly inhibit the physiological function of guard cells, leading to decreased gas exchange capacity in tomato leaves [54,55]. Conversely, excessive water can induce hypoxia stress, affecting the root absorption system by promoting reactive oxygen species production, which indirectly inhibits the photosynthetic rate of crops [56,57].

Irrigation interval significantly influences plant growth and development, impacting physiological functions such as photosynthetic capacity and efficiency. Missen et al. [58] demonstrated that irrigation interval notably affects the gas exchange parameters and water status of crops. Compared with infrequent irrigation, small irrigation interval enhances water status and absorption in crops compared to infrequent irrigation, thereby promoting cell expansion and photosynthesis [59]. Furthermore, some studies have shown that the Gs values are higher with smaller irrigation intervals [60], a finding corroborated by this experiment. On the contrary, a large irrigation interval expands the dry soil area and reduces soil moisture content, potentially lowering the light compensation and saturation points [61], which can increase stomatal limitation and may lead to non-stomatal limitation, thereby reducing photosynthetic efficiency [62].

In this study, under the treatment of low water and large irrigation interval, the stomatal conductance and photosynthetic activity decreased significantly, which might be caused by the drought stress resulting from the low irrigation threshold. The W2D1 treatment can maintain an appropriate soil moisture content, keep the stomata of tomato plants at a reasonable open level, and thereby enhance the photosynthetic capacity of the crops. However, small irrigation intervals combined with high irrigation levels may result in the level of water applied exceeding the water storage capacity of the soil [63], impairing oxygen diffusion from roots to soil and reducing enzyme activity and photosynthesis [64].

4.3. Effects of Irrigation Level and Interval on Tomato Yield, Crop Evapotranspiration, and Water-Nitrogen Use Efficiency

Tomato yield responds variably to different irrigation levels. Under a consistent irrigation interval, a moderate increase in irrigation volume significantly enhances tomato yield. However, excessive irrigation induces waterlogging in the soil profile, which impairs soil aeration and consequently reduces crop total yield [31,65]. Additionally, excessive water infiltration may diminish soil nitrogen retention and nitrogen availability to plants, further reducing nitrogen use efficiency and tomato yields [66,67]. In the present study, the total yield was positively correlated with irrigation level within a certain range (Table 3), consistent with previous studies [11,68]. The observed yield reduction under water stress was attributed to decreased fresh weight per fruit and the number of fruits per plant (Table 3), which may be due to the fact that prolonged soil dryness likely impaired water absorption by plants, limiting water accumulation in tomato fruits and fruit numbers. Similarly, different irrigation levels also have a certain effect on crop evapotranspiration, water productivity, and nitrogen use efficiency [13,69]. Moderate deficit irrigation improves WP and IWP while reducing overall water consumption (Figure 9). Drought stress in tomato plants leads to stomatal closure, limiting transpiration and enhancing water productivity [70]. Conversely, excessive irrigation increases leaf area, raising leaf temperature and intensifying transpiration, thus elevating water consumption and reducing water productivity. Appropriate water control can effectively improve the soil environment and reduce NO₃-leaching, thereby improving nutrient use efficiency [71]. Although a lower irrigation level can improve WP and NUE to some extent, the yields were significantly reduced. Therefore, it is crucial to optimize WP and NUE without compromising tomato growth and yield.

Liu et al. [72] pointed out that the yield was low under a large irrigation interval, which was due to the large drought area before irrigation, which affected the root elongation. Through our research, we found that the lateral extension of the tomato root system in the facility is large: the vertical root is about 30–40 cm, and the large-scale water shortage caused by a large irrigation interval hinders root growth. Other researchers have suggested that a large irrigation interval can lead to substrate salt accumulation, causing plant water deficiency [73]. In contrast, small irrigation interval maintains soil water in a smaller volume, thereby enhancing metabolic activity and cell division [74] and improving growth traits and the transport capacity of photosynthetic products, ultimately boosting the yields [75]. In our experiment, large irrigation intervals negatively impacted the average single fruit weight and fruit number compared to high and medium intervals, likely due to reduced soil water availability leading to flower abortion. The increase in irrigation interval had an adverse effect on NUE. This may be due to the prolonged drought environment of crops, which affects the ability of roots to absorb water, resulting in a significant yield reduction and a decrease in NUE. Similarly, both WP and IWP peaked at a small irrigation interval in two seasons, which was consistent with previous studies [76,77]. Since the difference in water consumption among the three irrigation intervals was minimal, the increase in WP with a smaller irrigation interval was primarily due to increased yield. In addition, in the experiments on tomato irrigation level and irrigation interval, the relationship between the cumulative irrigation level and the water requirement of crops can reflect the growth status of tomato plants. In this experiment, taking the spring experiment as an example, the cumulative irrigation water volume of W2D1 (with an interval of 4 days + 100% ET_c) was 295.6 mm, which was basically consistent with the theoretical water requirement of 291.8 mm, and the yield was optimal. However, the irrigation level and water requirement for other treatments vary greatly, which may lead to evaporation loss or water stress. Therefore, D1 (with a 4-day interval) is the time when the cumulative irrigation amount matches the water requirement to the highest degree, which is more conducive to the growth of tomatoes. Furthermore, the lower water amount per irrigation event under small irrigation interval treatment resulted in rapid surface soil drying, reducing evaporation rates and contributing to higher water productivity. It is speculated that this is also the reason for the higher water productivity under a small irrigation interval.

Overall, the tomato yield in spring was higher than in autumn due to lower temperatures in autumn [78]. In addition, adverse environmental factors in autumn negatively impact physiological processes such as photosynthesis and transpiration, thereby affecting plant growth and development [79]. Low temperatures in autumn, combined with increased irrigation, can raise humidity levels in greenhouses, but excessive humidity increases the risk of pest and disease outbreaks, accelerating disease and infection at the roots [80]. On the contrary, in spring, the overall temperature rises, and the solar radiation is stronger, which is more conducive to the photosynthesis of tomato leaves, thereby promoting the accumulation of aboveground biomass and the formation of yield. In this study, frequent rainy days in autumn and winter, coupled with low temperatures and high humidity in the greenhouse, led to the occurrence of bacterial spot disease. Unfavorable climatic conditions significantly reduced the plant height and leaf area index of tomato in autumn, limiting dry matter accumulation and thus potentially reducing the yield. Therefore, in the future, more greenhouse tomato experiments should be conducted in the same season to reduce the influence of seasonal factors.

4.4. Effects of Irrigation Level and Interval on Tomato Quality

Water plays a crucial role in improving tomato quality. Numerous studies have indicated that water deficit generally enhances the tomato quality [81,82,83]. Water balance is determined by phloem flow and leaf transpiration to regulate fruit quality [70]. Under water stress, the transport of phloem sap to the fruit is hindered [84], resulting in reduced dilution of quality indicators such as sugar and acid [85]. This leads to the accumulation of assimilates without affecting solute synthesis [86,87], which is an important reason why water deficit improves the content of soluble sugar and organic acid in tomato fruit. Additionally, a water deficit activated galactose metabolism and sucrose metabolism pathways, enhancing the activities of sucrose synthase and sucrose phosphate synthase [88], increasing the conversion rate of starch to sugar, and subsequently increasing the soluble sugar content and soluble solids [89].

In addition, small irrigation intervals also improve tomato quality, likely because they facilitate the transfer of photosynthetic products in the form of sucrose to reproductive organs, thereby increasing the content of soluble sugars [72]. Contrary to these findings, Colimba-Limaico et al. [90] did not observe a significant effect of different irrigation intervals (2-day interval and 3-day interval) on soluble solids. Other researchers suggest that increasing irrigation intervals is more conducive to increasing total soluble solids and organic acid concentrations in tomatoes [76,91]. This discrepancy may be attributed to differences in soil texture and irrigation interval measurement methods. The irrigation strategy also affects the VC concentration in tomato fruit. Low irrigation levels reduce the leaf area index, thereby increasing the light exposure and intensity on the fruit [92], which promotes VC synthesis. Additionally, the VC content can be influenced by season and environmental factors [93,94]. In this study, the VC content of tomato was higher in spring than in autumn, likely due to longer sunshine duration in spring, which facilitates compound accumulation in tomato fruits.

5. Conclusions

The irrigation strategy significantly affects the aboveground growth and photosynthetic characteristics of greenhouse tomato, which in turn influence fruit yield and quality. Decreasing irrigation interval promoted the root growth of tomato under drip irrigation in the greenhouse, effectively accelerated the absorption capacity of nitrogen fertilizer and nitrogen accumulation and affecting plant height, leaf area index, and stem diameter, thereby enhancing the photosynthetic capacity and transpiration rate of tomatoes, and improving the yield, quality, and water-nitrogen use efficiency. Moderately reducing irrigation water can increase the yield, quality, and water productivity of tomatoes. The W2D1 treatment provided the best overall benefits in the principal component analysis, achieving high yields and efficient resource utilization (especially in semi-humid and drought-prone climates). The results of this study have clarified the optimal irrigation system for solar greenhouses in Northwest China, providing a theoretical basis and technical support for the sustainable production strategy of greenhouse tomatoes and public policies on efficient water use in protected agriculture. In the future, we can verify the conclusions obtained in this paper in other crops or environments. In addition, the impact of irrigation level and irrigation interval on tomato yield and quality is also influenced by many other factors such as tomato variety, soil type, and fertilization levels, which need further investigation.