Chloride (Cl^–) has traditionally been considered harmful to agriculture because of its toxic effects in saline soils and its antagonistic interaction with nitrate (NO₃^–), which impairs NO₃^– nutrition. It has been largely believed that Cl^– antagonizes NO₃^– 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 plant growth, the excessive NO₃^– fertilization frequently used in agriculture becomes a major environmental concern worldwide, causing excessive leaf NO₃^– accumulation in crops like vegetables and, consequently, a potential risk to human health. New farming practices aimed to enhance plant NUE by reducing NO₃^– fertilization should promote a healthier and more sustainable agriculture. Given the strong interaction between Cl^– and NO₃^– homeostasis in plants, we have verified if indeed Cl^– affects NO₃^– accumulation and NUE in plants. For the first time to our knowledge, we provide a direct demonstration which shows that Cl^–, contrary to impairing of NO₃^– nutrition, facilitates NO₃^– utilization and improves NUE in plants. This is largely due to Cl^– improvement of the N–NO₃^– utilization efficiency (NU_TE), having little or moderate effect on N–NO₃^– uptake efficiency (NU_PE) when NO₃^– is used as the sole N source. Clear positive correlations between leaf Cl^– content vs. NUE/NU_TE or plant growth have been established at both intra- and interspecies levels. Optimal NO₃^– vs. Cl^– ratios become a useful tool to increase crop yield and quality, agricultural sustainability and reducing the negative ecological impact of NO₃^– on the environment and on human health.

Chloride (Cl⁻), traditionally considered harmful for agriculture, has recently been defined as a beneficial macronutrient with specific roles that result in more efficient use of water (WUE), nitrogen (NUE), and CO₂ in well-watered plants. When supplied in a beneficial range of 1–5 mM, Cl⁻ increases leaf cell size, improves leaf osmoregulation, and reduces water consumption without impairing photosynthetic efficiency, resulting in overall higher WUE. Thus, adequate management of Cl⁻ nutrition arises as a potential strategy to increase the ability of plants to withstand water deficit. To study the relationship between Cl⁻ nutrition and drought resistance, tobacco plants treated with 0.5–5 mM Cl⁻ salts were subjected to sustained water deficit (WD; 60% field capacity) and water deprivation/rehydration treatments, in comparison with plants treated with equivalent concentrations of nitrate, sulfate, and phosphate salts. The results showed that Cl⁻ application reduced stress symptoms and improved plant growth during water deficit. Drought resistance promoted by Cl⁻ nutrition resulted from the simultaneous occurrence of water deficit avoidance and tolerance mechanisms, which improved leaf turgor, water balance, photosynthesis performance, and WUE. Thus, it is proposed that beneficial Cl⁻ levels increase the ability of crops to withstand drought, promoting a more sustainable and resilient agriculture.

Wild subspecies of Olea europaea constitute a source of genetic variability with huge potential for olive breeding to face global changes in Mediterranean-climate regions. We intend to identify wild olive genotypes with optimal adaptability to different environmental conditions to serve as a source of rootstocks and resistance genes for olive breeding. The SILVOLIVE collection includes 146 wild genotypes representative of the six O. europaea subspecies and early-generations hybrids. These genotypes came either from olive germplasm collections or from direct prospection in Spain, continental Africa and the Macaronesian archipelago. The collection was genotyped with plastid and nuclear markers, confirming the origin of the genotypes and their high genetic variability. Morphological and architectural parameters were quantified in 103 genotypes allowing the identification of three major groups of correlative traits including vigor, branching habits and the belowground-to-aboveground ratio. The occurrence of strong phenotypic variability in these traits within the germplasm collection has been shown. Furthermore, wild olive relatives are of great significance to be used as rootstocks for olive cultivation. Thus, as a proof of concept, different wild genotypes used as rootstocks were shown to regulate vigor parameters of the grafted cultivar “Picual” scion, which could improve the productivity of high-density hedgerow orchards.

The hydraulic traits of plants, or the efficiency of water transport throughout the plant hydraulic system, could help to anticipate the impact of climate change and improve crop productivity. However, the mechanisms explaining the role of hydraulic traits on plant photosynthesis and thus, plant growth and yield, are just beginning to emerge. We conducted an experiment to identify differences in growth patterns at leaf, root and whole plant level among four wild olive genotypes and to determine whether hydraulic traits may help to explain such differences through their effect on photosynthesis. We estimated the relative growth rate (RGR), and its components, leaf gas exchange and hydraulic traits both at the leaf and whole-plant level in the olive genotypes over a full year. Photosynthetic capacity parameters were also measured. We observed different responses to water stress in the RGRs of the genotypes studied being best explained by changes in the net CO₂ assimilation rate (NAR). Further, net photosynthesis, closely related to NAR, was mainly determined by hydraulic traits, both at leaf and whole-plant levels. This was mediated through the effects of hydraulic traits on stomatal conductance. We observed a decrease in leaf area: sapwood area and leaf area: root area ratios in water-stressed plants, which was more evident in the olive genotype Olea europaea subsp. guanchica (GUA8), whose RGR was less affected by water deficit than the other olive genotypes. In addition, at the leaf level, GUA8 water-stressed plants presented a better photosynthetic capacity due to a higher mesophyll conductance to CO₂ and a higher foliar N. We conclude that hydraulic allometry adjustments of whole plant and leaf physiological response were well coordinated, buffering the water stress experienced by GUA8 plants. In turn, this explained their higher relative growth rates compared to the rest of the genotypes under water-stress conditions.