Phosphorus (P) use in agriculture remains inefficient and poses long-term environmental concerns. We investigate the uptake, redistribution, and efficacy of foliar-applied nano-hydroxyapatite (nHAp) in P-deficient barley (Hordeum vulgare), aiming to clarify its mode of action and evaluate its potential as a practical alternative to conventional phosphate salts. Using a fast and facile wet synthesis, we produced chemically-labeled nHAp with uniform elongated morphology (median length and width of 33.6 and 5.3 nm, respectively), excellent colloidal stability, pH-dependent solubility, and full redispersibility, suitable for highly concentrated foliar formulations. Infiltration experiments reveal that nHAp dissolves gradually in situ, with peak phosphate release between 1-3 days post-treatment, enabling local and systemic P recovery without inducing leaf scorching, unlike conventional P. Bioimaging reveals that nHAp, applied as droplets to the leaf surface, penetrates via stomata and diffuses through the apoplast, where it dissolves and releases P without entering mesophyll cells. We screened three commercial surfactants and examined how surfactant type, surfactant concentration, and NP concentration affect the surface tension of nHAp formulations. Foliar uptake efficiency increased as the formulation surface tension decreased, with the strongest effect under dry conditions. Notably, an inverse relation was observed, with high humidity improving uptake at high surface tension, and low humidity improving uptake at low surface tension. Increasing NP concentration markedly reduced uptake efficiency in barley but not in potato (Solanum tuberosum), which consistently showed higher uptake, highlighting strong species-dependent differences in leaf traits governing NP uptake. Our findings confirm the potential of nHAp as a viable slow-release foliar P source, with high uptake in potato enabling single applications, while barley requires repeated treatments due to lower efficiency.

Phosphorus fertilization remains inefficient in many cropping systems, motivating the search for alternative delivery methods that minimize losses and boost plant uptake. Nano-sized hydroxyapatite particles have become a promising phosphorus source, though their effectiveness heavily relies on particle design. In this study, we synthesized nano-sized hydroxyapatite by hydrothermal method with controlled crystal growth and physicochemical properties to better suit foliar applications. We systematically examined how synthesis parameters affect particle size, morphology, and crystal growth to identify an optimal suspension formulation for foliar fertilization. Foliar application trials of engineered nano-sized hydroxyapatite on potato plants showed successful uptake. These findings indicate that controlling the crystal growth of nano-sized hydroxyapatite could provide a basis for developing advanced nanofertilizers with improved nutrient efficiency.

Potato (Solanum tuberosum L.) has a high phosphorus (P) requirement, yet its shallow root system and the strong P-fixing capacity of many soils limit the effectiveness of soil fertilisation. Foliar application of nano-hydroxyapatite particles (nHAPs) has emerged as a promising alternative P fertilisation practice. In this study, we compared foliar uptake mechanisms of nHAPs and orthophosphate ions (Pi) in potato using advanced bioimaging techniques and quantified their P uptake efficiency (PUE) with ³³P radioisotopes in controlled-environment and field experiments. Imaging revealed distinct uptake pathways: Pi penetrated directly through the cuticle, whereas nHAPs primarily entered through hydraulically activated stomata. Foliar application of nHAPs achieved a PUE of approximately 50% after 7 days - far exceeding global averages for soil-applied P fertilisers - while avoiding the leaf scorching commonly associated with Pi sprays. Pi uptake was insensitive to leaf surface polarity but increased when the surface tension of the foliar spray was low. In contrast, nHAP uptake was greater on adaxial leaf surfaces and favoured by higher surface tension. These findings demonstrate that aligning foliar nano particle formulations with their respective uptake pathways, spray properties, and leaf traits can substantially enhance nano-particle uptake efficacy. This work provides new mechanistic insight to support more efficient and sustainable P management practices in potato production and broader agricultural systems.

Definitions of plant nutrient elements, historically anchored in the essentiality criteria of Arnon and Stout, have provided a robust basis for identifying essential mineral nutrients. Yet, they are increasingly challenged by evidence for context-dependent benefits of additional elements and by regulatory demands that tie fertiliser constituents to officially recognized ‘nutrients’. Recent proposals to expand the nutrient concept to include beneficial elements and nutrient effects on quality attributes of harvested products have intensified this debate. This opinion paper evaluates such proposed revisions from a plant-centred perspective. We argue that, while inclusion of elements that improve plant performance under defined conditions is justified, incorporating the broadly used and target-group-dependent concept of ‘quality’ introduces ambiguity and may foster misleading claims, including alleged ‘quality improvements’ driven by stress responses or concentration effects. To improve precision, we propose the term ‘conditionally beneficial elements’ for elements that enhance growth, development or reproduction only in specific species and/or under specified growth conditions. Building on classical essentiality while acknowledging practical experimental constraints, we advance an updated definition for vascular plants, namely: A plant nutrient element, considering natural or managed environments, is essential if deprivation impairs growth, development, or reproduction, or conditionally beneficial if it improves these performance traits in a given species under specified conditions. In both cases, the element must have an established performance-improving role in plant biology. The essential mineral elements comprise the macronutrients N, P, S, K, Ca, and Mg and the micronutrients Fe, Mn, Zn, Cu, Ni, B, Cl, and Mo, whereas Na, Si, Co, Al, and Se are currently classified as conditionally beneficial elements. We argue that this definition supports conceptual clarity in research as well as recognition of nutrient elements in fertiliser legislation, while protecting against unfounded claims.

[This corrects the article DOI: 10.3389/fpls.2025.1610402.].

Efficient foliar delivery of manganese (Mn) remains a major challenge in crop nutrition due to the low phloem mobility of Mn2 + ions and the risk of leaf scorching from conventional Mn salts. This study investigates the uptake and assimilation of foliar-applied ∼25 nm polyacrylic acid-coated manganese dioxide nanoparticles (nPAA-MnO2) in 4-week-old Mn-deficient barley (Hordeum vulgare) and compares them to ionic Mn. Complementary imaging methods showed rapid (2 h) leaf penetration of nPAA-MnO2 through stomata, facilitated by the formulation´s low surface tension and the addition of 3% glycerol as a humectant. Although ionic Mn displayed higher uptake efficiency than nPAA MnO2 (∼90% vs ∼11%), the latter restored Mn-dependent photosynthetic functionality without inducing phytotoxicity, even at high Mn concentrations (4 g L-1). Labeling experiments with tracer ions revealed nPAA-MnO2 hotspots around vascular bundles and a small (1.9%), yet significant, basipetal translocation of Mn out of the foliar application zone after 4 days, compared to only 0.1% for ionic Mn. By delineating the distinct uptake pathways and metabolic fate of nPAA-MnO2 versus ionic Mn, these findings provide the mechanistic foundation for developing a novel generation of nano-enabled strategies to address Mn and other micronutrient deficiencies in crops.

Potato (Solanum tuberosum L.) has a high phosphorus (P) requirement, yet its shallow root system and the strong P-fixing capacity of many soils limit the effectiveness of soil fertilisation. Foliar application of nano-hydroxyapatite particles (nHAPs) has emerged as a promising alternative P fertilisation practice. In this study, we compared foliar uptake mechanisms of nHAPs and orthophosphate ions (Pi) in potato using advanced bioimaging techniques and quantified their P uptake efficiency (PUE) with & sup3;& sup3;P radioisotopes in controlled-environment and field experiments. Imaging revealed distinct uptake pathways: Pi penetrated directly through the cuticle, whereas nHAPs primarily entered through hydraulically activated stomata. Foliar application of nHAPs achieved a PUE of approximately 50% after 7 days - far exceeding global averages for soil-applied P fertilisers - while avoiding the leaf scorching commonly associated with Pi sprays. Pi uptake was insensitive to leaf surface polarity but increased when the surface tension of the foliar spray was low. In contrast, nHAP uptake was greater on adaxial leaf surfaces and favoured by higher surface tension. These findings demonstrate that aligning foliar nano particle formulations with their respective uptake pathways, spray properties, and leaf traits can substantially enhance nano-particle uptake efficacy. This work provides new mechanistic insight to support more efficient and sustainable P management practices in potato production and broader agricultural systems.

The integration of nanotechnology in agriculture allows for more precise nutrient delivery through nanoparticles (NPs), particularly via foliar application. To mature this technology for enhancing fertilizer efficiency, it is essential to shed new light on the transport and dissolution of NPs in plants. Available analytical methods struggle to address this challenge in a direct manner. We introduce correlative X-ray imaging as a novel analytical tool capable of tracking NP pathways, dissolution and hence nutrient release in plants. By utilizing three complementary X-ray techniques, we offer a unique insight into the plant processes associated with foliar fertilization. We demonstrate that small-angle X-ray scattering enables the characterization of NP size and concentration, while X-ray fluorescence imaging, maps the distribution of elements within the sample. Finally, micro-computed tomography integrates these findings into a complete three-dimensional digital representation of the plant’s microstructure, revealing regions of apparent densification associated with NP accumulation. Using freeze-dried barley plants infiltrated with nano-hydroxyapatite (nHAP), we observed rapid dissolution of NPs, and we are able to associate time and space attributes to the translocation process of nutrients up to three days following foliar application of NPs. With the first pilot study of applying correlative X-ray imaging to live plants, we sought to indicate the potential of this new analytical approach for future nano-enabled agricultural research.

The discovery of new plant fertilization strategies heavily relies on our capabilities to probe physiological processes in living plants with subcellular precision. State-of-the-art microscopy techniques are in general limited to surface investigation or require elaborate tissue preparation and often destruction. X-ray microscopy has the potential to resolve some of these limitations by generating micro- to nanometer-scale 3D images deep in the tissue. We introduce experimental designs and quantitative analysis methodologies, pioneering in vivo 3D microscopy of plant tissue down to 50 nm isotropic voxel size. We show direct in vivo visualization of foliar-applied untagged clustered nanoparticulate fertilizers deep under the leaf surface, not accessible by other microscopy methods. The clustering of nanoparticles occurs naturally for MSN particles and is induced by CaCl2 for nPAA-MnO2 particles. Ultimately, our approach provides the means for direct observation of nanoparticle transport and dissolution in living plant tissue, a step critical for developing sustainable plant fertilization approaches.

Formation of an aqueous continuum from the leaf surface to the sub-stomatal cavity is a key process, affecting the foliar entry of solutes, particles, and pathogens. However, the factors controlling the transition from a water droplet to the formation of a continuous water film remain poorly understood. To address current limitations in state-of-the-art bioimaging methods, we developed an X-ray micro-CT technique that enables nondestructive, time-resolved visualization of water films on live barley (Hordeum vulgare) and potato (Solanum lycopersicum) plants under controlled environmental conditions. We compare droplet behavior, leaf wetting, and the formation of foliar water films on two important crop plants, potato and barley, which differ markedly in their leaf surface characteristics, in terms of hydrophobicity of the cuticle, as well as stomatal topography and trichome morphology and density. We show that continuous water films, from the cuticle into stomata, may form within a few hours, and that a given set of environmental conditions may trigger hydraulic activation of stomata in one crop but not in another, depending largely on the physicochemical properties of the liquid and leaf surface morphological features.

The application of nanotechnology in plant science is unlocking innovative approaches to enhance nutrient use efficiency in crops, particularly through foliar fertilization. This study demonstrates that colloidally stable, pH-responsive polyacrylic acid (PAA)-coated manganese dioxide (MnO2) nanoparticles (nPAA-MnO2) can be designed to significantly restore key metabolic functionalities in manganese (Mn)-deficient barley ( Hordeum vulgare ) within a few days. Using a combination of advanced bioimaging techniques - including confocal laser scanning microscopy (CLSM), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and X-ray nano-computed tomography (nano-CT), we mapped the uptake and distribution pathways of nPAA-MnO2 compared to ionic Mn. While soluble Mn2+ ions primarily enter through hydrophilic cuticular pores, nPAA-MnO2 penetrates leaves via stomata, facilitated by the application of an organosilicone surfactant and glycerol to enhance wetting and hydraulic activation of stomatal pores. Within a few hours, nPAA-MnO2 accumulated in the sub-stomatal cavity and mesophyll apoplast, gradually releasing bioavailable Mn ions in the acidic apoplast environment. Moreover, labeling experiments with tracer ions revealed nPAA-MnO2 hot-spots around the vascular bundles and a limited but significant basipetal translocation of intact nanoparticles out of the foliar application zone, a pivotal step towards converting immobile nutrients such as Mn into mobile ones. Importantly, and unlike ionic Mn solutions, nPAA-MnO2 could be applied at high doses without causing leaf scorching and cytotoxicity, paving the way for more sustainable and efficient foliar fertilization practices. These novel aspects of nanoparticle uptake, translocation, and assimilation underscores the potential of nanotechnology to address nutrient mobility challenges in agriculture, representing an important contribution to the green transition of modern crop production. ### Competing Interest Statement The authors have declared no competing interest. Novo Nordisk Foundation, NNF21OC0066114

Plant nutrient deficiencies can modify leaf surface properties and may affect the absorption of foliar fertilisers. This study examined how plant P-deficiency modified the adaxial leaf surface morphology of four barley cultivars and whether these modifications could be linked to quantitative differences in foliar-applied P absorption. Four Australian barley cultivars were grown hydroponically under P-sufficient and P-deficient conditions. A 32P radiolabelled foliar phosphate solution was applied to adaxial leaf surfaces for 2 or 7 days to investigate absorption and translocation. All cultivars showed different responses to P-deficiency (stomatal density, trichome density, thickness of the epidermal cell wall and cuticle). However, no clear trends were observed among the cultivars in their responses to P deficiency. Cultivars absorbed foliar-applied P regardless of plant P status. Remobilisation occurred from the treated leaf to untreated shoots in all but one cultivar. While P-deficient plants absorbed and accumulated significantly less foliar-applied P after 7 days, this was not linked to measured changes in stomatal or trichome density or the thickness of the epidermal cell wall and cuticle. Autoradiographs revealed that 32P accumulation was limited to newly emerging leaves in P-deficient plants, while P-sufficient plants also remobilized and accumulated 32P into older leaves and tillers. Relatively high P absorption (> 65% of foliar-applied P) in both P-sufficient and P-deficient plants suggests that foliar-applied P may be a useful fertiliser top-up strategy for barley. Due to the lower absorption in P-deficient barley, foliar applications should be made before severe P-deficiency symptoms are apparent to improve absorption.

Recent advances in plant science have greatly enhanced agronomic practices involving the foliar application of agrochemicals such as fertilizers and pesticides. However, the limited phloem mobility of certain nutrients and nonsystemic pesticides reduces the effectiveness of these strategies. Nanoparticles (NPs) have emerged as promising carriers to improve nutrient use efficiency (NUE) and crop protection by enabling long-distance transport and targeted delivery of essential nutrients and active ingredients. While increasing evidence suggests that foliar-applied NPs can translocate within plants, a mechanistic understanding and agronomically relevant case studies remain scarce. As a result, the extent to which NP translocation can facilitate significant distribution and cargo release within plants is yet to be fully established. This review critically evaluates existing research on foliar NP translocation, emphasizing key findings within a plant science framework. Specifically, we examine how NP design can influence translocation and assess the existing quantitative data of NP remobilization within plants. Additionally, we explore how physiological processes affect NP transport and highlight alternative, often overlooked translocation pathways. Lastly, we assess current techniques used to study NP transport, discussing their applicability and limitations. This review identifies significant research gaps that must be addressed to advance nano-enabled plant nutrition as well as crop protection and can therefore be used to inspire future research.

An urgent challenge within crop production is to maintain productivity in a world plagued by climate change and its associated plant stresses, such as heat, drought and salinity. A key factor in this endeavor is to understand the dynamics of root suberization, and its role in plant-water relations and nutrient transport. This study focuses on the hypothesis that endodermal suberin, acts as a physical barrier preventing radial potassium (K) movement out of the vascular tissues during translocation. Previous attempts to experimentally support this idea have produced inconsistent results. We developed a Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) method, allowing us to visualize the distribution of mineral elements and track K movement. Cesium (Cs), dosed in optimized concentrations, was found to be an ideal tracer for K, due to its low background and similar chemical/biological properties. In suberin mutants of Arabidopsis thaliana, we observed a positive correlation between suberin levels and K translocation efficiency, indicating that suberin enhances the plant's ability to retain K within the vascular tissues during translocation from root to shoot. In barley (Hordeum vulgare), fully suberized seminal roots maintained higher K concentrations in the stele compared to younger, less suberized root zones. This suggests that suberization increases with root maturity, enhancing the barrier against K leakage. In nodal roots, suberin was scattered towards the phloem in mature root zones. Despite this incomplete suberization, nodal roots still restrict outward K movement, demonstrating that even partial suberin barriers can significantly reduce K loss. Our findings provide evidence that suberin is a barrier to K leakage during root-to-shoot translocation. This understanding is crucial to maintain crop productivity in the face of climate change.

Bacteria can be applied as biofertilizers to improve crop growth in phosphorus (P)-limited conditions. However, their mode of action in a soil environment is still elusive. We used the strain ALC_02 as a case study to elucidate how Bacillus subtilis affects dwarf tomato cultivated in soil-filled rhizoboxes over time. ALC_02 improved plant P acquisition by increasing the size and P content of P-limited plants. We assessed three possible mechanisms, namely root growth stimulation, root hair elongation, and solubilization of soil P. ALC_02 produced auxin, and inoculation with ALC_02 promoted root growth. ALC_02 promoted root hair elongation as the earliest observed response and colonized root hairs specifically. Root and root hair growth stimulation was associated with a subsequent increase in plant P content, indicating that a better soil exploration by the root system improved plant P acquisition. Furthermore, ALC_02 affected the plant-available P content in sterilized soil differently over time and released P from native P pools in the soil. Collectively, ALC_02 exhibited all three mechanisms in a soil environment. To our knowledge, bacterial P biofertilizers have not been reported to colonize and elongate root hairs in the soil so far, and we propose that these traits contribute to the overall effect of ALC_02. The knowledge gained in this research can be applied in the future quest for bacterial P biofertilizers, where we recommend assessing all three parameters, not only root growth and P solubilization, but also root hair elongation. This will ultimately support the development of sustainable agricultural practices.

In this review, we untangle the physiological key functions of the essential micronutrients and link them to the deficiency responses in plants. Knowledge of these responses at the mechanistic level, and the resulting deficiency symptoms, have improved over the last decade and it appears timely to review recent insights for each of them. A proper understanding of the links between function and symptom is indispensable for an accurate and timely identification of nutritional disorders, thereby informing the design and development of sustainable fertilization strategies. Similarly, improved knowledge of the molecular and physiological functions of micronutrients will be important for breeding programmes aiming to develop new crop genotypes with improved nutrient-use efficiency and resilience in the face of changing soil and climate conditions.

Plant growth-promoting microbes (PGPM) can enhance crop yield and health, but knowledge of their mode-of-action is limited. We studied the influence of two Bacillus subtilis strains, the natural isolate ALC_02 and the domesticated 168 Gö, on Arabidopsis and hypothesized that they modify the root architecture by modulating hormone transport or signaling. Both bacteria promoted increase of shoot and root surface area in vitro, but through different root anatomical traits. Mutant plants deficient in auxin transport or signaling responded less to the bacterial strains than the wild-type, and application of the auxin transport inhibitor NPA strongly reduced the influence of the strains. Both bacteria produced auxin and enhanced shoot auxin levels in DR5::GUS reporter plants. Accordingly, most of the beneficial effects of the strains were dependent on functional auxin transport and signaling, while only 168 Gö depended on functional ethylene signaling. As expected, only ALC_02 stimulated plant growth in soil, unlike 168 Gö that was previously reported to have reduced biofilms. Collectively, the results highlight that B. subtilis strains can have strikingly different plant growth-promoting properties, dependent on what experimental setup they are tested in, and the importance of choosing the right PGPM for a desired root phenotype.