Seasonal Zinc Storage and a Strategy for Its Use in Buds of Fruit Trees

Abstract

Bud dormancy allows deciduous perennial plants to rapidly grow following seasonal cold conditions. Although many studies have examined the hormonal regulation of bud growth, the role of nutrients remains unclear. Insufficient accumulation of the key micronutrient zinc (Zn) in dormant buds affects the vegetative and reproductive growth of perennial plants during the subsequent year, requiring the application of Zn fertilizers in orchard management to avoid growth defects in fruit trees. However, the mechanisms of seasonal Zn homeostasis in perennial plants remain poorly understood. Here, we provide new insights into Zn distribution and speciation within reproductive and vegetative buds of apple (Malus domestica) and four other deciduous fruit trees (peach [Amygdalus persica], grape [Vitis vinifera], pistachio [Pistacia vera], and blueberry [Vaccinium spp.]) using microscopic and spectroscopic characterization techniques comprising synchrotron-based x-ray fluorescence and x-ray absorption near-edge-structure analyses. By establishing a link between bud development and Zn distribution, we identified the following important steps of Zn storage and use in deciduous plants: Zn is preferentially deposited in the stem nodes subtending apical and axillary buds; Zn may then be sequestered as Zn-phytate prior to dormancy; in spring, Zn effectively releases for use during budbreak and subsequent meristematic growth. The mechanisms of Zn homeostasis during the seasonal cycles of plant growth and dormancy described here will contribute to improving orchard management, and to selection and breeding of deciduous perennial species.

Figure 1.

μ-XRF images of the cross sections of stem nodes from apple trees. A, Microscope image of the cross section of an apple tree stem node. B, Micro-XRF images of stem nodes at upper, middle, and lower sections of apple tree stems, and fluorescence intensity values of Zn through the vascular bundles of stem nodes. The selected scanning sites from points (a) to (b) are marked by white dotted lines in μ-XRF images. Fluorescence intensity values of elements were normalized, such that the brightest spots correspond to the highest concentrations. Images were digitally extracted and made into a composite for comparison. Xy, Xylem; Ph, phloem.

Figure 2.

μ-XRF images of the cross sections of stem nodes with axillary buds and node-associated leaf from apple trees. A, Schematic showing the collection sites of samples. B and C, Zn distribution in the cross sections of a stem node (B) and the corresponding attached leaf (C) collected from an apple tree. The color-merge images show the relative locations of Zn (red), potassium (K; blue), and calcium (Ca; green). Fluorescence intensity values of Zn were normalized, such that the brightest spots correspond to the highest concentrations. Images were digitally extracted and made into a composite for comparison. Xy, Xylem; Ph, phloem.

Figure 3.

Zn concentration in leaves, internodes, and nodes in an apple tree. Samples were collected from the upper, middle, and lower sections of apple tree stems. Data are means ± se of five biological replicates. Lowercase letters indicate significant differences in Zn concentration among the leaf, internode, and node at P < 0.05. DW, Dry weight.

Figure 4.

μ-XRF images of the longitudinal sections of a dormant terminal bud from an apple tree. Color-merge images showing the relative concentrations of Zn (red), potassium (K; blue) and calcium (Ca; green). Fluorescence intensity values of Zn were normalized such that the brightest spots correspond to the highest concentrations. Images were digitally extracted and made into a composite for comparison. BS, Bud scales; LP, leaf primordium; Pi, pith.

Figure 5.

Distribution and speciation of Zn in a bud on an apple tree. A, Nano-XRF images of the incipient procambial region within a terminal dormant apple bud showing Zn accumulation within the cells around incipient procambium, with P localized in the same cells. Transverse sections were cut from the shoot apex (inset). B, Nano-XRF images of the vascular tissues within a terminal dormant apple bud showing strong localization of Zn in the procambial region and colocalization of P with Zn; transverse sections were cut at the bottom of a dormant bud (inset). C, The corresponding localization of Zn and P in a dormant bud and the correlation between XRF intensity of Zn versus P. Pixel brightness is displayed in RGB, such that the brightest spots correspond to the highest contents for the element depicted. D, Zn K-edge XANES recorded for Zn model compounds and apple buds at dormant and bud flush stages (solid lines), and the corresponding linear combination fits (dotted lines). E, Proportion (percent mole fraction) of Zn species in the samples. Xy, Xylem; Ph, phloem; Pr, procambium.

Figure 6.

P concentration in apple buds/leaves during budbreak. Concentrations of P_tot (A), P_i (B), P_org (C), and Zn (D) in plant samples collected at four different budbreak stages. Data shown are means ± se of five biological replicates. Different lowercase letters indicate significant difference between samples collected from different stages at P < 0.05. DW, Dry weight.

Figure 7.

Movement of Zn in an apple bud during budbreak. A and B, Microscope images show the morphology (A) and cross sections (B) of a bud at five different developmental stages. LP, Leaf primordium; Pr, procambium. C, Color-merge images show the relative location of Zn (red), potassium (K; blue), and calcium (Ca; green). D, Zn fluorescence intensity values were normalized for each map. Zn intensity values of the selected areas (marked with two white scanning lines) across the leaf primordium (L1) and vascular tissues (L2) are shown. Images were digitally extracted and made into a composite for comparison.

Figure 8.

μ-XRF images of longitudinal sections of dormant buds from different types of fruit tree crops. For each type, a microscopic image is shown (left) with its corresponding color-merge image indicating the relative location of Zn and other elements. Pixel brightness is displayed in red, green, and blue such that the brightest spots correspond to the highest content of the element depicted. LP, Leaf primordium; IP, inflorescence primordium; Pr, procambium; Pi, pith.

Figure 9.

Conceptual model of Zn storage and utilization in deciduous fruit tree species. A, During active growth, Zn inflow from the roots is allocated to the stem nodes via phloem vessels, where it is available for developing buds. B, During bud development and dormancy, Zn is sequestered as Zn-phytate, thus remaining as an overwintering Zn storage pool. C, When growth-promoting conditions are restored in early spring, Zn-phytate is decationized and hydrolyzed concomitantly with the initiation of budbreak. The growth-related function of Zn further mediates the development of new vascular system connections, thereby ultimately promoting efficient nutrient flow into the bud to support the continuation of growth. Xy, Xylem; Ph, phloem; LP, leaf primordium; Pr, procambium.