Understanding photosynthetic and metabolic adjustments in iron hyperaccumulators grass

Setaria parviflora and Paspalum urvillei are iron hyperaccumulating species. In order to understand how they are iron resistant, this study evaluated the adjustments in photosynthetic apparatus, metabolic pathways, and antioxidant response after exposure to iron excess. Two assays were carried out with different purposes. S. parviflora and P. urvillei were grown in Hoagland & Arnon nutrient solution containing 0.009, 1, 2, 4 and 7 mM Fe-EDTA to evaluate which Fe concentration causes accumulation of this element above the toxicity threshold. In addition, leaf samples were collected from plants growing in 0.009 and 7 mM iron-EDTA solution on the 6°, 10°, 14° and 18° day of growth in order to evaluate the adjustments in terms of primary metabolite concentrations and the antioxidant capacity. Only the 7 mM iron-EDTA treatment caused a toxic iron accumulation in the shoot of both species. P. urvillei had decreased photosynthetic rate only upon exposure to 7 mM iron-EDTA, in addition, adjustments in the electron transport rate through photosystem II and oxidation state of quinones were observed. Meanwhile, S. parviflora presented a gradual reduction in photosynthetic rate, although with higher non-photochemical quenching of fluorescence values. P. urvillei showed inhibition of tricarboxylic acid cycle. Based on the activity of antioxidant enzymes, S. parviflora showed higher ascorbate peroxidase, peroxidase values, and lipid peroxidation. We additionally identified that malate can also function to neutralize the free-reactive Fe and may contribute to the tolerance mechanism in P. urvillei. Therefore, these species have the ability to accumulate high concentrations of iron within a few days using effective and different adjustments in photosynthetic, antioxidant and carbon metabolism to cope with iron excess.