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^–) is a micronutrient that accumulates to macronutrient levels since it is normally available in nature and actively taken up by higher plants. Besides a role as an unspecific cell osmoticum, no clear biological roles have been explicitly associated with Cl^– when accumulated to macronutrient concentrations. To address this question, the glycophyte tobacco (Nicotiana tabacum L. var. Habana) has been treated with a basal nutrient solution supplemented with one of three salt combinations containing the same cationic balance: Cl^–-based (CL), nitrate-based (N), and sulphate+phosphate-based (SP) treatments. Under non-saline conditions (up to 5mM Cl^–) and no water limitation, Cl^– specifically stimulated higher leaf cell size and led to a moderate increase of plant fresh and dry biomass mainly due to higher shoot expansion. When applied in the 1–5mM range, Cl^– played specific roles in regulating leaf osmotic potential and turgor, allowing plants to improve leaf water balance parameters. In addition, Cl^– also altered water relations at the whole-plant level through reduction of plant transpiration. This was a consequence of a lower stomatal conductance, which resulted in lower water loss and greater photosynthetic and integrated water-use efficiency. In contrast to Cl^–, these effects were not observed for essential anionic macronutrients such as nitrate, sulphate, and phosphate. We propose that the abundant uptake and accumulation of Cl^– responds to adaptive functions improving water homeostasis in higher plants.

Páginas.-- 6 figuras.-- 2 tablas.-- 56 referencias.-- Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.catena.2016.07.037.Three biochars (B1: pine wood, B2: paper-sludge, B3: sewage-sludge) produced under controlled pyrolysis conditions and one produced in kilns (B4: grapevine wood) were used as organic ameliorants in a Calcic Cambisol, which represents a typical agricultural soil of the Mediterranean region. This field study was performed with plants of sunflower (Helianthus annuus L.) at the experimental station “La Hampa”, located in the Guadalquivir river valley (SW Spain). The soil was amended with doses equivalent to 1.5 and 15 t ha− 1 of the four biochars in two independent plantations. In addition, un-amended plots were prepared in both experiments for comparison purposes. The major goal of this study was the assessment of the effect of biochar amendment on the physiology and development of sunflower plants at field conditions. During most of the growing period plants of un-amended and amended plots showed no stress symptoms either by their appearance or by stress-sensitive biochemical parameters such as the stability of the photosystem II (QY). Biochar addition had no effect on seed germination. Addition of 1.5 t ha− 1 biochar did not significantly change the pH of the soil, its electrical conductivity (EC) or its water holding capacity (WHC). Concomitantly the plant development and plant biomass production remained unaltered. Amendments with 15 t ha− 1 slightly increased the WHC of the soil but showed no lasting impact on the soil pH. It stimulated plant growth and led to a greater leaf area, larger plant stems and wider inflorescences of the sunflower plants than those cultivated on the un-amended soil. At the end of the experiment, biochar amendment of soil caused no significant increase of the total biomass production excepting B4, the biochar with the lowest capacity of water retention, which exhibited the highest vegetative growth and seed production. The lack of rain during the last weeks caused a water shortage in the culture that produced greater QY loss in non-amended plants. Interestingly, better growth of amended plants during the drought period correlated with higher reduction of stomatal conductance, indicating that the greater water use efficiency is at the origin of the better crop performance of biochar-amended plants. This finding points to the agronomic relevance of biochar amendment of Mediterranean rain fed crops.The Marie Skłodowska-Curie actions of the European Union's FP7 People Programme (REA grant agreement no PCIG12-GA-2012-333784-Biocharisma project) and the Spanish Ministry of Economy and Competitiveness (MINECO) (project PCGL2012-37041) are thanked for the financial support of the present study. The Spanish Ministry of Education, Culture and Sport is acknowledged for funding the FPU fellowship (FPU 13/05831) of Marina Paneque, and the contract of Juan De Dios Franco-Navarro (AGL2009-08339/AGR). The European Biochar Network (Biochar as option for sustainable resource management-COST action TD1107) and Bodegas Torres (Spain) are acknowledged for providing the biochar samples. M. Velasco is gratefully acknowledged for her technical assistance at field.Peer reviewe

Drought stress, which is one of the most critical environmental constraints affecting global crop productivity, is exacerbated by climate change and increased atmospheric water demand. This review comprehensively examines plant responses to drought, integrating physiological, morphological, biochemical, and genetic adaptations that contribute to water‐use efficiency and stress tolerance. Key mechanisms such as osmotic adjustment, stomatal regulation, antioxidant defense, and hormonal signaling are analyzed, highlighting their role in mitigating drought‐induced cellular damage. Advances in plant breeding and biotechnological approaches, including transgenic strategies, genome editing, and marker‐assisted selection, are discussed in the context of improving drought resilience. The importance of root system architecture, leaf anatomical modifications, and stress‐responsive transcription factors is underscored as essential components of drought adaptation. Additionally, agronomic innovations such as precision irrigation, soil management techniques, and plant‐microbe interactions are reviewed due to their potential to enhance sustainable water use in agriculture. The role of epigenetic modifications and long‐distance signaling mechanisms in drought acclimation is explored, shedding light on emerging strategies for engineering multi‐stress tolerant crops. Furthermore, we assess the impact of drought on crop nutritional quality, the trade‐offs between drought tolerance and pest resistance, and the socio‐economic implications of water scarcity on global food security. This review provides a roadmap for integrating cutting‐edge scientific knowledge with practical agricultural applications, aiming to develop resilient cropping systems capable of sustaining productivity under increasingly unpredictable climatic conditions.

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.