Silicon nutrition mitigates the negative impacts of iron toxicity on rice photosynthesis and grain yield.

Excess iron (Fe) is commonly observed in wetland rice (Oryza sativa L.) plants, impairing crop growth and productivity. Some information suggests that silicon (Si) can reduce Fe content in leaves and roots of rice (vegetative phase), but nothing is known if Si could mitigate the effects of Fe toxicity on rice production and photosynthesis. Here, we assessed the role of Si in alleviating the well-known effects of Fe toxicity on nutritional imbalances, biomass accumulation, photosynthesis and grain yield using two rice cultivars having differential abilities to tolerate excess Fe. Plants were hydroponically grown under two Fe levels (25 µM or 5 mM) and the nutrient solutions were amended with Si (0 or 2 mM). Under excess Fe were detected (i) nutritional deficiencies, especially of calcium and magnesium in leaves; (ii) negligible changes in grain nutritional composition, independently of Si application; (iii) decreases in net photosynthetic rates, stomatal conductance and electron transport rate, in parallel to decreased grain yield components (total grain biomass, 1000-grain mass, percentage of filled grains, number of grains per plant and harvest index), especially in the Fe-sensitive cultivar. These impairments were partially reversed by the application of Si. Results also suggest that Si alleviated the negative impacts of Fe on spikelet sterility. In summary, we conclude that the use of Si can be recommended as an effective management strategy to reduce the negative impacts of Fe toxicity on rice photosynthetic performance and crop yield.

The interplay between irrigation and fruiting on branch growth and mortality, gas exchange and water relations of coffee trees.

The overall coordination between gas exchanges and plant hydraulics may be affected by soil water availability and source-to-sink relationships. Here we evaluated how branch growth and mortality, leaf gas exchange and metabolism are affected in coffee (Coffea arabica L.) trees by drought and fruiting. Field-grown plants were irrigated or not, and maintained with full or no fruit load. Under mild water deficit, irrigation per se did not significantly impact growth but markedly reduced branch mortality in fruiting trees, despite similar leaf assimilate pools and water status. Fruiting increased net photosynthetic rate in parallel with an enhanced stomatal conductance, particularly in irrigated plants. Mesophyll conductance and maximum RuBisCO carboxylation rate remained unchanged across treatments. The increased stomatal conductance in fruiting trees over nonfruiting ones was unrelated to internal CO2 concentration, foliar abscisic acid (ABA) levels or differential ABA sensitivity. However, stomatal conductance was associated with higher stomatal density, lower stomatal sensitivity to vapor pressure deficit, and higher leaf hydraulic conductance and capacitance. Increased leaf transpiration rate in fruiting trees was supported by coordinated alterations in plant hydraulics, which explained the maintenance of plant water status. Finally, by preventing branch mortality, irrigation can mitigate biennial production fluctuations and improve the sustainability of coffee plantations.

Elevated [CO2] benefits coffee growth and photosynthetic performance regardless of light availability.

Despite being evolved in shaded environments, most coffee (Coffea arabica L.) is cultivated worldwide under sparse shade or at full sunlight. Coffee is ranked as greatly responsive to climate change (CC), and shading has been considered an important management strategy for mitigating the harmful CC outcomes on the crop. However, there is no information on the effects of enhanced [CO2] (eCa) on coffee performance in response to light availability. Here, we examined how carbon assimilation and use are affected by eCa in combination with contrasting light levels. For that, greenhouse-grown plants were submitted to varying light levels (16 or 7.5 mol photons m-2 day-1) and [CO2] (ca. 380 or 740 µmol mol-1 air) over six months. We demonstrated that both high light and eCa improved growth and photosynthetic performance, independently. Despite marginal alterations in biomass partitioning, some allometric changes, such as higher root biomass-to-total leaf area and lower leaf area ratio under the combination of eCa and high light were found. Stimulation of photosynthetic rates by eCa occurred with no direct effect on stomatal and mesophyll conductances, and no signs of photosynthetic down-regulation were found irrespective of treatments. Particularly at high light, eCa led to decreases in both photorespiration rates and oxidative pressure. Overall, our novel findings suggest that eCa could tandemly act with shading to mitigate the harmful CC effects on coffee sustainability.

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 µM 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 (gs) and mesophyll (gm) 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 gs and gm, 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.