## 2 Materials and Methods  
Healthy, similar-sized tomato plants (_Solanum lycopersicum_), chosen for applicability for follow up studies, were placed in two chambers (target and control) of a dark tent in the laboratory (Figure 1a). On Day 1 of the experiment, a dose of a photosystem II (PSII) inhibitor, an algicide/herbicide called 3-(3\({}^{\prime}\),\({}^{\prime}\)-dichlorophenyl)-1,1-dimethylurea (DCMU) mixed with 300 mL water was applied to the soil of the target plant. DCMU blocks the electron flow from photosystem II (PSII) during photosynthesis, thus inducing stress when plants cannot effectively convert incoming energy to useful forms for carrying out proper functioning. For this reason, DCMU makes an efficient weed killer, for instance. The large application amount relative to what might be used for weed control in a natural setting was chosen to illicit an intentional stress response due to photosynthetic system shutdown in order to test the capability to detect and track [51]. In this scenario, CF would be expected to rise dramatically in the beginning stages because the energy supplied by incoming illumination would be blocked from entering PSII and a greater amount of excess energy would be given off as CF. After this initial increase, a decline in function results in a decline in CF. The HH2 was mounted on a standard tripod above the target tomato plant canopy in the dark tent enabling a field of view of approximately 29 cm diameter (Figure 1a). An LED was positioned in each chamber using flexible bulb holders from a single lamp base to provide incoming photosynthetically active radiation (PAR; \(\sim\)400-680 nm), one illuminating the target tomato plant and one illuminating the control. The LED emits light only in the blue and red regions, leaving a \"window\" in the upper red and far-red regions where plants emit CF as a by-product of photosynthesis (Figure 1b). Using a timer, the LEDs were turned on at 7:00 a.m. CDST each day and turned off at 7:00 p.m. CDST each evening, during which the HH2 was set to record a spectral measurement every 30 min. Figure 1c shows examples CF spectra on Day 1 of the experiment and the typical two-peak feature of a CF spectrum are distinguishable. For reference, an incoming (LED) spectrum (measured using a Spectralon(r) white reference panel (WR)) is overplotted to illustrate the CF and LED spectra are clearly distinguishable from one another. However, it should be noted that the magnitude of the LED spectrum is lower when illuminating the plant than when illuminating the WR (albedo of \(\sim\)1, while the plant albedo \(\sim\)0.4). Once daily, a Spectralon(r) white reference panel (WR) was placed above the target plant canopy and an irradiance measurement of the LED was made. In addition, a mea  
Figure 1: (**a**) The experimental instrument setup included the handheld ASD/Malvern HandHeld-2 Pro spectroradiometer (Boulder, CO, USA; HH2) and a blue-red LED grow light. A full spectrum lamp is added here for the purposes of a clear photo only. (**b**) Plot of blue-red light emitting diode grow light (MiracleLED(tm) 2.2 Watt; LED) spectrum, measured by recording the spectrum of the blue-red LED incident on a Spectralon® white reference panel (Boulder, CO, USA; WR). Inset: zoom in to a portion of the fluorescence range from 670–800 nm. (**c**) Two sample spectra of the tomato plant within the fluorescence range (approximately 670 nm–780 nm with the spectrum of the WR measurement of the LED for reference. surement of the target and control plant and the WR were made under an ASD/Malvern Illuminator Reflectance Lamp (IRL, 70 W stable quartz-tungsten-halogen calibrated light source, ASD/Malvern, Boulder, CO, USA). The spectra under IRL were used to calculate daily normalized difference vegetation index (NDVI) and photochemical reflectance index (PRI) measurements. The daily reflectance of the control plant did not change throughout the experiment (not shown). The CF portion (650-800 nm region) was extracted from each observation made between 8:00 a.m. CDST and 6:00 p.m. CDST.