Foliar Calcium Absorption by Tomato Plants: Comparing the Effects of Calcium Sources and Adjuvant Usage.

The deficiency of calcium (Ca) reduces the quality and shelf life of fruits. In this scenario, although foliar spraying of Ca2+ has been used, altogether with soil fertilization, as an alternative to prevent deficiencies, little is known regarding its absorption dynamics by plant leaves. Herein, in vivo microprobe X-ray fluorescence was employed aiming to monitor the foliar absorption of CaCl2, Ca-citrate complex, and Ca3(PO4)2 nanoparticles with and without using adjuvant. We also investigated whether Sr2+ can be employed as Ca2+ proxy in foliar absorption studies. Moreover, the impact of treatments on the cuticle structure was evaluated by scanning electron microscopy. For this study, 45-day-old tomato (Solanum lycopersicum L., cv. Micro-Tom) plants were used as a model species. After 100 h, the leaves absorbed 90, 18, and 4% of aqueous CaCl2, Ca-citrate, and Ca3(PO4)2 nanoparticles, respectively. The addition of adjuvant increased the absorption of Ca-citrate to 28%, decreased that of CaCl2 to 77%, and did not affect Ca3(PO4)2. CaCl2 displayed an exponential decay absorption profile with half-lives of 15 h and 5 h without and with adjuvant, respectively. Ca-citrate and Ca3(PO4)2 exhibited absorption profiles that were closer to a linear behavior. Sr2+ was a suitable Ca2+ tracer because of its similar absorption profiles. Furthermore, the use of adjuvant affected the epicuticular crystal structure. Our findings reveal that CaCl2 was the most efficient Ca2+ source. The effects caused by adjuvant suggest that CaCl2 and Ca-citrate were absorbed mostly through hydrophilic and lipophilic pathways.

Physiological responses of plants to in vivo X-ray damage from X-ray fluorescence measurements: insights from anatomical, elemental, histochemical, and ultrastructural analyses.

X-ray fluorescence spectroscopy (XRF) is a powerful technique for the in vivo assessment of plant tissues. However, the potential X-ray exposure damages might affect the structure and elemental composition of living plant tissues, leading to artefacts in the recorded data. Herein, we exposed in vivo soybean (Glycine max (L.) Merrill) leaves to several X-ray doses through a polychromatic benchtop microprobe X-ray fluorescence spectrometer, modulating the photon flux density by adjusting either the beam size, current, or exposure time. Changes in the irradiated plant tissues' structure, ultrastructure, and physiology were investigated through light and transmission electron microscopy (TEM). Depending on X-ray exposure dose, decreased K and X-ray scattering intensities and increased Ca, P, and Mn signals on soybean leaves were recorded. Anatomical analysis indicated the necrosis of epidermal and mesophyll cells on the irradiated spots, where TEM images revealed the collapse of cytoplasm and cell wall breaking. Furthermore, the histochemical analysis detected the production of reactive oxygen species and the inhibition of chlorophyll autofluorescence in these areas. Under certain X-ray exposure conditions, e.g. high photon flux density and long exposure time, XRF measurements may affect the soybean leaves structures, elemental composition, and cellular ultrastructure, inducing programmed cell death. Our characterization shed light on the plant's responses to the X-ray-induced radiation damage and might help to establish proper X-ray radiation limits and novel strategies for in vivo benchtop-XRF analysis of vegetal materials.

Selenium increases photosynthetic capacity, daidzein biosynthesis, nodulation and yield of peanuts plants (Arachis hypogaea L.).

This study aimed to investigate the roles of selenium (Se) application on the profile of photosynthetic pigments, oxidant metabolism, flavonoids biosynthesis, nodulation, and its relation to agronomic traits of peanut plants. Two independent experiments were carried out: one conducted in soil and the other in a nutrient solution. When the plants reached the V2 growth stage, five Se doses (0, 7.5, 15, 30, and 45 µg kg-1) and four Se concentrations (0, 5, 10, and 15 µmol L-1) were supplied as sodium selenate. The concentration of photosynthetic pigments, activity of antioxidant enzymes and the concentration of total sugars in peanut leaves increased in response to Se fertilization. In addition, Se improves nitrogen assimilation efficiency by increasing nitrate reductase activity which results in a higher concentration of ureides, amino acids and proteins. Se increases the synthesis of daidzein and genistein in the root, resulting in a greater number of nodules and concentration and transport of ureides to the leaves. Se-treated plants showed greater growth, biomass accumulation in shoots and roots, yield and Se concentration in leaves and grains. Our results contribute to food security and also to increase knowledge about the effects of Se on physiology, biochemistry and biological nitrogen fixation in legume plants.

Identification of potential plant species hyperaccumulating light rare earth elements (LREE) in a mining area in Minas Gerais, Brazil.

Phytoextraction of rare earth elements (REE) from contaminated soils has gained importance during the last few decades. The Poços de Caldas municipality in Brazil is known for its mineral richness, including large reserves of REE. In this study, we report light REE (La, Ce, Sm, Pr, and Nd) in soils and plants collected in an area. Composite soil samples and plant individuals were collected, and total concentrations of LREE in soils were determined by wavelength dispersive X-ray fluorescence (WDXRF). The plant available LREE concentrations in soils were estimated upon the acetic acid method (F1 fractions) of the stepwise sequential extraction procedure, together with plant content that was analysed by inductively coupled plasma mass spectrometry (ICP-MS). The total sum concentrations of tested LREE in soils varied from 5.6 up to 37.9 g kg-1, the bioavailable fraction was ca. 1%, and a linear relationship was found between them. The only exception was Sm, whose availability was lesser and did not show a linear relationship. The concentration of LREE in non-accumulator plants varied from 1.3-950 mg kg-1 for Ce, La 1.1-99 mg kg-1, Sm 0.04-9.31 mg kg-1, Pr 0.1-24.1 mg kg-1, and Nd 0.55-81 mg kg-1. The concentration of LREE among shoots did not show a linear relation either with the available fraction or total content. The screening also revealed Christella dentata (Forssk.) Brownsey & Jermy, Thelypteridaceae family, as a promising hyperaccumulator species. The concentrations of LREE among shoots of six individuals of this species were in the ranges from 115 to 1872 mg kg-1 for Ce, La 190-703 mg kg-1, Sm 9-48 mg kg-1, Pr 32-144 mg kg-1, and Nd 105-478 mg kg-1.

Assessment of selenium spatial distribution using µ-XFR in cowpea (Vigna unguiculata (L.) Walp.) plants: Integration of physiological and biochemical responses.

Low concentrations of selenium (Se) are beneficial for plant growth. Foliar Se application at high concentrations is toxic to plants due to the formation of reactive oxygen species (ROS). This study characterized Se toxicity symptoms using X-ray fluorescence (XRF) technique in response to foliar Se application in cowpea plants. Five Se concentrations (0, 10, 25, 50, 100 e 150 g ha-1) were sprayed on leaves as sodium selenate. The visual symptoms of Se toxicity in cowpea leaves were separated into two stages: I) necrotic points with an irregular distribution and internerval chlorosis at the leaf limb border (50-100 g ha-1); II) total chlorosis with the formation of dark brown necrotic lesions (150 g ha-1). Foliar Se application at 50 g ha-1 increased photosynthetic pigments and yield. Ultrastructural analyses showed that Se foliar application above 50 g ha-1 disarranged the upper epidermis of cowpea leaves. Furthermore, Se application above 100 g ha-1 significantly increased the hydrogen peroxide concentration and lipid peroxidation inducing necrotic leaf lesions. Mapping of the elements in leaves using the XRF revealed high Se intensity, specifically in leaf necrotic lesions accompanied by calcium (Ca) as a possible attenuating mechanism of plant stress. The distribution of Se intensities in the seeds was homogeneous, without specific accumulation sites. Phosphorus (P) and sulfur (S) were found primarily located in the embryonic region. Understanding the factors involved in Se accumulation and its interaction with Ca support new preventive measurement technologies to prevent Se toxicity in plants.

X-ray fluorescence spectroscopy (XRF) applied to plant science: challenges towards in vivo analysis of plants.

X-ray fluorescence spectroscopy (XRF) is an analytical tool used to determine the elemental composition in a myriad of sample matrices. Due to the XRF non-destructive feature, this technique may allow time-resolved plant tissue analyses under in vivo conditions, and additionally, the combination with other non-destructive techniques. In this study, we employed handheld and benchtop XRF to evaluate the elemental distribution changes in living plant tissues exposed to X-rays, as well as real-time uptake kinetics of Zn(aq) and Mn(aq) in soybean (Glycine max (L.) Merrill) stem and leaves, for 48 hours, combined with transpiration rate assessment on leaves by an infrared gas analyzer (IRGA). We found higher Zn content than Mn in stems. The latter micronutrient, in turn, presented higher concentration in leaf veins. Besides, both micronutrients were more concentrated in the first trifolium (i.e., youngest leaf) of soybean plants. Moreover, the transpiration rate was more influenced by circadian cycles than Zn and Mn uptake. Thus, XRF represents a convenient tool for in vivo nutritional studies in plants, and it can be coupled successfully to other analytical techniques.

In Vivo Evaluation of Zn Foliar Uptake and Transport in Soybean Using X-ray Absorption and Fluorescence Spectroscopy.

Understanding the mechanisms of absorption and transport of foliar nutrition is a key step towards the development of advanced fertilization methods. This study employed X-ray fluorescence (XRF) and X-ray absorption near edge spectroscopy (XANES) to trace the in vivo absorption and transport of ZnO and ZnSO4(aq) to soybean leaves (Glycine max). XRF maps monitored over 48 h showed a shape change of the dried ZnSO4(aq) droplet, indicating Zn2+ absorption. Conversely, these maps did not show short movement of Zn from ZnO. XRF measurements on petioles of leaves that received Zn2+ treatments clarified that the Zn absorption and transport in the form of ZnSO4(aq) was faster that of ZnO. Solubility was the major factor driving ZnSO4(aq) absorption. XANES speciation showed that in planta Zn is transported coordinated with organic acids. Because plants demand Zn during their entire lifecycle, the utilization of sources with different solubilities can increase Zn use efficiency.