Photosynthetic response mechanisms in typical C3 and C4 plants upon La2O3 nanoparticle exposure

Lanthanum oxide nanoparticles (La2O3 NPs) have raised enormous concerns due to the potential environmental risks they pose to agricultural production. Previous studies have demonstrated that the growth and photosynthesis of higher plants could be inhibited by La2O3 NPs. Due to the different photosynthetic pathways of C3 and C4 plants, their photosynthetic responses and defense mechanisms under NP exposure have not previously been investigated. To examine different regulatory strategies for photosynthesis in C3 and C4 plants, typical C3 (soybean) and C4 (maize) plants were selected in the present study, and their photosynthetic performance upon La2O3 NP exposure was compared. After 10 mg L-1 La2O3 NP exposure for 4 days, the net photosynthetic rate was reduced significantly by 8.77% and 55.52% in soybean and maize, respectively. Soybean's maximum quantum yield of PSII (Fv/Fm) was decreased by 10.53% and electron transport was blocked, resulting in low utilization efficiency of solar energy. Meanwhile for maize, the relative gene expressions for phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were downregulated, and the activity of PEPC and Rubisco were reduced by 19.08% and 7.16%, respectively. Thus, carbon fixation was restricted. Consequently, excess reactive oxygen species (ROS) were generated in the chloroplasts of maize. This is the first study focusing on differences in photosynthetic responses between C3 and C4 plants upon NP exposure, which provides useful knowledge for developing targeted regulation strategies to apply nano-agricultural technology in higher plants with different photosynthetic pathways.