Visualization of water usage and photosynthetic activity of street trees exposed to 2 ppm of SO2—A combined evaluation by cold neutron and chlorophyll fluorescence imaging

Photosynthetic efficacy and auto-exhaust-fume resistance of street trees were evaluated by cold neutron radiography (CNR) with D2O tracer and chlorophyll fluorescence (CF) imaging. With these techniques, information on the responses of water usage and photosynthetic activity of plants exposed to simulate toxic auto-exhaust fumes (2 ppm SO2 in air) were obtained. Branches of hibiscus trees were detached, placed into a tub with aerated water and used for the experiments after rooting. A CF image was taken before SO2 was applied for 1 h. During the experiment, CNR and CF imaging were conduced. H2O and D2O in the plant container were exchanged every 30 min to observe water uptake. D2O tracer clearly showed water uptake into the hibiscus stem during each treatment. When the atmosphere was changed from simulated auto-exhaust fumes to normal air again, the amount of D2O and, hence, water uptake increased. CF imaging was well suited to evaluate the effects of SO2 as simulated toxic auto-exhaust fumes on plants. The maximum photochemical efficiency (Fv/Fm), a sensitive indicator of the efficacy and the integrity of plants’ photosynthesis, immediately dropped by 30% after supplying the simulated auto-exhaust fumes. This indicates that toxic auto-exhaust fumes negatively affected the photosynthetic activity of hibiscus leaves. Simultaneous CNR and CF imaging successfully visualized variations of photosynthetic activity and water uptake in the sample. Thus, this combination method was effective to non-destructive analyze the physiological status of plants.