Supplemental macronutrients and microbial fermentation products improve the uptake and transport of foliar applied zinc in sunflower (Helianthus annuus L.) plants. Studies utilizing micro X-ray florescence

Abstract

Enhancing nutrient uptake and the subsequent elemental transport from the sites of application to sites of utilization is of great importance to the science and practical field application of foliar fertilizers. The aim of this study was to investigate the mobility of various foliar applied zinc (Zn) formulations in sunflower (Helianthus annuus L.) and to evaluate the effects of the addition of an organic biostimulant on phloem loading and elemental mobility. This was achieved by application of foliar formulations to the blade of sunflower (H. annuus L.) and high-resolution elemental imaging with micro X-ray fluorescence (μ-XRF) to visualize Zn within the vascular system of the leaf petiole. Although no significant increase of total Zn in petioles was determined by inductively-coupled plasma mass-spectrometer, μ-XRF elemental imaging showed a clear enrichment of Zn in the vascular tissues within the sunflower petioles treated with foliar fertilizers containing Zn. The concentration of Zn in the vascular of sunflower petioles was increased when Zn was applied with other microelements with EDTA (commercial product Kick-Off) as compared with an equimolar concentration of ZnSO4 alone. The addition of macronutrients N, P, K (commercial product CleanStart) to the Kick-Off Zn fertilizer, further increased vascular system Zn concentrations while the addition of the microbially derived organic biostimulant "GroZyme" resulted in a remarkable enhancement of Zn concentrations in the petiole vascular system. The study provides direct visualized evidence for phloem transport of foliar applied Zn out of sites of application in plants by using μ-XRF technique, and suggests that the formulation of the foliar applied Zn and the addition of the organic biostimulant GroZyme increases the mobility of Zn following its absorption by the leaf of sunflower.

Keywords: biostimulant; foliar fertilizer; microbial fermentation; remobilization; sunflower; zinc; μ-XRF.

FIGURE 1

Microscope image of petiole cross-section collected from sunflowers plants after foliar application. Fully expanded leaves of 1-month old sunflower were treated with different foliar fertilizers, then cross-sections of the petiole were cut by cryotome at -20^∘C 7 days after final foliar fertilization treatment.

FIGURE 2

Microscope cross sections (a–f) and μ-XRF elemental maps (A–F) for Zn (red), Ca (green), and K (blue) of petioles collected from sunflowers subject to different foliar fertilizers. Cross-sections of leaf veins were cut from sunflower plants treated with (a, A) control, (b, B) CleanStart, (c, C) Kick-Off, (d, D) CleanStart + Kick-Off, (e, E) CleanStart + Kick-Off + GroZyme, and (f, F) ZnSO₄, and then analyzed by μ-XRF imaging. Compositions of the nutrients in different treatments are shown in Table 1. Pixel brightness for μ-XRF map (A–F) is displayed in RGB, with the brightest spots corresponding to the highest element fluorescence. Scale bar: 500 μm.

FIGURE 3

Micro X-ray fluorescence elemental maps for Zn (red), Ca (green), and K (blue) focused on vascular bundles of petioles collected from sunflowers subject to different foliar fertilizers. Full expanded leaves of sunflower were treated with (A) control, (B) CleanStart, (C) Kick-Off, (D) CleanStart + Kick-Off, (E) CleanStart + Kick-Off + GroZyme, and (F) ZnSO₄. Compositions of the nutrients in different treatments were shown in Table 1. Pixel brightness for μ-XRF map is displayed in RGB, with the brightest spots corresponding to the highest element fluorescence.

FIGURE 4

Zinc intensities (counts/s) of 30–40 scan lines through the vascular bundles of petioles collected from sunflower plants treated with different foliar fertilizers. The selected scanning sites from point A to B are marked by yellow lines in Figure 3, with 30–40 scanning different lines selected for each plant sample.

FIGURE 5

Concentrations of Zn (mg kg^–1 DW) in the petioles collected from sunflower plants treated with different foliar fertilizers. Full expanded leaves of sunflower were treated with control, CleanStart, Kick-Off, CleanStart + Kick-Off, CleanStart + Kick-Off + GroZyme, and ZnSO₄, and the Zn concentration of leaf veins were analyzed by ICP-MS. Compositions of the nutrients in different treatments were shown in Table 1. Data points and error bars represent means and SEs of three replicates (n = 3).

FIGURE 6

Concentrations of Mn, Mg, B, Fe (mg kg^–1 DW), and Ca, K (g kg^–1 DW) in the petioles collected from sunflower plants treated with different foliar fertilizers. Full expanded leaves of sunflower were treated with control, CleanStart, Kick-Off, CleanStart + Kick-Off, CleanStart + Kick-Off + GroZyme, and ZnSO₄, and the concentrations of Mn, Mg, B, Fe, Ca, K of leaf veins were analyzed by ICP-MS. Compositions of the nutrients in different treatments were shown in Table 1. Data points and error bars represent means and SEs of three replicates (n = 3).