Chloride (Cl−) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO3−), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl− has gone from being considered a harmful ion, accidentally absorbed through NO3− transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl− determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO2, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl− and NO3− molecules, the NO3−/Cl− plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl− influx (ZmNPF6.4 and ZmNPF6.6), Cl− efflux (AtSLAH3 and AtSLAH1), and Cl− compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl−, as well as for cell turgor regulation and stress tolerance in plants.

Chloride (Cl − ) has traditionally been considered harmful to agriculture because of its toxic effects in saline soils and its antagonistic interaction with nitrate (NO 3 − ), which impairs NO 3 − nutrition. It has been largely believed that Cl − antagonizes NO 3 − uptake and accumulation in higher plants, reducing crop yield. However, we have recently uncovered that Cl − has new beneficial macronutrient functions that improve plant growth, tissue water balance, plant water relations, photosynthetic performance, and water-use efficiency. The increased plant biomass indicates in turn that Cl − may also improve nitrogen use efficiency (NUE). Considering that N availability is a bottleneck for the growth of land plants excessive NO 3 − fertilization frequently used in agriculture becomes a major environmental concern worldwide, causing excessive leaf NO 3 − accumulation in crops such as vegetables, which poses a potential risk to human health. New farming practices aimed to enhance plant NUE by reducing NO 3 − fertilization should promote a healthier and more sustainable agriculture. Given the strong interaction between Cl − and NO 3 − homeostasis in plants, we have verified if indeed Cl − affects NO 3 − accumulation and NUE in plants. For the first time to our knowledge, we provide a direct demonstration which shows that Cl − , contrary to impairing NO 3 − nutrition, facilitates NO 3 − utilization and improves NUE in plants. This is largely due to Cl − improvement of the N-NO 3 − utilization efficiency (NU T E), having little or moderate effect on N-NO 3 − uptake efficiency (NU P E) when NO 3 − is used as the sole N source. Clear positive correlations between leaf Cl − content vs. NUE/NU T E or plant growth have been established at both intra-and interspecies levels. Optimal NO 3 − vs. Cl − ratios become a useful tool to increase crop yield and quality, agricultural sustainability and to reduce the negative ecological impact of NO 3 − on the environment and on human health.

Abstract Sea level rise (SLR) imposes increasing salinity and inundation stresses in salt marshes which simultaneously face invasions by exotic plant species. We aimed to improve and apply knowledge on the ecophysiological responses of halophytes to SLR to conservation management of salt marshes. In a mesocosm experiment, we measured and compared phosphoenolpyruvate carboxylase (PEPC) activity and related functional traits of the California-native Spartina foliosa, invasive S. densiflora and their hybrid S. densiflora × foliosa in response to increasing levels of salinity and inundation. S. foliosa was moderately sensitive to salinity, showing a 57% reduction in PEPC specific activity from freshwater to hypersalinity. This native species compensated for the reduction of PEPC activity with increased salinity through 80% higher enzyme activation by phosphorylation. PEPC functional trait responses of S. foliosa were mostly independent of inundation depth. In view of these results, managers should conserve undeveloped lands for accommodation space above current high tide lines to facilitate colonization of stress-tolerant S. foliosa. Our results on functional responses of PEPC traits recorded high sensitivity to salinity for S. densiflora. This was reflected by 65% lower PEPC specific activity together with increasing accumulation of free proline (+96%) and total proteins (+23%) with elevated salinity. These results suggest prioritized eradication of S. densiflora populations in brackish habitats. Measured PEPC responses support the high stress tolerance of the S. densiflora × foliosa hybrid. PEPC traits for the hybrid were mostly independent of salinity and inundation. The hybrid showed higher PEPC-specific activity than S. foliosa (+70%) and S. densiflora (+15%) in freshwater under intermediate inundation. Results suggest that eradication of the hybrid should be the highest management priority. Our study shows that the responses of key functional physiological traits to environmental stresses serve as biological indicators that can guide ecosystem management practices in a scenario of climate change.