Titanium dioxide nanoparticles impaired both photochemical and non-photochemical phases of photosynthesis in wheat.

Titanium dioxide nanoparticles (TiO2-NP) are increasingly being proposed for nanoagriculture but their effect on photosynthesis is limited and contradictory, mostly regarding putative chronical effects associated to the exposure to the commercial P25 formulation (anatase:rutile). This research aims at evaluating how chronical exposure to P25-NP affect photosynthetic processes in Triticum aestivum. Wheat plants were exposed (from the germination stage) to 0, 5, 50, and 150 mg L-1 P25-NP for 20 days. P25-NP impaired both light-dependent and -independent phases of photosynthesis, decreased chlorophyll a content, maximal and effective efficiency of PSII, net photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO2 concentration, and starch content. On the other hand, no effects were observed in photochemical and in non-photochemical quenching values, on total soluble sugar (TSS) content or in RuBisCO activity. Our results support that the induced decay in chlorophyll a content compromised the electron transport through PSII and that stomatal limitations impaired CO2 assimilation. The decline of starch content seems to be a consequence of its degradation as a mechanism to maintain the TSS levels. Consequently, we propose that photosynthetic related endpoints are sensitive and valuable biomarkers to assess TiO2-NP toxicity.