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

Abstract

Background: Most nanoparticles (NPs) have a significant impact on the structure and function of the plant photosynthetic apparatus. However, their spectrum of action varies significantly, from beneficial stimulation to toxicity, depending on the type of NPs, the concentration used and plant genotypic diversity. Photosynthetic performance can be assessed through chlorophyll a fluorescence (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 reflectance performance, to evaluate photosynthesis sensitivity to stress stimuli.

Results: We investigated effects of different metal and metal(oid) oxide nanoparticles on photosynthesis of oakleaf lettuce seedlings by monitoring the chlorophyll a fluorescence light radiation and reflectance 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 9^th 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 9^th day. Leaf reflectance analysis showed an increase in FRI for SiO₂-NPs and CeO₂-NPs treatments and ARI2 for Fe₂O₃, however, WBI and PRI coefficients for the latter nanoparticle were lower than in control. Chlorophyll a fluorescence parameters have changed due to NPs treatment. Fe₂O₃-NPs caused an increase in F_v/F₀, PI_ABS, ET₀/RC, DI₀/RC, ABS/RC in different time points in comparison to control, also Ag, Au and SnO₂ treatment caused an increase in F_v/F₀, PI_ABS or ET₀/RC, respectively. On the other hand, TiO₂-NPs caused a decrease in F_v/F_m and F_v/F₀ parameters, but an increase in DI₀/RC value was observed. SnO₂-NPs decreased PI_ABS, but increased ET₀/RC than compared to control. Nanoparticles affected 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 reflectance values clearly proved the significant influence 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 significant 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 9^th day were comparable to the control curve.

Keywords: Lactuca sativa L. var. foliosa; Non-organic nanoparticles; O-J-I-P; Photosystem II; Reflectance.