Agriculture represents a major consumer of freshwater, accounting for about 70% of the worldwide total water withdrawal [1,2]. In addition, climate change may result in more frequent occurrences of water shortage and increase the competition for water among urban, industrial, and agricultural demands [3,4]. With increasing water demands from other sectors, irrigation agriculture must increase food production with limited water allocation [1,5]. Crop breeding technologies have been developed for this purpose [6,7]. A major point of crop breeding is to develop ideotypes in irrigation agriculture for more efficient acquisition of water and nutrient in the irrigated area [4,8]. Hydrotropism allows roots to grow actively towards water source for drought avoidance [4,9,10]. Hence, precise analysis of this response in plant and its relation to plant water use efficiency (WUE) is important for breeding of drought avoidance species.
Water-saving irrigation technologies have also been developed to increase WUE, and one such technology is subsurface irrigation [11-15]. This technology uses emitters buried in soil to deliver irrigation water directly to the crop root zone [12,13,15]. Proper management of irrigation water can prevent water logging in topsoil and reduce surface evaporation, run-off, and deep percolation, resulting in improved efficiency of irrigation water use and nutrient uptake [12,13,15]. So far, this technology has shown great performance in field irrigation practice, resulting in higher crop yields and quality with less irrigation water, preventing weed growth, mitigating soil N2O emissions, and facilitating the use of degraded-quality water .
When performing subsurface irrigation, a localized zone of wet soil can be produced. Studies have shown that the geometry of the wet zone under subsurface irrigation can be influenced by irrigation rate, soil hydraulic properties, and root water uptake . In the case of bulk soil, the wet soil particles adhere to each other and form a harder part due to the cohesive force of water molecules. A threshold of penetration pressure exists at the border between wet and dry soil. Therefore, water will not flow into the surrounding dry soil when the penetration pressure is lower than the threshold [16,17]. As water supply continues, the wet zone can be enlarged due to the meniscus phenomenon. Previous studies showed that a spherical wet zone can be stably formed in homogeneous dry soil from a point water source and the size of the wet zone depends on the volume of the supplied water [16,17].
Since plants absorb water from the roots, a negative pressure difference is generated between the root and the surrounding soil. The formed wet zone may be smaller than the state without plant due to the root water uptake. The water volume of the narrowed region is considered the available water that absorbed by the plant [16,18]. If the volume of available water is controlled through irrigation to balance the moisture flow between the plant and the surrounding soil, the penetration pressure may not exceed the threshold and the wet zone can be maintained at a constant volume. Increasing water supply may increase the gravitational water flow due to the expansion of the wet zone. But since the root zone may also expand due to plant growth and the corresponding increase in water uptake, it can be considered that the plant water uptake dynamically equilibrates with the penetration pressure generated by capillary forces to stabilize the wet zone. In other words, the wet zone changes dynamically because of the influence of the pressure fluctuation caused by the root water uptake that changes the amount of water retained in the wet zone . Thus, how roots respond to the dynamics of the wet zone is important for high WUE irrigation practice.
Roots are essential organs for water and nutrient absorption . Research showed that plants can adapt and regulate water uptake capacity by changing the root system architecture according to changes in their local environment [4,8,19,20]. Studies of the physiological characteristics of the root system can be dated back to 19th century Darwin’s study “The power of movements in plants,” which stated that the root tips of plants can sense the surrounding soil moisture and modify the direction of root elongation . Root hydrotropism, which allows roots to exploit and intercept localized water resources in soil, may facilitate the utilization of limited water resources [9,10,19,22-24]. In the past decades, two systems have often been used to observe this phenomenon [9,25-29]. In one system, air is placed in an enclosed environment with a concentrated salt solution, and seedlings are mounted on a support, often foam or agar blocks, with the very root tip suspended in air. A water potential gradient can be produced in the root tip between the wet support and the surrounding air [25,26]. Another is the agar-sorbitol system, which places root tips near the border between two growth media with a water potential gradient produced by adding sorbitol to one of the media [26,27]. In both cases, root bending towards higher water potential is considered a hydrotropic phenotype of the test seedling. These systems have been used to identify genes involved in hydrotropism that characterize cellular and molecular events of the response [9,27,30-32]. However, a major issue with these studies is that the ways plants regulate hydrotropism in the laboratory may be different from how they operate in the field condition [10,24,28,33].
To test whether this response happens in natural soil, a few experimental systems have been established. Cole and Mahall  tested hydrotropic responses of two coastal dune shrubs under soil conditions. They produced a water potential gradient in soil by injecting water during the seedling growth to create a moisture rich patch laterally next to the growing pot, and roots growing into the patch were considered a hydrotropic response. In the study, they found no compelling evidence for hydrotropic root behavior of the test seedlings. Iwata et al.  developed an experimental system to measure hydrotropic response in soil and investigate its role in root system development and crop biomass production. They grew Arabidopsis seedlings in rectangular plates covered with soil. A water potential gradient was produced through natural drought by placing a wetter plastic foam on one side of the soil that was half-covered with a lid. Root architecture was scanned by a scanner to study its relationship with moisture distribution. They found that hydrotropism plays an important role in root system development and crop growth. Li et al.  developed a sand system to study hydrotropism of Arabidopsis and tomato plants. They created water potential gradients in both oblique and vertical directions in soils and found gravity significantly influenced the hydrotropic response in both cases. These systems are efficient to study hydrotropism of primary roots in the early growing stages. But how lateral roots (or fine roots), which form the main root system [3,24,34], respond to water potential gradients has been little studied .
Last, a recent study tested the synergic effect of root biomass and hydrotropism on grain yield . The researchers identified hydrotropic phenotypes of maize hybrids using the conventional air system and then performed field trials using hybrids with robust and weak hydrotropic responses. They found a positive interaction between root biomass and hydrotropism in enhancing grain yield . Although the study showed that roots have a positive relationship with water distribution in soil, the root hydrotropic behavior in response to the dynamics of wet zone under subsurface irrigation has not been studied.
Due to the opaque nature of soil-grown roots and the highly varied soil water in time and space, a major difficulty in study root hydrotropism in natural soil is observing and analyzing root growth in response to the dynamics of soil moisture distribution. Modern high-throughput phenotypic technology based on computer vision and machine learning enabled high-resolution measurement of root traits. But this technology requires extracting roots from soil to obtain the high-resolution root images [36,37]. X-ray Computed Tomography (CT) has been widely used to visualize roots in situ. This technology uses a non-destructive technique to visualize the interior of objects in 2-D and 3-D based on the attenuation of an electromagnetic wave . Since the attenuation density of root and soil matrices are similar and highly dependent on soil water content, efforts to visualize root system architecture in soil have focused on the segmentation of roots from the soil pore area [36,38-40]. But so far this technology has limitations in the detection of fine roots and has low contrast in heterogeneous soil and time-consuming user interaction .
To test the hydrotropism theory, rigorous experimental design concerning the observation and control of soil water dynamics is important. A major emphasis should be on forming a steep wetting front around the root system, while water supply inside the wet zone should sustain the crop growth. To implement such a study, high-resolution measurement of soil water dynamics around the rooting zone is prerequisite. Previous research has used high-resolution, nondestructive imaging technologies such as x-ray radiation, magnetic resonance techniques, and electrical resistance [41-45]. However, these techniques require tedious calibration of soil parameters to obtain spatial soil moisture distribution , and the difficulty in accessing such devices also limits their use . Moreover, most studies of the interaction between soil and root water uptake use single-point soil moisture measurement based on neutron probes or tension meters, which lacks accuracy and representativeness due to the spatially and temporally variant nature of soil-root system [47-49]. As a result, high-resolution measurement of spatiotemporal soil water dynamics has not been achieved.
This paper suggests using a high-resolution soil moisture sensor matrix to measure the micro soil water dynamics in the crop root zone. The system uses minimally sized soil moisture sensors placed in a matrix inside and outside the root zone to obtain precise, real-time signals of the temporal and spatial moisture dynamics around the growing roots. Prior to this study, the authors have used this method to confirm the existence of a wet zone in water-saving cultivation of tomato plants [17,18]. The objective of this paper is to verify the feasibility of using the sensors to observe hydrotropic response in the wet zone by measuring the dynamics of soil water content caused by water absorption of the growing plant.