Leaf chlorophyll fluorescence and reflectance of oakleaf lettuce exposed to metal and metal(oid) oxide nanoparticles

Background Most nanoparticles (NPs) have a signifcant impact on the structure and function of the plant photosynthetic apparatus. However, their spectrum of action varies signifcantly, from benefcial stimulation to toxicity, depending on the type of NPs, the concentration used and plant genotypic diversity. Photosynthetic performance can be assessed through chlorophyll a fuorescence (ChlF) measurements. These data allow to indirectly obtain detailed information about primary light reactions, thylakoid electron transport reactions, dark enzymatic stroma reactions, slow regulatory processes, processes at the pigment level. It makes possible, together with leaf refectance performance, to evaluate photosynthesis sensitivity to stress stimuli. Results We investigated efects of diferent metal and metal(oid) oxide nanoparticles on photosynthesis of oakleaf lettuce seedlings by monitoring the chlorophyll a fuorescence light radiation and refectance from the leaves. Observations of ChlF parameters and changes in leaf morphology were carried out for 9 days in two-day intervals. Spectrophotometric studies were performed at 9th day. Suspensions of NPs with the following concentrations were used: 6% TiO2, SiO2; 3% CeO2, SnO2, Fe2O3; 0.004% (40 ppm) Ag; 0.002% (20 ppm) Au. Nanoparticles were applied directly on the leaves which caused small symptoms of chlorosis, necrosis and leaf veins deformation, but the plants fully recovered to the initial morphological state at 9th day. Leaf refectance analysis showed an increase in FRI for SiO2-NPs and CeO2-NPs treatments and ARI2 for Fe2O3, however, WBI and PRI coefcients for the latter nanoparticle were lower than in control. Chlorophyll a fuorescence parameters have changed due to NPs treatment. Fe2O3-NPs caused an increase in Fv/F0, PIABS, ET0/RC, DI0/RC, ABS/RC in diferent time points in comparison to control, also Ag, Au and SnO2 treatment caused an increase in Fv/F0, PIABS or ET0/RC, respectively. On the other hand, TiO2-NPs caused a decrease in Fv/Fm and Fv/F0 parameters, but an increase in DI0/RC value was observed. SnO2-NPs decreased PIABS, but increased ET0/RC than compared to control. Nanoparticles afected the shape of the O-J-I-P curve in slight manner, however, further analyses showed unfavourable changes within the PSII antenna, manifested by a slowdown in the transport of electrons between the Chl molecules of the light-harvesting complex II and the active center of PSII due to NPs application. Conclusion Changes in ChlF parameters and leaf refectance values clearly proved the signifcant infuence of NPs on the functioning of the photosynthetic apparatus, especially right after NPs application. The nature of these changes was strictly depended on the type of nanoparticles and sometimes underwent very signifcant changes over time. The greatest changes in ChlF parameters were caused by Fe2O3 nanoparticles, followed by TiO2-NPs. After slight response of O-J-I-P curves to treatment of the plants with NPs the course of the light phase of photosynthesis stabilized and at 9th day were comparable to the control curve.