## 1 Introduction  
Mineral resources are an indispensable primary raw material for social and economic development. However, mining inevitably causes damage to the environment and brings many adverse effects on the ecological environment and human health [1]. When mining mineral resources such as coal and metals, a large amount of mine wastewater is generated due to the oxidation of sulfide minerals contained in the minerals [2]. Such mine wastewater has the characteristics of low pH (2-3), high level of Fe ions, and high SO4\({}^{2-}\), thus, it is called acid mine drainage (AMD) [3]. The infiltration of AMD into the soil could cause soil acidification, resulting in vegetation wither, even causing death.  
Furthermore, AMD discharged into rivers will cause the pH of the water body to decrease rapidly, leading to the death of aquatic organisms and weakening the self-purification ability of the water body [4, 5]. AMD is one of the most severe problems for the global mining industry. Due to the large output, the long-time span of formation, and the scattered production sources, the AMD problem has attracted widespread attention from mine environmental remediators worldwide.  
There are physical methods for AMD remediation, including ion exchange, adsorption, and membrane technology [6]. Chemical methods [7,8] include acid-base neutralization, chemical precipitation, and an electrochemical process. Biological methods [9] include phytoremediation and microbiological methods. Phytoremediation, where certain plants are grown to remediate the contaminated soil, is attracting researchers because of its low cost and non-generation of secondary pollution [10]. Traditional chemical monitoring methods usually employs enzyme activity and metabolite analyses in selecting remediation plants. However, these methods require a lot of monitoring time and are invasive, and thus, are not conducive to dynamic monitoring of different stages of the same sample. Therefore, there is a need to develop techniques that can monitor the effects of AMD speedily and non-invasively.  
Optical coherence tomography (OCT) has the advantage of non-invasively detecting the internal structure of biological samples with a few-microns-level resolution, and the technique allows the mapping of 2D or 3D in vivo depth-resolved tomographic images of internal tissue structures [11]. OCT is now widely used in biomedical research [12,13] and is primarily applied in ophthalmology [14]. Several studies on plants using OCT have been published. For example, microstructural changes in rice leaves at different pH levels [15], monitoring of in vivo growth of pepper seeds treated with different NaCl concentrations [16], root growth in soil [15], assessment of apple initial bitter rot process [17], screening of melon seeds for diseases [18], and cucumber green mottle mosaic virus (CG-MMV) infestation of cucumber seeds [19]. In addition, there are applications for portable OCT [20]. In particular, our group has developed biospeckle OCT (biospeckle optical coherence tomography), or bOCT, and demonstrated the potential of bOCT in the visualization of the response of plant leaves under ozone exposure [21,22], biological activities in pea seed germination [23], the phenotyping of increasing Zn concentrations on seed [24], and speedy visualization of application of phytohormone on plant leaves [25]. Therefore, in this study, we propose the use of bOCT to monitor the response of plants to AMD. To our knowledge, the application of OCT in the study of AMD response on plants has never been reported before. bOCT can be a sensitive and efficient method, thus paving the way to speedily select remediation plants.  
We used Kaiware daikon seeds as the sample for the study. They are popular and widely grown in Japan because of their suitable taste and high nutritional value. Furthermore, it has a reasonable seed size, germinating within two days.