A Century‐Long Journey of Boron Research in Plant Science

In the early 20th century, plant nutrition was dominated by the soil chemistry paradigm of nitrogen, phosphorus, and potassium. Micronutrients were largely ignored and poorly understood, partly because existing methods did not allow precise adjustment of micronutrient levels in plant growth media. The importance of most of today's known essential micronutrients, including boron (B), was largely overlooked, even though abnormal crop growth on certain soils could be “rescued” by adding B. In 1923, a century ago, the situation changed when Katherine Warington showed that certain vascular plants, such as pea, cannot reproduce without B, and that different plant species had different quantitative B demands (Warington 1923). Boron was then added to the list of essential mineral elements for plants, marking the beginning of a new chapter in plant nutrition research. In the first decades (1920–1950s), B research was pragmatic and descriptive. Agronomists documented B deficiency symptoms in many crops and fruit trees, including cracked and hollow stems, curled and lanceolated leaves, deformed vascular tissue in stems, leaves, and roots, deformed and withered flowers, poor fruit set, and reduced seed formation. In many seed plants, insufficient B supply caused reduced shoot and root growth and lower fertility. Critical B concentrations in soil or aboveground tissues at which development is impaired and deficiency symptoms appear were determined for various plants. On the basis of these in terra and in planta data, B requirements were defined and plants classified as low-B-demanding (mainly graminaceous monocots) or high-B-demanding (non-graminaceous monocots and eudicots). Consistent with Warington (1923), large variation in both critical B concentrations and B toxicity thresholds was found among different species or even cultivars. It was shown that bioavailable B levels in soils determine plant health, and the effectiveness of different B fertilizer forms and soil and foliar B fertilization management strategies were tested to improve yields. In the following decades (1960–1980s), as understanding of plant physiology grew and new experimental and analytical methods emerged, B research rapidly expanded beyond agronomy and focused on B's essential role in plants and the cause of specific B deficiency symptoms. Boron was shown to be required for cell division and elongation, especially in root and shoot meristems and newly formed tissues, and for pollen germination and tube growth. Controlled experiments distinguished B's role from other micronutrients, reinforcing its status as essential. Boron deficiency was linked to altered cell differentiation and disrupted cell wall structure, particularly the integrity and physicochemical properties of the pectin-rich primary cell walls of growing tissues. By the late 20th century, B's essentiality and many physiological consequences of its deficiency had been described, but its biochemical functions remained unknown. The molecular era (1990–2010s) further transformed B research from descriptive agronomy to fundamental biology, revealing molecular aspects of B function and fine-regulation of plant B status. Breakthroughs in cell wall chemistry showed that B, in the form of boric acid, forms di-ester cross-links with rhamnogalacturonan II (RG-II) in primary cell walls, stabilizing and flexibilizing the pectic network. A positive correlation between RG-II abundance in cell walls and tissue B concentrations across species suggested that the amount of RG-II quantitatively influences plant B requirements. Advances in molecular biology and genetics also greatly improved the understanding of B transporters, membrane proteins essential for B uptake and translocation within plants. Since the 2010s, B research has become integrated into many areas of plant science. Genomics and transcriptomics have mapped gene expression responses to B deficiency and toxicity. In the context of a changing climate, emerging work focuses on interactions of B with abiotic stresses such as drought, salinity, and nutrient imbalances, as well as on B efficiency mechanisms and underlying genes that enhance abiotic stress resilience and pathogen defense. Agricultural research continues to refine precision fertilization, tailoring B management and fertilizer forms to crops, soil types, and weather, and systems biology reveals effects of B on translational processes, providing evidence for cell-wall-independent B functions. This Special Issue entitled “Boron celebrates its 100th birthday as an essential element in plant nutrition” comprises nine articles and one review covering various aspects of exactly these research areas: Boron deficiency is a major constraint for crop production. Soil B availability is influenced by adsorption to short-range-order minerals and soil organic matter and by interactions with other nutrients. Fallas-Corrales et al. (2025) analyzed B adsorption and surface speciation in a volcanic soil from the humid tropics, where B availability is very low. They show that short-range-order minerals are key B-binding surfaces, phosphate competition is limited, and a mechanistic multisurface geochemical model insufficiently describes soil-B interactions. Their findings will contribute to optimize B management in volcanic tropical soils. The depletion of nutrients in agricultural soils threatens food security and ecosystem health. Although macronutrient depletion is well studied, the magnitude and drivers of B depletion are not. Eyherabide et al. (2024) compared extractable B in pristine soils (Mollisols) with cultivated soils from a temperate region to estimate annual B depletion rates and identify edaphic, productive, and/or climatic controlling factors. They showed that current depletion rates from agricultural Mollisols may soon compromise B availability, depending on regional edaphoclimatic and productive differences. The study calls for urgent measures to halt or reverse soil degradation. Soil-based micronutrient fertilization recommendations, including B, can be improved by models that integrate soil properties associated with the micronutrient dynamics. The extraction of B with hot water (HW-B) is the most commonly used method to estimate available B, but field calibrations of HW-B for maize and critical thresholds for Mollisols are lacking. Barbieri et al. (2024) quantified maize grain yield responses to B fertilization under field conditions, calibrated HW-B as a predictor of this response, and evaluated clay, pH, and soil organic matter as additional predictors. The identified critical B threshold for maize will improve identification of B-deficient soils. Low B availability is typical for strongly weathered, coarse-textured, low organic matter soils, such as many occuring in sub-Saharan Africa. The yield benefit of B fertilization on such soils is unclear. Breure et al. (2024) conducted B omission trials with maize at 15 sites in Kenya, Zambia, and Zimbabwe to identify soil properties controlling B availability. Although B fertilization increased B uptake at 11 sites, it did surprisingly not increase yields at any site and even reduced yield at five sites, likely due to toxicity. Improving soil organic matter may alleviate B deficiency in maize, and the authors provide recommendations to overcome the identified challenges for studying B availability in tropical soils. Soybean (Glycine max), one of the major protein crops worldwide, suffers from B deficiency, but the predictive value of soil or plant tissue tests for yield response to B fertilization is poorly known. Bustos et al. (2025) analyzed field data from 30 site-years (2004–2022) to evaluate soil and tissue tests as predictors of seed yield response. Soil tests using the CaCl2-B or the NH4Ac-B methods and derived critical thresholds reliably predicted yield response, whereas early and late-stage leaf or seed tissue B concentrations did not. This study recommends soil testing for predicting seed yield response to B fertilization in soybean. Oilseed rape (Brassica napus) is highly susceptible to B deficiency; thus, studies dealing with optimum B supply to oilseed rape fields have a long history. Increasing frequencies of water limitation reduce B availability, calling for fertilization strategies that consider the release speed of different B fertilizers throughout the season, especially during flowering, and that are robust to variable water supply. Tölle et al. (2025) compared different B fertilizer species under single and combined B-deficiency and water-limitation stresses in oilseed rape. Borax sustained fertility better than boric acid, colemanite, ulexite, or zinc borate at suboptimal B supply in a peat-based substrate under both well-watered and water-limited conditions. However, soil water availability remained the dominant factor for sufficient B supply, independent of the applied B species. Eudicot and monocot species differ in their B demand and in shoot responses to B deficiency. Although eudicots often show strong root architectural changes, responses of root systems of low-B-demanding monocots are less known. Bienert et al. (2023) analyzed root responses to B deficiency in maize and a root-hairless mutant. Atypical for plants, maize showed neither reduced root growth nor compensatory foraging behavior under severe B deficiency. The root-hairless mutant had lower shoot B concentrations under both deficient and surplus B but did not compensate by increased root length, fine roots, or higher expression of B transporters. To maintain appropriate B levels, plants require B-sensing mechanisms that fine-regulate B transport. Takano and Tanaka (2023) reviewed recent work on B sensing and its role in B transport regulation, highlighting mechanisms of ubiquitination, degradation, translation suppression, and mRNA degradation of B transporters under high B conditions. They argue that understanding B sensing in crops will clarify genotypic differences in tolerance to B limitation and excess and support breeding for improved crop performance under changing B availability. The authors further propose future directions for research and pinpoint research gaps. Knowledge of B transport is crucial for effective foliar and soil B applications. Although benefits of B fertilization in fruit trees are known, data on B distribution in sweet cherries are lacking. Arredondo et al. (2024) traced 10B applied either to flowers or soil under different timings and soil B levels. Boron applied to flowers at full bloom largely remained in the fruit, making floral application an effective complementary strategy to meet fruit demand regardless of the soil B status. Plant responses to B deficiency are diverse and likely extend beyond transporter regulation and the B function in cell wall crosslinking, and interactions of B with phytohormones have been proposed. Matthes et al. (2023) used the boron-deficient tassel-less1 mutant and the auxin-deficient vanishing tassel2 mutant to study interactions between B status, auxin biosynthesis, and auxin transport in leaves and shoot meristems. They demonstrate a link between polar auxin transport and B-deficiency phenotypes and show that B deficiency-induced developmental defects are sensitive to low auxin levels. Collectively, the papers in this Special Issue advance our understanding of B in plant nutrition and soil science. Continued progress will be essential to address soil degradation, maintain plant productivity and yield stability, and safeguard food security. The authors declare no conflicts of interest. The editorial contains no experimental data which can be made available.