Silicon alleviates the impairments of iron toxicity on the rice photosynthetic performance via alterations in leaf diffusive conductance with minimal impacts on carbon metabolism

Iron (Fe) toxicity is often observed in lowland rice (Oryza sativa L.) plants, disrupting cell homeostasis and impairing growth and crop yields. Silicon (Si) can mitigate the effects of Fe excess on rice by decreasing tissue Fe concentrations, but no information exists whether Si could prevent the harmful effects of Fe toxicity on the photosynthesis and carbon metabolism. Two rice cultivars with contrasting abilities to tolerate Fe excess were hydroponically grown under two Fe levels (25 mu M or 5 mM) and amended or not with Si (0 or 2 mM). Fe toxicity caused decreases in net photosynthetic rate (A), particularly in the sensitive cultivar. These decreases were correlated with reductions in stomatal (g(s)) and mesophyll (g(m)) conductances, as well as with increasing photorespiration. Photochemical (e.g. electron transport rate) and biochemical (e.g., maximum RuBisCO carboxylation capacity and RuBisCO activity) parameters of photosynthesis, and activities of a range of carbon metabolism enzymes, were minimally, if at all, affected by the treatments. Si attenuated the decreases in A by presumably reducing the Fe content. In fact, A as well as g(s) and g(m), correlated significantly with leaf Fe contents. In summary, our data suggest a remarkable metabolic homeostasis under Fe toxicity, and that Si attenuated the impairments of Fe excess on the photosynthetic apparatus by affecting the leaf diffusive conductance with minimal impacts on carbon metabolism.