# Quantifying Moss Response to Metal Contaminant Exposure Using Laser-Induced Fluorescence
## 1 Introduction  
While many metals naturally occur and are important for the growth of organisms, metals and radionuclides in excess, also termed potentially toxic elements (PTEs), can negatively impact the metabolic functions of biota, resulting in physiologic changes due to metal-induced stress [1, 2, 3]. Because of their simple morphology, bryophytes are viewed as model organisms for observing physiological changes as a result of exposure to metal toxicity [4, 5]. Despite their extent, which is limited to wet environments, mosses, a type of bryophyte (clade: Embryophyta, division: Bryophyta), are commonly used as biomonitors for atmospheric deposition of PTEs due to their lack of a true root system that limits metal absorption only to their surface [6, 7]. The tissue structure in mosses allows for more rapid uptake of metals when compared to vascular plants, which also results in a noticeablephysiological response [8]. Because their cell walls lack a cuticle layer [5; 9], metals can easily interact and accumulate, leading to moss's large surface-to-weight ratio [10]. Mosses also have the benefit of being able to capture metals from single deposition events and retain a history of exposure due to their slower growth rate [11] and less specialized connective tissue [12]. There has been success for decades in using moss for environmental monitoring, which is largely due to their robust and resilient nature as a plant species that survives in even the harshest of environments [13]. Metals can be distributed via the atmosphere and then delivered to terrestrial mosses by wet and dry deposition [6; 14; 15; 16]. Retention of dissolved copper (Cu) in wet deposition or Cu-carrying dry particles onto the surfaces of plants depends on plant size and its cell wall lipid structure [11]. After metal uptake, total chlorophyll content and chloroplast alteration [5] have been observed in moss [1; 8]. Cu is one of the most commonly tested metals in the literature on moss and found to create a measurable decrease in overall chlorophyll content and increases in the chlorophyll-b-to-chlorophyll-a ratio [17]. Due to these results, and the known history of its natural uptake in plants, this study used Cu in evaluating the potential application of a new laser-induced fluorescence (LIF) technique to aid in identification of metal contamination in the environment via moss. LIF is a promising new tool which offers a less invasive, in situ method for a more efficient alternative, or complement, to more laborious and costly traditional moss collection and analyses. In this research, we propose the use of laser-induced fluorescence (LIF), an emerging imaging and spectroscopic technique that can be used to document a change in the atomic/molecular energy level in plants. Atoms or molecules are excited to a higher energy state via a laser, and after absorption, the energy is released as a spontaneous emission of light or fluorescence [18; 19]. When compared to other methods currently being used, for example, absorption spectroscopy, LIF has been found to have a high signal-to-noise ratio while also maintaining better sensitivity for detection [20]. Near-IR (infrared) spectroscopy, widely used in agriculture, has similar benefits to LIF, being a non-destructive and inexpensive way of monitoring plant stress or nutrient levels. However, at higher wavelengths, near-IR can have reduced sensitivity, which limits the distance at which the technique can be used or in what light conditions measurements can be performed [21; 22]. Though solar reflectance is traditionally used for remote assessment of plant health and observing photosynthesis efficiency in biomass, the technique is useful at larger resolution using satellites but has reduced plant specificity [23]. When compared to these technologies, LIF could fill existing gaps or offer more efficient alternatives. LIF shows promise in successful testing to monitor stress changes as a result of biochemical and physiological factors with a measurable impact on the photosynthesis rate [24].