Foliar uptake of zinc by vascular plants: radiometric study

The behavior, transport, and positive regulation mechanism of ZnO nanoparticles in a plant-soil-microbe environment

ZnO nanoparticles (ZnO NPs) have been widely used in several fields, and they have the potential to be a novel fertilizer to promote plant growth. For the effective use of ZnO NPs, it is necessary to understand their influence mechanisms and key interactions with the soil physical and biological environment. In this review, we summarize the fate and transport of ZnO NPs applied via soil treatment or foliar spray in plant-soil systems and discuss their positive regulation mechanisms in plants and microbes. The latest research shows that the formation, bioavailability, and location of ZnO NPs experience complicated changes during the transport in soil-plant systems and that this depends on many factors. ZnO NPs can improve plant photosynthesis, nutrient element uptake, enzyme activity, and the related gene expression as well as modulate carbon/nitrogen metabolism, secondary metabolites, and the antioxidant systems in plants. Several microbial groups related to plant growth, disease biocontrol, and nutrient cycling in soil can be altered with ZnO NP treatment. In this work, we present a systematic comparison between ZnO NP fertilizer and conventional zinc salt fertilizer. We also fill several knowledge gaps in current studies with the hope of providing guidance for future research.

Spraying high concentrations of chelated zinc enhances zinc biofortification in wheat grain

BACKGROUND

Foliar application of highly concentrated ZnSO₄ fertilizer improves Zn biofortification in wheat grains. However, excess ZnSO₄ ·7H₂ O concentration (≥5 g kg^-1 , w v^-1 ) has been associated with leaf burn and yield loss, necessitating Zn sources with a high threshold concentration. The aim of this study, based on a 2 year field experiment conducted on wheat cultivated in acidic and alkaline soil, was to identify a suitable Zn formulation with a high Zn concentration or efficient adjuvant to achieve optimal Zn biofortification levels without compromising agronomic performance.

RESULTS

There was a continued increase in the Zn concentration in wheat grains and a decrease in grain yield with an increase in the concentration of the Zn foliar sprays in both soil types examined. Wheats treated with chelated Zn foliar sprays - Zn glycine chelate (ZnGly) and Zn-ethylenediaminetetraacetic acid (ZnEDTA) - had less foliar injury than those treated with unchelated Zn fertilizers. Furthermore, irrespective of wheat cultivars and soil types, ZnEDTA applied to wheat at a concentration of 10 g kg^-1 achieved the highest grain Zn concentration without negatively affecting the wheat performance. Adjuvant type and concentration caused no significant variation in grain Zn concentration.

CONCLUSION

Overall, without foliar burn, wheat treated with 10 g kg^-1 ZnEDTA foliar spray had the best performance with regard to grain Zn concentration and grain yield, which could have considerable implications for Zn biofortification of wheat grain. © 2021 Society of Chemical Industry.

Glycine-chelated zinc rather than glycine-mixed zinc has lower foliar phytotoxicity than zinc sulfate and enhances zinc biofortification in waxy corn

To determine whether high spraying concentrations of Zn sources increase the Zn concentration in waxy corn (Zea mays L. var. ceratina Kulesh) seeds without compromising agronomic performance, field experiments were conducted between 2018 and 2020. Excess ZnSO₄ application caused foliar burn, barren ear tip, and grain yield loss. ZnEDTA and Glycine-chelated Zn (ZnGly) caused less foliar burn, but Glycine-mixed Zn caused more foliar burn than ZnSO₄. The seed Zn concentration increased with spraying Zn concentration. ZnEDTA (≤0.8%) had a higher threshold concentration than ZnGly (≤0.4%). Nevertheless, Zn biofortification efficacy did not significantly differ between 0.4% ZnGly and 0.8% ZnEDTA, and the grain Zn recovery rate of 0.4% ZnGly was much higher than that of 0.8% ZnEDTA. Additionally, dual-isotope labelling tests confirmed that ¹⁵N-glycine and ⁶⁸Zn in ZnGly interacted. In the future, chelating technology is essential for developing new Zn fertilizers to optimize Zn biofortification efficacy.

Zinc Accumulates in the Nodes of Wheat Following the Foliar Application of 65Zn Oxide Nano- and Microparticles

Using zinc (Zn) foliar fertilizers to enhance the grain quality of wheat (Triticum aestivum) can be an effective alternative or supplement to Zn soil fertilizers. However, knowledge about the mechanisms of Zn absorption and translocation following foliar application is scarce. Here, autoradiography and γ-spectrometry were used to investigate the behavior of ⁶⁵Zn applied to wheat leaves as soluble ⁶⁵Zn chloride (⁶⁵ZnCl₂), chelated ⁶⁵Zn (⁶⁵ZnEDTA), ⁶⁵Zn oxide nanoparticle (⁶⁵ZnO-NP) suspensions, and ⁶⁵ZnO microparticle (⁶⁵ZnO-MP) suspensions. The largest amount of ⁶⁵Zn absorption occurred in ⁶⁵ZnCl₂ treated leaves. However, this treatment (⁶⁵ZnCl₂) also had the lowest proportion of absorbed ⁶⁵Zn translocated away from the treated leaf after 15 d due to leaf scorching (p = 0.0007). Foliar-applied ⁶⁵ZnO-NPs and ⁶⁵ZnO-MPs had the lowest absorption, but ⁶⁵ZnO-NPs had the highest relative translocation. ⁶⁵Zinc EDTA was intermediate, with higher ⁶⁵Zn absorption than ⁶⁵ZnO treatments but similar translocation. Regardless, the majority of the foliar-applied ⁶⁵Zn remained in the treated leaf for all treatments. Furthermore, ⁶⁵ZnO-NPs and ⁶⁵ZnO-MPs accumulated in plant nodes, suggesting that Zn was absorbed as dissolved ⁶⁵Zn and particulate ⁶⁵ZnO. Overall, the form and amount of absorbed ⁶⁵Zn affected translocation.

Effects of changes in leaf properties mediated by methyl jasmonate (MeJA) on foliar absorption of Zn, Mn and Fe

Background and Aims

Foliar fertilization to overcome nutritional deficiencies is becoming increasingly widespread. However, the processes of foliar nutrient absorption and translocation are poorly understood. The present study aimed to investigate how cuticular leaf properties affect the absorption of foliar-applied nutrients in leaf tissues.

Methods

Given that methyl jasmonate (MeJA) can cause alterations in leaf properties, we applied 1 mm MeJA to sunflower (Helianthus annuus), tomato (Solanum lycopersicum) and soybean (Glycine max) to assess changes in leaf properties. Using traditionally analytical approaches and synchrotron-based X-ray fluorescence microscopy, the effects of these changes on the absorption and translocation of foliar-applied Zn, Mn and Fe were examined.

Key Results

The changes in leaf properties caused by the application of MeJA increased foliar absorption of Zn, Mn and Fe up to 3- to 5-fold in sunflower but decreased it by 0·5- to 0·9-fold in tomato, with no effect in soybean. These changes in the foliar absorption of nutrients could not be explained by changes in overall trichome density, which increased in both sunflower (86%) and tomato (76%) (with no change in soybean). Similarly, the changes could be not attributed to changes in stomatal density or cuticle composition, given that these properties remained constant. Rather, the changes in the foliar absorption of Zn, Mn and Fe were related to the thickness of the cuticle and epidermal cell wall. Finally, the subsequent translocation of the absorbed nutrients within the leaf tissues was limited (<1·3mm) irrespective of treatment.

Conclusions

The present study highlights the potential importance of the combined thickness of the cuticle and epidermal cell wall in the absorption of foliar-applied nutrients. This information will assist in increasing the efficacy of foliar fertilization.

Imaging and analysis of thin structures using positron emission tomography: Thin phantoms and in vivo tobacco leaves study

In this work, a novel approach utilizing the designed phantoms imitating the plant tissues was applied for the evaluation of the relationships between the parameters of the prepared phantoms and/or quantitative variables obtained within the PET analysis. The microPET system developed for animal objects and approaches used made it possible to obtain the quantitative data in the form of (18)F radioactivity as well as the glucose (in µg) accumulated in leaf tissues within the dynamic in vivo study.

Bacteria-zinc co-localization implicates enhanced synthesis of cysteine-rich peptides in zinc detoxification when Brassica juncea is inoculated with Rhizobium leguminosarum

Some plant growth promoting bacteria (PGPB) are enigmatic in enhancing plant growth in the face of increased metal accumulation in plants. Since most PGPB colonize the plant root epidermis, we hypothesized that PGPB confer tolerance to metals through changes in speciation at the root epidermis. We employed a novel combination of fluorophore-based confocal laser scanning microscopic imaging and synchrotron based microscopic X-ray fluorescence mapping with X-ray absorption spectroscopy to characterize bacterial localization, zinc (Zn) distribution and speciation in the roots of Brassica juncea grown in Zn contaminated media (400 mg kg(-1) Zn) with the endophytic Pseudomonas brassicacearum and rhizospheric Rhizobium leguminosarum. PGPB enhanced epidermal Zn sequestration relative to PGBP-free controls while the extent of endophytic accumulation depended on the colonization mode of each PGBP. Increased root accumulation of Zn and increased tolerance to Zn was associated predominantly with R. leguminosarum and was likely due to the coordination of Zn with cysteine-rich peptides in the root endodermis, suggesting enhanced synthesis of phytochelatins or glutathione. Our mechanistic model of enhanced Zn accumulation and detoxification in plants inoculated with R. leguminosarum has particular relevance to PGPB enhanced phytoremediation of soils contaminated through mining and oxidation of sulphur-bearing Zn minerals or engineered nanomaterials such as ZnS.

In situ analysis of foliar zinc absorption and short-distance movement in fresh and hydrated leaves of tomato and citrus using synchrotron-based X-ray fluorescence microscopy

BACKGROUND AND AIMS

Globally, zinc deficiency is one of the most important nutritional factors limiting crop yield and quality. Despite widespread use of foliar-applied zinc fertilizers, much remains unknown regarding the movement of zinc from the foliar surface into the vascular structure for translocation into other tissues and the key factors affecting this diffusion.

METHODS

Using synchrotron-based X-ray fluorescence microscopy (µ-XRF), absorption of foliar-applied zinc nitrate or zinc hydroxide nitrate was examined in fresh leaves of tomato (Solanum lycopersicum) and citrus (Citrus reticulatus).

KEY RESULTS

The foliar absorption of zinc increased concentrations in the underlying tissues by up to 600-fold in tomato but only up to 5-fold in citrus. The magnitude of this absorption was influenced by the form of zinc applied, the zinc status of the treated leaf and the leaf surface to which it was applied (abaxial or adaxial). Once the zinc had moved through the leaf surface it appeared to bind strongly, with limited further redistribution. Regardless of this, in these underlying tissues zinc moved into the lower-order veins, with concentrations 2- to 10-fold higher than in the adjacent tissues. However, even once in higher-order veins, the movement of zinc was still comparatively limited, with concentrations decreasing to levels similar to the background within 1-10 mm.

CONCLUSIONS

The results advance our understanding of the factors that influence the efficacy of foliar zinc fertilizers and demonstrate the merits of an innovative methodology for studying foliar zinc translocation mechanisms.