1. Introduction
In the SPAC water vapor dynamic circulation system, plants are the main link connecting soil and atmosphere [
1,
2,
3,
4,
5]. Water reaches plant roots through soil, leaves through plant stems, then diffuses from leaf stomata to the static air layer, and finally participates in the atmospheric turbulent transformation to form a unified, dynamic, and mutual feedback continuous system [
6,
7]. Accordingly, the continuous monitoring of SPAC is of great significance for comprehensively exploring the law of crop water consumption.
Domestic and foreign scholars have conducted extensive research on SPAC. In the field of soil monitoring, Ekanayaka et al. [
8] have developed low-cost capacitive soil moisture sensors for data collection, with the aim of automatically monitoring soil moisture. Shen et al. [
9] used soil moisture sensors to measure the soil moisture content at different depths to determine the appropriate placement of soil moisture sensor. Stephan et al. [
10] utilized soil moisture sensors to measure the soil moisture content at different locations and found that after rainfall, the soil moisture in the corridor was the lowest in the weathered surface crust. However, in practical application, the aforementioned methods can only obtain single-point parameters and cannot completely describe the state of the overall soil moisture loss in the root-zone during crop growth. In crop monitoring, a portable photosynthetic instrument is utilized to determine the transpiration rate, stomatal conductance and other indicators of cotton [
11], hybrid winter wheat [
12] and young apricot trees [
13]. Current monitoring methods are mainly based on leaf-scale parameter information, and continuous online monitoring cannot be realized, thereby reducing the integrity of crop response research. Regarding atmospheric monitoring, Liu et al. [
14] and Wang et al. [
15] utilized a large-scale automatic weighing lysimeter to measure the actual crop evapotranspiration. Tyagi et al. [
16] used a weighing lysimeter to measure the hourly evapotranspiration of rice and sunflowers. However, the equipment has high cost, complicated installation, and difficult soil borrowing process, thereby limiting its wide-scale application.
Based on this, we propose a low-cost potted plant evapotranspiration measurement system based on Lora wireless technology for horticultural facilities [
17]. This system achieves the continuous weighing of potted plants in the whole growth cycle and real-time monitoring of the evapotranspiration of potted plants. As a kind of crop with large water demand and high sensitivity to water, vegetables are suitable for irrigation test based on our IWS. Lettuce (
Lactuca sativa var.
ramosa Hort.) as a representative of leafy vegetables, is rich in nutrition and has certain health and medicinal value. In recent years, the cultivation scale of lettuce has increased rapidly. The vegetable planting area in China is approximately 1.35 million hm
2 [
18]. Based on the IWS, this study monitors and analyzes the SPAC of lettuce under different water treatments, so as to explore the water consumption characteristics of potted crops under different water treatments. This study provides theoretical guidance for improving precise crop irrigation and provides a new direction for the development of irrigated agriculture.
4. Discussion
The IWS can sensitively reflect changes in crop weight during different growth periods at different times. The results indicate that WIWS and SWC have similar changing trends, and a significant correlation exists between WIWS and SW. The correlation in the second season is lower than that in the first season. This may be due to the different types of lettuce in the first and second seasons. The lettuce in the second season is three times as heavy as the lettuce in the first season. The daily weight gain of lettuce is ignored because it is small. However, under a similar SW, as the weight of the lettuce in the latter period increases, the variation range of WIWS is affected by the weight of the lettuce, resulting in scattered data and reduced correlation.
“E” refers to the amount of water transpired per unit leaf area of crops over a certain period of time. Plant water transpiration is mainly performed by the stomata of leaves, which is an important channel for the exchange of gas and water between leaves and the external environment. The opening and closing of stomata play an important role in plant transpiration. The results of the experiments indicate that as the irrigation volume decreases,
E and
gsc also decrease. Álvarez et al. [
25] believe that deficit irrigation treatment reduces stomatal conductance, which reduces the relative growth rate of plants, as well as transpiration and photosynthesis. This is because when crops suffer from soil water deficit, they cannot obtain the water required to meet the photosynthesis of plants. The photosynthesis of plants is weakened, the
gsc of crops will decrease, and
E will also decrease, so as to effectively reduce the water loss of crops during transpiration. The
E and
gsc of lettuce under T2 treatment are higher than those under other water treatments when the lettuce enters into the rosette and heading stages. This is because the SWC under T1 treatment is too high, which inhibits lettuce respiration, thus affecting transpiration and photosynthesis. Under T4 treatment, the SWC is too low to meet the water required for photosynthesis in lettuce, and photosynthesis is weakened [
26]. In addition, the leaf area of lettuce under T2 treatment is significantly higher than that of the other treatments, which increases the
gsc of lettuce. In the first season, the leaf area under T1, T2, T3, and T4 treatments were 464.67 cm
2, 481.64 cm
2, 473.30 cm
2, and 403.30 cm
2, respectively. In the second season, the leaf area under T1, T2, T3, and T4 treatments were: 2515.50 cm
2, 2609.63 cm
2, 2104.87 cm
2, and 1849.75 cm
2, respectively.
Based on the IWS, the weight of lettuce was monitored in real time and continuously, and the variation law of plant evapotranspiration during the growth period was analyzed. The change of
ETc is closely related to the change of environmental factors; hence, we further explore the relationship between
ETc and environment (
Table 5). There is a significant positive correlation between
ETc and T under different water treatments. With the increase of temperature, plant evapotranspiration also increases. The results of the first and second seasons indicate that the
ETc and water consumption of the T2 treatment are higher than those of other treatments in the rosette and heading stages. This is because with the passage of time, the leaf area of plants under T2 treatment gradually increases and photosynthesis is gradually enhanced, resulting in an increase in the
ETc of plants. This finding is similar to the results reported by Chang et al. [
27].
Water regulation has an important effect on plant growth and output.
Kc is an important parameter for calculating crop
ETc and reflects the influence of the biological characteristics of the crop itself, crop type, and yield level on crop water demand. Obtaining the
Kc accurately is difficult. Currently, this process is mainly based on experience values [
28,
29,
30]. This study analyzes the
ETc of lettuce based on the IWS and calculates the
ET0 through the environmental parameters obtained by the weather station to analyze the daily
Kc of lettuce. Studies have indicated that under different water treatments,
Kc decreases as the amount of irrigation decreases. This may be because the crops under T4 treatment have been in severe water deficit for a long time during the rosette and heading stages, which inhibits the growth of crops. In the rosette and heading stages, the
Kc under T2 treatment is higher than that under the other treatments, because the leaf area gradually increases over time.
Table 6 indicates that the yield under T2 treatment has significantly increased by 11.55%, 5.72%, and 15.80% compared with other treatments in the first season. In the second season, the yield under T2 treatment has significantly increased by 9.59%, 39.18%, and 61.36% compared with other treatments. From the perspective of IWUE, the T2 treatment significantly increases by 11.07%, 0.91%, and 2.16% compared with other treatments in the first season. In the second season, the T2 treatment significantly increases by 21.05%, 9.89%, and 13.47% compared with other treatments. From the perspective of WUE, lettuce in the second season has significantly higher water use efficiency than in the first season. This result may be because of the high temperature of the greenhouse in the second season, which increased transpiration of the lettuce, and increased the water demand, resulting in a reduction in water loss from irrigation. From the perspective of plant physiological monitoring by the IWS, the higher photosynthetic rate of T2 treatment in the first and second seasons promoted plant growth. Based on the results of the first and second seasons, it can be seen that the lower limit of soil water irrigation for lettuce planting is 80%. A very high or very low water level is not good for growth and development. Excessive water irrigation will cause the soil to be in a state of water saturation for a long time, resulting in soil waterlogging, and thereby forming hypoxia deficit in the root system, which affects the growth above ground. At a very low irrigation level, the water in the soil will not reach the environment required for the growth of lettuce, thus forming soil water deficit and affecting its growth rate. This finding is consistent with the results of previous studies [
31,
32].
The results of growth physiological parameters of leaf-expansion lettuce in the first season and heading lettuce in the second season showed that the E and gsc of lettuce in the second season were lower than those in the first season. This was because the greenhouse temperature of lettuce in the heading stage in the second season was higher than that in the first season, which inhibited gsc and E. The ΔPWn of lettuce in the second season was faster than that in the first season, and the Kc and ETc of lettuce in the second season were higher than that in the first season. This is due to the different varieties and types of lettuce in the first season and the second season. The heading lettuce in the second season was heavier than the leaf-expansion lettuce in the first season, and the greenhouse temperature in the second season was higher, so the lettuce in the second season needed more water. However, the ΔPWn, E, gsc, yield, WUE, and other growth physiological parameters of lettuce in both of the first and second seasons were the highest under T2 treatment among the four different water treatments. The results show that T2 treatment has higher irrigation efficiency for different types of lettuce. Under the condition of water resource shortage, it can be used as a method for farmers to improve water use efficiency of lettuce.
The advantages of our IWS are as follows: 1. It can realize continuous online monitoring of SPAC, and the
Kc analytical method is established. 2. It is not affected by soil texture. Currently, dielectric method is commonly used to measure soil moisture content [
33]. Due to the complexity of the soil dielectric properties of different soil types, the high-precision measurement of soil moisture at different scales is affected in the measurement process. However, the IWS measures the changes of the entire soil. So, it would not be affected by the soil moisture accuracy measurement. 3. It is applicable to a wide range of plants, such as ornamental potted plants, leafy vegetables, flowers, etc. However, the application of IWS is also limited by specific crops. For crops such as tomatoes and cucumbers, vine hangings are required during the growth process. The hanging vines split the load with symmetrical weight, resulting in inaccurate measurements. Therefore, IWS can be improved and optimized by adding a plant-hanging scale in future research.
5. Conclusions
(1) During this study, an intelligent weighing system was used to realize real-time monitoring and analysis of soil moisture weight, weight gain, transpiration rate, stomatal conductance, evapotranspiration, and crop coefficients of potted lettuce. According to the online monitoring data of the intelligent weighing system, the changes in the soil-plant-atmosphere continuum at different times were reflected in real time. Simultaneously, the accuracy and feasibility of the data acquired through the intelligent weighing system were verified.
(2) Different water treatments had a significant impact on the soil-plant-atmosphere continuum of lettuce. Regarding soil, the relative system weight of the intelligent weighing system and the soil volumetric moisture content showed a consistent trend of increase and decrease, with a significant linear correlation (R2 = 0.639–0.941). Soil volumetric moisture content was higher under high moisture treatment, and the soil volumetric moisture content changed less. Regarding plants, the transpiration rate and stomata conductance of lettuce under the lowest water treatment in the first and second seasons were the lowest, which were 2.62 g·h−1 and 2.12 g·h−1, 364.60 mmol m−2 s−1 and 162.74 mmol m−2 s−1, respectively. In the atmospheric environment, with an increase in temperature, lettuce transpiration gradually increases. Under T2 treatment, the evapotranspiration of lettuce was 2.70%, 2.35%, 5.21% in the first season, and 32.07%, 23.21%, 31.05% in the second season, which was higher than that of the other treatments.
(3) An appropriate irrigation low limit could increase the yield of potted lettuce. Based on research results of the first and second seasons, the stomatal conductance, transpiration rate, weight gain, individual plant weight, and irrigation water use efficiency of lettuce all improved when the irrigation low limit was 80% field capacity. Moreover, the irrigation water use efficiency of lettuce under T2 treatment in the first and second seasons was significantly increased by 11.07%, 0.91%, 8.08%, and 21.05%, 9.89%, 13.47%, respectively, compared with the other treatments. The yields obtained were 0.425 kg and 1.428 kg, respectively.