Photosynthetic and Antioxidant Responses of Lolium perenne to Aluminum Oxide Nanoparticles, Their Supernatant, and Aluminum Chloride Under Acidic Conditions

Aluminum oxide nanoparticles (Al2O3 NPs) are frequently incorporated into fertilizers and pesticides. Nevertheless, the effects of Al2O3 NPs on plants growing in acidic soils with elevated aluminum toxicity remain poorly understood. In this context, we evaluated the effects of aluminum oxide nanoparticles (Al2O3 NPs) on photosynthetic activity, gene expression, and antioxidant defense mechanisms in the Lolium perenne genotype Halo-AR1 cultivated in nutrient solution under acidic conditions. Thus, plants were subjected to Al2O3 NPs, Al2O3 NP supernatant, and AlCl3 treatments. Photosynthetic performance was significantly impaired in all treatments, as reflected by reduced CO2 assimilation, stomatal conductance, effective PSII quantum yield of PSII (ΦPSII), and electron transport rate (ETR), with stronger effects in the Al2O3 NP supernatant and AlCl3 treatments. Furthermore, Al2O3 NPs and AlCl3 downregulated the expression of the photosynthesis-related genes psbA and rbcS after 48 h of exposure. Antioxidant activity, total phenols, and flavonoid contents increased in leaves and roots, indicating activation of antioxidant defenses, particularly under Al2O3 NPs and Al2O3 NP supernatant exposure. Overall, the Al2O3 NP supernatant and AlCl3 treatments produced more pronounced effects on photosynthetic performance than the Al2O3 NP suspension under the tested conditions. However, the contributions of particulate and dissolved Al to these responses remain unresolved.