Responses of the Growth Characteristics of Spinach to Different Moisture Contents in Soil under Irrigation with Magnetoelectric Water

Abstract

Spinach (Spinacia oleracea L.) is a worldwide vegetable crop with rich nutritional value, and drought is the main factor restricting its growth. Magnetized water and de-electronated water have shown potential for improving yield and quality in some crops. To assess the influence of magnetized-de-electronated water (denoted magnetoelectric water) on the growth characteristics of spinach, five soil moisture gradients were developed, including 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of field capacity (FC). The results demonstrated that the influence of irrigation by magnetoelectric water on the growth of spinach was obvious. All the spinach indicators with each soil moisture gradient under irrigation by magnetoelectric water were higher than those of irrigation by conventional water, including the fresh weight of shoots, chlorophyll content, and the total nitrogen content in the leaves. In particular, the improvement in fresh weight of shoots and the total nitrogen contents in the leaves had the highest values, as demonstrated by increases of 52.26% and 25.87%, respectively, under 65–75% of the gradient of FC. Additionally, the fitting results of the photo response curve by different light response models varied. The modified rectangular hyperbolic model was the most accurate for all the treatment groups and thus was the optimized model for the photosynthetic characteristics of spinach under irrigation by magnetoelectric water analysis. The parameters of the photo response curve showed that the dark respiration rate, apparent quantum efficiency, light saturation point, and maximum net photosynthetic rate all increased following irrigation by magnetoelectric water with different soil moisture gradients compared with conventional water irrigation. These research results can provide new technical support for improving the water use efficiency of irrigation water and increasing vegetable production.

1. Introduction

A shortage of water resources is a prominent problem in agricultural production worldwide, particularly in arid and semiarid regions, where agricultural irrigation and water usage frequently exceed the sustainable supply [1]. To improve the efficiency of the production of agricultural water resources, many researchers have conducted extensive and in-depth studies. Irrigation water activation technology, including magnetization and de-electronation treatments, has attracted the extensive attention of researchers owing to its use of simple equipment, low energy consumption and investment, lack of pollution, and high efficiency.

In 1930, Savostin first reported that the magnetic field could promote the growth of crop seedlings [2]. Indeed, treatment by magnetic fields could promote the growth and development of crop roots and has a greater effect on roots than shoots [3,4]. Nevertheless, it was difficult to directly apply a magnetic field to field production, while the treatment of irrigation water by a magnetic field was relatively convenient and effective. In a fixed magnetic field environment formed by permanent magnets or electromagnets, conventional water passes along the direction perpendicular to the magnetic field line at a certain flow rate. Conventional water that changes its physical and chemical properties owing to the treatment by a magnetic field is called magnetized water. The results of a study by Abdulrahman et al. showed that magnetized irrigation water could significantly promote an increase in the Pennisetum glaucum leaf area and increase the chlorophyll content [5]. Zareei et al. demonstrated that the use of magnetized water irrigation by conventional water could stimulate the biosynthesis of phenolic compounds in grape (Vitis vinifera) plants [6], which could strengthen the physiological and metabolic functions of crops. Mohammadi et al. found that magnetized water was able to improve the leaf area index, relative leaf water content, and the special product analysis division (SPAD) index of vegetation (i.e., Nitraria, Haloxylon, and Atriplex) in the arid region of northeastern Iran [7]. The results obtained by Li et al. showed that magnetized water irrigation effectively enhanced root growth, the root−shoot ratio, and water use efficiency of soybean, and alleviated the negative effects of drought stress under severe drought conditions [8]. Wang et al. reported that magnetized water irrigation improved plant height, leaf area, aboveground biomass, and photosynthetic characteristics of winter wheat at various growth stages [9]. Additionally, de-electronated water refers to the ground wire on the metal pipe; when the water flows through the metal pipe, a certain number of electrons in the water body are enriched on the pipe wall, and they are introduced into the ground through the ground wire to change the physical and chemical properties of the irrigation water [10]. De-electronated water treatment has been proposed by the ECO1ST Technology Group (Huntington Beach, CA, USA) and is widely applied in industrial water-oil separation [11], while its application in agriculture has also recently attracted attention. It is primarily used to neutralize the high salinity of poor-quality irrigation water, thereby reducing the adverse effects of irrigation with poor-quality water on crops and promoting crop growth. The results obtained by Wang et al. indicated that de-electronated brackish water could increase the absorption rate of the Philip infiltration formula, the saturated water conductivity of the Green-Ampt infiltration formula, and the suction power at the wet front, and significantly improved the water holding efficiency of soil and the leaching effect of upper soil salt [12].

To sum up, magnetized and de-electronated water irrigation is effective in promoting the growth of different crops. However, because magnetized or de-electronated water has a short maintenance time after its physical and chemical properties are changed, the continuous effect on the whole crop growth process is not as good as the combined treatment of the two [13,14]. Further analysis should be required for the effects of irrigation water treated with a combination of magnetization and de-energization on crop growth. Spinach (Spinacia oleracea L.) not only requires a large amount of water but is also very sensitive to moisture [15], so it is suitable as the test material for this experiment. Therefore, taking spinach as the research object, this study aims to clarify the response law of spinach growth to different soil moisture contents under magnetoelectric water irrigation, explore the influence mechanism of magnetoelectric water irrigation on spinach productivity, and provide a theoretical basis for magnetoelectric water irrigation of spinach crops.

2. Materials and Methods

2.1. Study Site and Soil Description

The experiment was carried out in the artificial climate greenhouse of Xi’an University of Science and Technology. The daytime temperature (8:00–18:00) was set to 25 °C, the night temperature (18:00–8:00) was set to 15 °C, the humidity was 50%, and the light was set to 40 Klux. The test soil samples were collected from Yangling, Shaanxi Province, and 0–20 cm topsoil of farmland soil was collected. A Mastersizer 2000 laser particle size analyzer (Malvern Instrument Co., Ltd., Shanghai, China) was used to determine the physical sand and silt volume fractions of the soil samples, which were 36.3% and 53.31%, respectively. According to the international soil texture classification standard, the soil sample was determined to be silty loam, and the mass field water holding capacity was 30%. The tested spinach seeds were Dayu ideal No. 1 (Hebei Dayu Seed Industry Co., Ltd; Cangzhou, Hebei, China). The pot experiment was conducted with 1.2 kg soil in each pot, and urea (150 mg·kg⁻¹ nitrogen), phosphorus dioxide (75 mg·kg⁻¹ phosphorus), and potassium sulfate (75 mg·kg⁻¹ potassium) were applied to the basal fertilizer, respectively. Six spinach seeds were planted in each pot. Three seedlings with the same growth vigor were retained in each pot at the four-leaf stage, and three replicates were set for each treatment.

Irrigation water was divided into two groups: conventional water (CK) and magnetoelectric water (MD). Soil moisture content was controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of field capacity (FC) by the daily weighing method, respectively. A total of 10 treatments (

Table 1. Experimental treatments combination.

Treatment	Irrigation Type	Soil Moisture Content
CK-1	conventional water	45–55%FC
CK-2	conventional water	55–65%FC
CK-3	conventional water	65–75%FC
CK-4	conventional water	75–85%FC
CK-5	conventional water	85–95%FC
MD-1	magnetoelectric water	45–55%FC
MD-2	magnetoelectric water	55–65%FC
MD-3	magnetoelectric water	65–75%FC
MD-4	magnetoelectric water	75–85%FC
MD-5	magnetoelectric water	85–95%FC

CK-1, CK-2, CK-3, CK-4, and CK-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with conventional water, respectively; MD-1, MD-2, MD-3, MD-4, and MD-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with magnetoelectric water, respectively. FC, field capacity.

) were repeated three times. The conventional water flowed through the permanent magnet (Baotou Xinda Magnetic Material Factory; Baotou, Inner Mongolia, China) with a magnetic field intensity of 3000 Gs at a fixed flow rate (0.5 m·s⁻¹), so that the conventional water was magnetized and flowed into the de-electronation device. The device exported the electrons in the magnetized water through the grounding resistance of the external connection, so as to complete the preparation of magnetoelectric water.

2.2. Determination Content and Method

The measurement method of spinach aboveground fresh weight (FWA): the fresh weight of the aboveground part of each treatment group was measured by an electronic scale (precision 0.01 g) during the harvest period.

The total nitrogen measurement method of spinach leaves (LTN): during the harvest period, the sample was dried, ground, and passed through a 0.2 mm sieve, and the LTN was determined by the salicylic acid-zinc powder reduction method.

The measurement method of spinach chlorophyll content: 0.5 g spinach leaves were weighed, and 100 mL extract (acetone:ethanol = 1:1, v/v) was added. The absorbance (A) values were measured by a UV spectrophotometer at 663 nm and 645 nm, respectively. The calculation method of chlorophyll mass fraction was as follows [16]:

$${\omega }_a=\frac{\left(12.70A_{663}-2.69A_{645}\right)V}{m\times 1000}$$

(1)

$${\omega }_b=\frac{\left(22.90A_{645}-4.68A_{663}\right)V}{m\times 1000}$$

(2)

$${\omega }_T=\frac{\left(20.21A_{645}+8.02A_{663}\right)V}{m\times 1000}$$

(3)

where V is the extraction volume, mL; m is the fresh weight of the sample; ω_a is the mass fraction of chlorophyll a (Chl a); ω_b is the mass fraction of chlorophyll b (Chl b); ω_T is the total chlorophyll mass fraction (Chl a + b).

The measurement method of the light response characteristics of spinach was as follows: During the vegetative growth period of spinach, the sunny weather was selected, and the observation time was 09:00−11:00. Each treatment randomly selected spinach leaves with good growth and fully expanded upper part for observation. Each leaf was observed three times and the average value was taken. The net photosynthetic rate, stomatal conductance, and intercellular CO₂ concentration of spinach leaves under different photosynthetically active radiation gradients were measured with the red and blue light source of the LC-pro portable photosynthesis instrument. The illumination range set a 15 light intensity gradient, due to the need for light induction in the measurement process, so according to 2500, 2200, 2000, 1800, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 100, 50, and 0 µmol m⁻²·s⁻¹ order for the automatic recording of data. According to the net photosynthetic rate under different photosynthetically active radiation, the light response curve of the photosynthetic rate could be plotted to determine the apparent quantum efficiency (α), maximum net photosynthetic rate (P_nmax), light compensation point (I_c), light saturation point (I_s) and dark respiration rate (R_d) of spinach.

2.3. Models Used

2.3.1. Logistic Model

The logistic model was used to fit the fresh weight of aboveground parts of spinach:

$$FWA=\frac{FW{A}_{\max }}{1+e^{A-Bt}}$$

(4)

where FWA is the fresh weight of the aboveground part; FWA_max is the theoretical maximum of aboveground fresh weight; and A, B are fitting parameters.

The relationship (v) between the growth rate of the aboveground fresh weight and time and the maximum growth rate (v_max) were obtained by derivation of Equation (4):

$$v=\frac{FW{A}_{\max }B{e}^{A-Bt}}{{(1+e^{A-Bt})}^2}$$

(5)

$$v_{\max }=\frac{FW{A}_{\max }B}{4}$$

(6)

2.3.2. Light Response Model

In this paper, four models were used to simulate and analyze the light response curve of spinach. The specific model formula was as given below.

The theoretical formula of the rectangular hyperbolic model [17]:

$$P_n=\frac{\alpha I{P}_{n\max }}{\alpha I+P_{n\max }}-R_d$$

(7)

The calculation formula of the light compensation point (I_c):

$$I_c=\frac{R_d{P}_{n\max }}{\alpha \left(P_{n\max }-R_d\right)}$$

(8)

The line y = P_nmax intersects with the line y = αI − R_d, and the value of x axis corresponding to the intersection point is the light saturation point (I_s).

The theoretical formula of the modified rectangular hyperbolic model:

$$P_n=\alpha \frac{1-\beta I}{1+\gamma I}\left(I-I_c^{}\right)$$

(9)

where β is the photoinhibition coefficient and γ is the coefficient independent of I.

For I = 0, the dark respiration rate (R_d):

$$R_d=-P_n\left(I=0\right)=-\alpha I_c$$

(10)

The light saturation point (I_s):

$$I_{sat}=\alpha \frac{\sqrt{\left(\beta +\gamma \right)\left(1+\gamma I_c\right)/\beta }-1}{\gamma }$$

(11)

The maximum net photosynthetic rate (P_nmax):

$$P_{n\max }=\alpha \frac{1-\beta I_{sat}}{1+\gamma I_{sat}}\left(I_{sat}-I_c\right)$$

(12)

The theoretical formula of the non−rectangular hyperbolic model [18]:

$$P_n=\frac{\alpha I+P_{n\max }-\sqrt{{\left(\alpha I+P_{n\max }\right)}^2-4I\alpha k{P}_{n\max }}}{2k}-R_d$$

(13)

where k is the curve angle of the light response curve.

The light compensation point (I_c):

$$I_c=\frac{R_d{P}_{n\max }-k{R}_d{}^2}{\alpha \left(P_{n\max }-R_d\right)}$$

(14)

The line y = P_nmax intersects with the line y = α − R_d, and the value of x axis corresponding to the intersection point is the light saturation point (I_s).

The theoretical formula of the exponential model [19]:

$$P_n=p_{n\max }\left(1-e^{-\frac{\alpha I}{P_{n\max }}}\right)-R_d$$

(15)

When I_s is estimated, it is assumed that the light intensity corresponding to P_n of 0.99P_nmax is the saturated light intensity.

2.4. Data Processing

The experimental data were processed and mapped by Microsoft Office 2016 (Redmond, Washington, DC, USA; Microsoft), while Origin 2021 (Northampton, MA, USA; OriginLab Corporation) and IBM SPSS Statistics 25 (Armonk, NY, USA; IBM Corp) were used for fitting and statistical analysis of model parameters, and the LSD method was used for the explicitness test (p < 0.05).

3. Results and Analysis

3.1. Dynamic Characteristics of the Total Nitrogen Content in Spinach Leaves

Figure 1 shows the trends of total nitrogen content in spinach leaves in different treatment groups. As observed, the total nitrogen content in the leaves in all the treatment groups first increased and then decreased with an increase in sowing time. In other words, the total nitrogen content in the leaves in all the treatment groups increased first and then decreased with the increase in soil moisture content. Indeed, magnetoelectric water could increase the total nitrogen content in leaves. Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the total nitrogen content in the spinach leaves in MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 20.23%, 11.21%, 25.87%, 15.56%, and 11.27%, respectively (p < 0.05).

3.2. Dynamic Characteristics of the Fresh Weight of Spinach Shoots

Figure 2 illustrates the dynamic changes in the fresh weight of spinach shoots in different treatment groups. As observed, the fresh weight of shoots of both the MD and CK groups first increased and then decreased as the soil moisture content increased during a constant sowing time. Among the CK groups, the shoots of the CK-4 treatment had the highest fresh weight. Among the MD groups, the MD-3 treatment had the largest fresh weight of the shoots. Compared with irrigation by conventional water, irrigation by magnetoelectric water could increase the fresh weight of spinach shoots. After 56 days of sowing, compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the fresh weight of the shoots of MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 19.03%, 30.18%, 52.26%, 3.02%, and 9.52%, respectively (p < 0.05). Thus, when the soil moisture content was 65–75% FC, irrigation by magnetoelectric water caused the largest increase in the yield of the fresh weight of spinach shoots.

Equation (4) was used to fit the curve of the fresh weight of shoots in different treatment groups with time, and the fitting results are shown in

Table 2. Parameters of the logistic model for the fresh weight of aerial part of different treatments.

Treatment	Logistic Model Parameter Values			RMSE	R2
	FWAmax	A	B
CK-1	3.62 ± 0.55 i	5.958	0.138	0.022	0.998
MD-1	3.81 ± 0.28 i	8.397	0.210	0.068	0.993
CK-2	5.3 ± 0.43 h	6.378	0.157	0.034	0.998
MD-2	7.69 ± 0.12 g	5.671	0.145	0.007	0.999
CK-3	8.41 ± 0.35 f	5.585	0.126	0.028	0.999
MD-3	12.86 ± 0.19 a	4.337	0.103	0.092	0.998
CK-4	11.43 ± 0.26 c	5.645	0.124	5.013	0.998
MD-4	12.17 ± 0.23 b	5.013	0.118	0.015	0.999
CK-5	9.74 ± 0.24 e	5.580	0.126	0.026	0.997
MD−5	10.59 ± 0.32 d	5.136	0.119	0.018	0.999

CK-1, CK-2, CK-3, CK-4, and CK-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with conventional water, respectively; MD-1, MD-2, MD-3, MD-4, and MD-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with magnetoelectric water, respectively. A; B, fitting parameters; FWA_max, theoretical maximum of aboveground fresh weight; RMSE, root mean square error. Mean ± SD (n = 3) followed by different lowercase letters in the same column differ significantly at p < 0.05.

. As observed, the fitted R² values of all the treatment groups were >0.99, indicating that the fitting was effective. The theoretical maximum of the shoots of both the MD and CK groups increased first and then decreased with the increase in the soil moisture content. The MD treatment promoted the theoretical maximum of the fresh weights of shoots under the same soil moisture content. Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the theoretical maximum fresh weight of the shoots of MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 5.25%, 45.09%, 52.91%, 6.47%, and 8.73%, respectively. Thus, when the soil moisture content is 65–75% of the FC, irrigation by magnetoelectric water could obtain the theoretical maximum fresh weight of spinach shoots.

Table 3. Analysis of variance of the total nitrogen in spinach leaves, fresh weight of aerial parts, and interaction between irrigation water type and soil moisture content.

Treatment	LTN		FWA
	F	P	F	P
Irrigation water type	49.285	8.24 × 10−7	175.38	2.33 × 10−11
Soil moisture content	81.619	4.28 × 10−12	505.36	8.88 × 10−20
Irrigation water type × Soil moisture content	0.707	0.597	35.66	7.73 × 10−9
R2	0.927		0.988

LTN, leaf total nitrogen; FWA, fresh weight of the aerial part. F; P, Indicates a significant difference.

shows the effect of the two factors of irrigation water type and soil moisture content on the total nitrogen content in the spinach leaves and the fresh weight of shoots. As observed, irrigation water type and soil moisture content had extremely significant effects on the total nitrogen content in spinach leaves and the fresh weight of shoots (p < 0.01). The interaction between the two had no significant effect on the total nitrogen content in spinach leaves (p > 0.05), but it had a significant effect on the fresh weight of spinach shoots (p < 0.01). By comparing the F and p values, the overall order of the main effects was as follows: soil moisture content > type of irrigation water.

To further analyze the effect of different treatments on the fresh weight of shoots, the curve of the growth rate of shoot fresh weight of spinach in different treatment groups with time was obtained based on Equation (5) (Figure 3). As shown in Figure 3, the growth rates of the fresh weight of spinach shoots in different treatment groups exhibited consistent trends, such as a slow initial growth rate, followed by the maximum growth rate, and then a decrease in the growth rate. In comparison with conventional water, the effect of magnetoelectric integrated water on the growth rate of spinach shoots as determined by the fresh weight differed based on different soil moisture conditions. Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the maximum growth rate of MD-1, MD-2, MD-3, MD-4, and MD-5 had increased by 60.27%, 34.41%, 24.95%, 1.24%, and 2.48%, respectively. Different treatment groups reached the maximum growth rate at different times, MD-1, MD-2, MD-3, MD-4, and MD-5 were 3 d, 2 d, 2 d, 3 d, and 1 d ahead of CK-1, CK-2, CK-3, CK-4, and CK-5, respectively.

3.3. Analysis of Chlorophyll Content in Spinach and Its Photosynthetic Characteristics

Since the maximum growth rate of fresh weight of spinach shoots in different treatment groups mostly appeared around day 42 after sowing, the chlorophyll content and photosynthetic characteristics, including the photoresponse curve, stomatal conductance, and intercellular CO₂ concentration measured at this culture time were selected for analysis.

Table 4. Chlorophyll content of spinach under different treatments.

Treatment	Chl a (mg·g−1)	Chl b (mg·g−1)	Chl a/b (mg·g−1)	Chl a + b (mg·g−1)
CK-1	0.87 ± 0.13 h	0.38 ± 0.02 e	2.29 ± 0.07 i	1.25 ± 0.12 i
MD-1	1.24 ± 0.06 g	0.51 ± 0.04 d	2.43 ± 0.05 f	1.75 ± 0.09 h
CK-2	1.49 ± 0.11 f	0.56 ± 0.04 d	2.66 ± 0.1 e	2.04 ± 0.06 g
MD-2	1.84 ± 0.05 e	0.62 ± 0.02 cd	2.97 ±0.06 cd	2.46 ± 0.07 f
CK-3	1.87 ± 0.08 de	0.63 ± 0.05 bcd	2.98 ±0.05 c	2.48 ± 0.09 ef
MD-3	2.41 ± 0.11 b	0.75 ± 0.0 8 b	3.21 ±0.07 ab	3.16 ± 0.15 c
CK-4	3.32 ± 0.31 a	1.05 ± 0.08 a	3.16 ±0.02 b	4.37 ± 0.11 b
MD-4	3.47 ± 0.22 a	1.11 ± 0.11 a	3.13 ±0.06 a	4.58 ± 0.12 a
CK-5	1.97 ± 0.09 cd	0.69 ± 0.06 bc	2.86 ±0.09 d	2.66 ± 0.11 de
MD-5	2.11 ± 0.08 c	0.7 ± 0.08 b	3.01 ±0.07 bc	2.81 ± 0.08 d

CK-1, CK-2, CK-3, CK-4, and CK-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with conventional water, respectively; MD-1, MD-2, MD-3, MD-4, and MD-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with magnetoelectric water, respectively. Chl a, chlorophyll a; chl b, chlorophyll b; chl a/b, chlorophyll a/b; chl a + b, chlorophyll a + b. Mean ± SD (n = 3) followed by different lowercase letters in the same column differ significantly at p < 0.05.

shows the chlorophyll contents in spinach in different treatment groups. The amount of chlorophyll a, b, and chlorophyll a + b in both the MD and CK groups increased first and then decreased with the increase in soil moisture content. Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the total mass fraction of chlorophyll in MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 38.14%, 19.82%, 27.10%, 9.74%, and 5.29%, respectively. The rules for change from large to small were as follows: MD-4, CK-4, MD-3, MD-5, CK-5, CK-3, MD-2, CK-2, MD-1, CK-1, which was basically similar to the growth rate change rule. For chlorophyll a, the MD groups were significantly higher than those of the CK in other soil moisture gradients (p < 0.05), except that the MD and CK groups did not differ significantly (p > 0.05) under the 75–85% FC and 85–95% FC gradients (p > 0.05). For chlorophyll b, the MD groups were only significantly higher than the CK at 45–55% FC (p < 0.05), and the difference was not significant at other soil moisture gradients (p > 0.05). It was apparent that the magnetoelectric water primarily affected the content of chlorophyll a in spinach leaves. Additionally, for chlorophyll a/b under different soil moisture gradients, the chlorophyll a/b of the MD groups was significantly higher than that of the CK groups (p < 0.05), suggesting that the magnetoelectric water enhanced the light-capturing efficiency of spinach leaves.

On day 42 during sowing, the relationship between the increase in chlorophyll content of the MD groups relative to the CK groups was fitted with the increase in total nitrogen content in the leaves and the shoot growth rate, as shown in Figure 4. Under different soil water contents, the increase in chlorophyll of the spinach in magnetoelectric water relative to conventional water showed a good linear relationship with the growth rate of aboveground fresh weight and the increase in leaf total nitrogen content. Respectively, the fitting accuracy was greater than 0.90, indicating that there was a correlation between chlorophyll, aerial fresh weight, and leaf total nitrogen content.

Figure 5a shows the trends of the net photosynthetic rate of spinach leaves in different treatment groups with photosynthetically active radiation. As observed, the change in dynamics of the photoresponse curves of MD and CK groups under different soil moisture contents and different photosynthetically active radiation treatments were similar. When the photosynthetically active radiation was constant, the spinach net photosynthetic rate first increased and then decreased with the increase in the soil moisture content. When the photosynthetically active radiation increased, and the soil moisture content was constant, the spinach net photosynthetic rate first increased rapidly and then began to level off when the photosynthetically active radiation reached 1000 µmol·m⁻²·s⁻¹, before finally decreasing. This indicated that spinach leaves would be subject to photoinhibition and light saturation under strong light irradiation. For example, when the photosynthetically active radiation was 1800 µmol·m⁻²·s⁻¹ in the range of photosynthetically active radiation with a relatively stable net photosynthetic rate, compared with MD-1, MD-2, MD-3, MD-4, and MD-5, the stomatal conductance of CK-1, CK-2, CK-3, CK-4, and CK-5 increased by 81.28%, 90.45%, 80.27%, 19.87%, and 8.08%, respectively. Figure 5c shows the variation in intercellular CO₂ concentration in the spinach leaves in different treatment groups with photosynthetically active radiation. As observed, the intercellular CO₂ concentration first decreased and then increased with the increase of soil moisture content. Magnetoelectric water reduced the intercellular CO₂ concentration in leaves. For example, when the photosynthetically active radiation was 1800 µmol·m⁻²·s⁻¹, compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the intercellular CO₂ concentrations of MD-1, MD-2, MD-3, MD-4, and MD-5 decreased by 9.0%, 6.76%, 35.74%, 22.89%, and 13.03%, respectively.

3.4. Determination of a Suitable Model for the Photoresponse Curve

To quantitatively analyze the effect of magnetoelectric water on spinach photosynthetic characteristics and determine the appropriate photoresponse curve, rectangular hyperbolic, non-rectangular hyperbolic, modified rectangular hyperbolic, and exponential models were used to fit the photoresponse curve of spinach leaves under different soil moisture contents.

Figure 6 shows the fitting curve of the four models to the spinach net photosynthetic rate. The net photosynthetic rates of spinach in all the treatment groups showed a trend of first decreasing and then increasing with the increase in photosynthetically active radiation. When the photosynthetically active radiation was 2500 µmol·m⁻²·s⁻¹, the fitted values of rectangular hyperbolic, non-rectangular hyperbolic, and exponential models were basically larger than the measured values. This could be attributed to the fact that the rectangular hyperbolic, non-rectangular hyperbolic, and exponential models were all monotonically increasing functions, which could not effectively fit the photo-inhibition and photo-saturation phenomena of spinach leaves under strong light. The modified rectangular hyperbolic model could effectively fit the process of change in the net photosynthetic rate of spinach leaves under different levels of photosynthetically active radiation.

Table 5. Precision comparison of simulation values of light response curves by four light response models.

Models	Treatment	RMSE µmol·m−2·s−1	MAE µmol·m−2·s−1	R2	Models	Treatments	RMSE µmol·m−2·s−1	MAE µmol·m−2·s−1	R2
Rectangular hyperbolic model	CK-1	0.583	0.453	0.992	Non- rectangular hyperbolic model	CK-1	0.373	0.302	0.997
	MD-1	0.656	0.533	0.991		MD-1	0.510	0.409	0.994
	CK-2	0.795	0.635	0.989		CK-2	0.512	0.434	0.995
	MD-2	0.777	0.599	0.991		MD-2	0.534	0.44	0.996
	CK-3	0.758	0.555	0.993		CK-3	0.664	0.49	0.995
	MD-3	0.863	0.695	0.993		MD-3	0.771	0.662	0.995
	CK-4	0.768	0.641	0.995		CK-4	0.674	0.555	0.996
	MD-4	0.588	0.478	0.998		MD-4	0.534	0.449	0.998
	CK-5	0.529	0.392	0.997		CK-5	0.458	0.354	0.998
	MD-5	0.511	0.407	0.998		MD-5	0.469	0.395	0.998
Modified rectangular hyperbolic model	CK-1	0.315	0.292	0.998	Exponential model	CK-1	0.355	0.31	0.997
	MD-1	0.221	0.19	0.999		MD-1	0.389	0.302	0.997
	CK-2	0.233	0.185	0.999		CK-2	0.392	0.327	0.997
	MD-2	0.217	0.174	0.999		MD-2	0.385	0.298	0.998
	CK-3	0.439	0.369	0.998		CK-3	0.602	0.504	0.996
	MD-3	0.445	0.364	0.998		MD-3	0.63	0.546	0.997
	CK-4	0.429	0.296	0.999		CK-4	0.586	0.432	0.997
	MD-4	0.367	0.303	0.999		MD-4	0.723	0.578	0.996
	CK-5	0.359	0.301	0.999		CK-5	0.577	0.487	0.996
	MD-5	0.316	0.258	0.999		MD-5	0.502	0.411	0.998

CK-1, CK-2, CK-3, CK-4, and CK-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with conventional water, respectively; MD-1, MD-2, MD-3, MD-4, and MD-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with magnetoelectric water, respectively. RMSE, root mean square error; MAE, mean absolute error.

shows the comparison of the simulation accuracy of the photoresponse curve by the four models. A comparative analysis indicated that, compared with the other models, the fitting curve of the modified rectangular hyperbolic model was the closest to the measured value, while R² was closer to 1, and the root mean square error (RMSE) and mean absolute error (MAE) were smaller.

3.5. Light Response Fitting and Parameter Variation Characteristics of Spinach as Shown by the Modified Rectangular Hyperbolic Model

The modified rectangular hyperbolic model was used to simulate and calculate the net photosynthetic rate of spinach leaves of the CK and MD groups under five types of soil moisture content, and the fitted values were compared with the measured values. The results are shown in Figure 7. The error calculation and analysis indicated that the coefficient of determination R² between the simulated and measured values of CK and MD was >0.99 in both cases; the RMSEs were 0.425 and 0.391 µmol·m⁻²·s⁻¹, respectively; and the MAEs were 0.316 and 0.295 µmol·m⁻²·s⁻¹, respectively. Therefore, there was a strong goodness of fit index between the simulated and measured values. This indicated that the modified rectangular hyperbolic model could be used to simulate the net photosynthetic rate of spinach leaves under different soil moisture contents.

The photoresponse curve mathematical model was used to calculate P_nmax. Parameters that reflect the physiological significance of crops, such as the light compensation point I_c, light saturation point I_sat, and dark respiration rate R_d, among others, are widely used in the research on crop growth and agricultural production [20]. To accurately compare and analyze the effects of magnetoelectric water and conventional water on spinach photosynthesis under different soil moisture contents, the photosynthetic characteristic parameters, such as α, P_nmax, I_c, I_s, and R_d, of spinach in different treatment groups (

Table 6. Parameters of the spinach light response curve from the modified rectangular hyperbolic model.

Treatment	α	Pnmax µmol·m−2·s−1	Ic µmol·m−2·s−1	Is µmol·m−2·s−1	Rd µmol·m−2·s−1	R2	ΔI = Is − Ic µmol·m−2·s−1
CK-1	0.038 ± 0.003 i	13.88 ± 0.59 g	29.98 ± 0.42 a	1680.73 ± 12.35 h	1.15 ± 0.05 h	0.998	1650.75
MD-1	0.049 ± 0.004 h	16.99 ± 0.97 f	26.96 ± 0.36 c	1748.88 ± 8.52 f	1.32 ± 0.06 g	0.999	1721.91
CK-2	0.048 ± 0.005 h	16.97 ± 0.68 f	27.80 ± 0.51 b	1726.72 ± 10.37 g	1.36 ± 0.04 g	0.999	1698.92
MD-2	0.061 ± 0.002 g	22.77 ± 0.51 e	24.90 ± 1.14 e	2001.42 ± 45.54 e	1.51 ± 0.06 f	0.999	1976.5
CK-3	0.067 ± 0.003 f	24.68 ± 0.33 d	26.82 ± 0.63 e	1991.70 ± 28.73 e	1.80 ± 0.06 e	0.998	1964.88
MD-3	0.091 ± 0.003 c	26.53 ± 0.82 c	23.26 ± 0.31 f	2278.81 ± 16.22 c	2.12 ± 0.02 c	0.998	2255.54
CK-4	0.099 ± 0.003 b	31.13 ± 1.25 b	22.76 ± 0.24 g	2325.34 ±14.84 b	2.24 ± 0.05 b	0.999	2302.58
MD-4	0.129 ± 0.005 a	33.73 ± 1.13 a	20.29 ± 0.53 h	2517.71 ± 65.41 a	2.61 ± 0.14 a	0.999	2497.43
CK-5	0.072 ± 0.002 e	24.46 ± 0.65 d	25.09 ± 0.22 d	2212.51 ± 23.68 d	1.80 ± 0.04 e	0.999	2187.42
MD-5	0.079 ± 0.003 d	31.03 ± 1.37 b	24.64 ± 0.21 e	2251.21 ± 9.34 d	1.95 ± 0.08 d	0.999	2226.61

CK-1, CK-2, CK-3, CK-4, and CK-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with conventional water, respectively; MD-1, MD-2, MD-3, MD-4, and MD-5 indicate that the soil moisture content is controlled at 45–55%, 55–65%, 65–75%, 75–85%, and 85–95% of the field capacity with magnetoelectric water, respectively. α, apparent quantum efficiency; P_nmax, maximum net photosynthetic rate; I_c, light compensation point; I_s, light saturation point; R_d, dark respiration rate; ΔI, available light intensity range. Mean ± SD (n = 3) followed by different lowercase letters in the same column differ significantly at p < 0.05.

) were calculated based on the fitting results of the modified rectangular hyperbolic model.

Apparent quantum efficiency is an indicator of the ability of a crop to absorb, convert and utilize light energy in low light [21]. As shown in Table 6, the apparent quantum efficiency of the spinach leaves of both the CK and MD groups increased first and then decreased with the increase in the soil moisture content. Under different soil moisture contents, the apparent quantum efficiency of spinach leaves in the MD groups was greater than that of the CK groups, indicating that irrigation by magnetoelectric water could increase the ability of spinach leaves to utilize weak light. Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the spinach leaf apparent quantum efficiency of MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 27.60%, 23.93%, 35.62%, 30.43%, and 10.17%, respectively. The apparent quantum efficiency of MD-4 was the highest of spinach leaves.

The light compensation and light saturation points of crop leaves reflect the light conditions requirements of a crop and are an important indicator for evaluating whether a crop can tolerate shade and utilize strong light [22]. As the light compensation point decreases, the ability of a crop to utilize low light intensity increases [23], while a higher light saturation point indicates a stronger ability of the crop to utilize strong light [24]. As shown in Table 6, the spinach leaf light compensation points of both the CK and MD groups showed a trend of first decreasing and then increasing with the increase in soil moisture content, and the light saturation point increased first and then decreased with the increase in the soil moisture content. The spinach leaf light compensation point of the MD groups was lower than that of the CK under different soil moisture contents, and the light saturation point was higher than that of the CK. The results demonstrated that irrigation by magnetoelectric water could increase the range of light intensity utilized by spinach leaves. Compared with the CK-1, CK-2, CK-3, CK-4, and CK-5, the spinach leaf light compensation points of MD-1, MD-2, MD-3, MD-4, and MD-5 decreased by 10.06%, 10.44%, 13.25%, 10.85%, and 1.94%, respectively; and the light saturation point increased by 4.05%, 15.91%, 14.41%, 8.27%, and 1.75%, respectively. The MD−4 treatment had the lowest spinach leaf light compensation point and the highest light saturation point.

The R_d of crops refers to the respiration rate of crops when there is no light. Most of the energy released by crops during R_d is lost in the form of heat, and a small portion is used for the physiological activities of crops [25]. Therefore, to some extent, a larger R_d indicates a higher physiological activity of the crop leaves. As shown in Table 6, the spinach leaf R_d of both the CK and MD groups first increased and then decreased with the increase in the soil moisture content. Under the same soil moisture content, the R_d of the MD groups was significantly higher than that of the CK groups (p < 0.05). Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the spinach leaf R_d of MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 14.59%, 10.96%, 17.60%, 16.22%, and 8.05%, respectively. The maximum net photosynthetic rate of both the CK and MD groups first increased and then decreased with the increase in the soil moisture content, and the maximum net photosynthetic rates of the MD groups were higher than those of the CK groups. Compared with CK-1, CK-2, CK-3, CK-4, and CK-5, the maximum net photosynthetic rate of the spinach leaves of MD-1, MD-2, MD-3, MD-4, and MD-5 increased by 22.34%, 34.22%, 7.51%, 8.34%, and 26.38%, respectively. Among them, the MD−4 treatment exhibited the highest maximum net photosynthetic rate of the spinach leaves.

4. Discussion

4.1. Effects of Soil Moisture Content and Magnetoelectric Water on the Total Nitrogen Content and Chlorophyll in Spinach Leaves

Nitrogen is a key macroelement that is necessary for crop growth and development, primarily in chloroplasts and proteins [26], and it is an important factor that limits the rate of crop growth and photosynthetic capacity [27]. In this experiment, the dynamics of total nitrogen content in the spinach leaves in different treatment groups were similar. At the early stage of growth, the nitrogen content of spinach leaves was at a high level, and with the rapid growth of leaves, the photosynthetic capacity of the spinach increased. Then, the rate of nitrogen consumption was greater than the rate of absorption [28], and the nitrogen content of the leaves began to decrease. The soil moisture condition affected the nitrogen content of spinach. With the increase in the soil moisture content, the total nitrogen content of spinach tended to first increase and then decrease. This could be attributed to the fact that more nutrients were absorbed by crops while absorbing water, and the increase in soil water content could make crops absorb more nitrogen. When the soil moisture contents were excessively high, the soil pores would be filled with water, making it difficult for outside air to enter, causing soil hypoxia, increasing soil acidity, and hindering the ability of crops to absorb nitrogen [29].

This study found that under different soil moisture gradients, the total nitrogen content in spinach leaves under irrigation by magnetoelectric water was significantly higher than that of irrigation by conventional water (Figure 1), demonstrating that magnetoelectric water could promote the absorption and utilization of nitrogen by spinach. Previous research also found that magnetization could increase the contents of N, P, K, Mg, Ca, and other mineral elements in crops, such as wheat (Triticum aestivum), tomato (Solanum lycopersicum), and soybean (Glycine max) [30,31,32]. This is probably because the surface tension of water decreases, and the amount of dissolved oxygen increases after conventional water is magnetized and de-electronated. The hydrogen bond between water molecules is weakened, and monomer water molecules with higher activity and permeability are generated. The improved solubility of various mineral salts and the availability of soil nutrients help the crop to absorb and utilize mineral nutrients, which may be the reason for the increase in the nitrogen content of spinach leaves under magnetoelectric water irrigation.

Higher plants primarily contain chlorophyll a and chlorophyll b, which are important pigments that convert light energy into chemical energy. The function of chlorophyll b is primarily to collect the light energy, while chlorophyll a primarily uses the collected light energy for photochemical activity. The chlorophyll a/b ratio reflects the stacking degree of thylakoids, and as the stacking degree increases, the photoinhibition effect becomes weaker. The soil moisture conditions affected the synthesis of spinach chlorophyll. This study found that suitable soil moisture content (75–85% FC) could maximize the chlorophyll content in spinach, while higher or lower soil moisture contents would reduce the chlorophyll content in spinach.

Severe soil moisture stress causes the thylakoids to gradually disintegrate, resulting in a complete loss of chloroplast activity and a decrease in chlorophyll content [33]. By contrast, when the soil moisture is high, the chlorophyll is diluted, and the chlorophyll content will decrease accordingly. Moreover, nitrogen is a component of chloroplasts, particularly in chlorophyll, and a lack of nitrogen in crops will inhibit the synthesis of chlorophyll. This experiment found that magnetoelectric water increased the chlorophyll content in spinach (Table 4), which was consistent with the findings of previous studies [34]. Additionally, this study found that irrigation by magnetoelectric water correlated to some degree with irrigation by conventional water in the growth rate of chlorophyll content and total nitrogen content in leaves (Figure 4). This result indicated that irrigation by magnetoelectric water could increase the total nitrogen content in the spinach plant, so that more nitrogen would be distributed to chloroplasts and participate in photosynthesis [35]. An increase in the amount of nitrogen that was involved in photosynthesis resulted in a higher proportion of nitrogen allocated to chlorophyll and 1,5-bisphosphate carboxylase/oxygenase (RuBisCo) [36], which would increase physiological metabolism, thereby increasing the growth rate of spinach.

4.2. Effects of Soil Moisture Content and Magnetoelectric Water on Photosynthetic Characteristics and Aboveground Fresh Weight of Spinach

Soil moisture affects crop photosynthesis, which can be divided into stomatal factors and non-stomatal factors [37]. The former refers to the lack of water absorption and water supply capacity of roots owing to soil drought, resulting in a decrease in the content of leaf guard cytosol; the water potential increases and the water loss diminishes, thus, decreasing the turgor pressure [38]. Some stomata are closed, or their degree of opening is reduced, resulting in a low ability of the leaf stomata to conduct the transport of CO₂. A reduction in the concentration of CO₂ in the photosynthetic raw material will eventually reduce the photosynthetic capacity. The latter refers to the decrease in photosynthetic capacity, such as the chlorophyll RuBisCo activities of mesophyll cells, resulting in an insufficient supply of ATP and nicotinamide adenine dinucleotide phosphate (NADPH). The increase in the concentration of intercellular CO₂ directly affects the photosynthetic capacity of leaves. Therefore, the change in the concentration of intercellular CO₂ can be used as the basis for evaluating whether the reduction in photosynthetic rate is caused by stomatal or non-stomatal factors [39]. In this study, analyses of the changes in spinach stomatal conductance and intercellular CO₂ concentration with the soil moisture content (Figure 5) indicated that the intercellular CO₂ concentration increased when the soil moisture content decreased. Therefore, the non-stomatal factors mainly dominated the effect of soil moisture content on spinach leaf photosynthesis, which indicated that the stress of soil moisture led to the inhibition of spinach photosynthesis.

Under stress conditions, such as drought, waterlogging, low temperature, or salinity, active oxygen will accumulate in crops, resulting in peroxidative damage to lipids, nucleic acids, and sugars. Crops primarily rely on antioxidase activities in vivo to resist the damage caused by peroxidation [40,41,42], so the level of antioxidase activity in crops reflects their tolerance to adversity. Some researchers have found that when water is magnetized or de-electronated, the positive and negative ions in the water will move, and the generated electron flow will produce O₂ in the water, which generates superoxide ions (O₂⁻) and hydrogen peroxide (H₂O₂) [43]. H₂O₂ is a toxic molecule of active oxygen produced during the metabolism of crop cells [44]. However, H₂O₂ can also be used as an exogenous signal molecule, involved in the regulation of crop growth and development [45] and the process of responding to various adversity stresses. Thus, H₂O₂ has a dual role of protection and toxicity on crops [46]. Ahmed et al. reported that low concentrations of H₂O₂ could induce the establishment of antioxidant defense mechanisms in crops [47]. Additionally, the use of exogenous H₂O₂ could increase the tolerance of tomatoes, bell peppers, and watermelons to cold stress [48,49,50], improve the drought tolerance of soybeans, tobacco, and rapeseed [51,52,53], and improve the salt tolerance of wheat, tomatoes, and other crops [54,55,56,57]. Darmanti et al. reported that under drought conditions, exogenous H₂O₂ treatment enhanced the net photosynthetic rate and stomatal conductance of soybean leaves [58], while reducing the contents of the oxidative product malondialdehyde (MDA), which was induced by drought stress and H₂O₂.

This experiment found that when the soil moisture content was low or high, the apparent quantum efficiency, light saturation point, and stomatal conductance of spinach leaf decreased, while the light compensation point and intercellular concentration of CO₂ increased. This result suggested that the photosynthesis of spinach was inhibited. However, the apparent quantum efficiency, light saturation point, and stomatal conductance of spinach under irrigation by magnetoelectric water were higher than those in spinach plants irrigated with conventional water. Both the light compensation point and intercellular CO₂ concentration were lower than those under irrigation by conventional water (Figure 5 and Table 6), indicating that irrigation by magnetoelectric water could effectively improve the stomatal regulatory ability of spinach leaves under water stress, reduce stomatal resistance, increase the range of available light intensity and the fluorescence efficiency of chlorophyll, and maintain normal photosynthesis. An additional combination of the data on the fresh weight of spinach shoots (Figure 2) under the same soil moisture gradient indicated that the fresh weight of spinach shoots under irrigation by magnetoelectric water was significantly higher than that of shoots irrigated by conventional water. This could be attributed to the fact that the H₂O₂ in the magnetoelectric water promoted the establishment of the spinach antioxidant defense mechanism, alleviated the damage to the spinach photosynthetic apparatus caused by active oxygen accumulation induced by water stress, and thus improved the ability of the spinach to assimilate photosynthetic carbon. Experiments have demonstrated that magnetized water could enhance antioxidase activities in kidney beans (Phaseolus vulgaris) and lettuce (Lactuca sativa) to enhance stress resistance [59,60].

Through this experiment, it is found that the use of magnetoelectric water irrigation can improve the photosynthetic capacity and yield of spinach, which provides new technical approaches for the effective use of water resources, and the large-scale use of magnetoelectric water for spinach irrigation in the future is expected to increase the yield by 15–20%. However, the effect of magnetoelectric water on spinach growth under a fixed magnetic field intensity (3000 Gs) was determined in this study, and only one magnetoelectric water was used as the irrigation water. In the future, the influence of different magnetic field intensities and different activation times on spinach growth can be explored.

5. Conclusions

Through experimental research, we found that under different soil water content conditions, magnetoelectric water irrigation could increase the total nitrogen content, chlorophyll content, and net photosynthetic rate of spinach leaves, thereby accelerating the growth process of spinach, and ultimately leading to an increase in yield. Among them, spinach yield was the highest when the soil water content was maintained at 65–75% FC with magnetoelectric water irrigation. In addition, the modified right-angle hyperbolic model fits the spinach light response curve well. Therefore, under magnetoelectric water irrigation, the soil water content of 65–75% FC was more conducive to increasing vegetable yield. The results of this study can provide a theoretical basis and method support for improving the efficiency of irrigation water resources.