Handbook of Plant Nutrition - chapter 15 doc

bun-15.3 ZINC DEFICIENCY Zinc deficiency is common in plants growing in highly weathered acid or calcareous soils 16.Roots of zinc-deficient trees often exude a gummy material.. Zinc defici

15 Zinc

J Benton Storey

Texas A&M University, College Station, Texas

CONTENTS

15.1 Introduction 411

15.1.1 Early Research on Zinc Nutrition of Crops 411

15.2 Absorption and Function of Zinc in Plants 412

15.3 Zinc Deficiency 412

15.4 Zinc Tolerance 415

15.5 Trunk Injection 422

15.6 Zinc in Soils 422

15.7 Phosphorus–Zinc Interactions 423

15.8 Tryptophan and Indole Acetic and Synthesis 423

15.9 Root Uptake 423

15.10 Foliar Absorption 424

15.10.1 Influence of Humidity on Foliar Absorption 427

15.11 Role of Zinc in DNA and RNA Metabolism and Protein Synthesis 428

15.12 Zinc Transporters and Zinc Efficiency 428

15.13 Summary 429

References 430

15.1 INTRODUCTION

15.1.1 E ARLY R ESEARCH ON Z INC N UTRITION OF C ROPS

Discovery of zinc as an essential element for higher plants was made by Sommer and Lipman (1)

while working with barley (Hordeum vulgare L.) and sun flower (Helianthus annuus L.) However,

Chandler et al (2) stated that Raulin, as early as 1869, reported zinc to be essential in the culture media for some fungi, and speculated that zinc was probably essential in higher plants Skinner and

Demaree (3) reported on a typical Dougherty county pecan (Carya illinoinensis K Koch) orchard

in Georgia Pecan trees that were placed in a study that started in 1918 increased in trunk diameter, but their tops had dieback each year, and their condition ‘appeared hopeless’ in 1922 Fertilizers (N,

P, K), cover crops, and all known means were of no avail Rosette, or related dieback, had been rec-ognized since around 1900, but it was in 1932 before zinc was found to be the corrective element (4,5) The common assumption among pecan growers was that a deficiency of iron was responsible for rosette as pecans were brought into cultivation in the early 1900s Alben used 0.8 to 1.0% solu-tions of FeCl2and FeSO4in his rosette treatments in 1931 and obtained conflicting results The

1932 treatments included injections into dormant trees, soil applications while the trees were dor-mant and after the foliage was well developed, and foliar spraying and dipping The only favorable results were obtained when Alben mixed the iron solutions in zinc-galvanized containers Analysis proved that the solutions contained considerable quantities of zinc These experiments led to the use

411

results were obtained with trees located on alkaline or acid soils The most satisfactory results wereobtained with a foliar spray of 0.18% ZnSO4and a 0.012% ZnCl2solution Roberts and Dunegan(6) also observed a bactericidal effect when using a ZnSO4-hydrated lime mixture that controlled

bacterial leaf spot (Xanthomonas pruni), which later became a serious pest for susceptible peach (Prunus persica Batsch.) cultivars like ‘Burbank July Elberta’ in the 1940s, ‘Sam Houston’ in the

1960s, and ‘O-Henry’ in the 1990s (personal experience) Hydrated lime was necessary to preventdefoliation of peach trees by ZnSO4toxicity

15.2 ABSORPTION AND FUNCTION OF ZINC IN PLANTS

Zinc is taken up predominantly as a divalent cation (Zn2 ⫹), but at high pH it is probably absorbed as

a monovalent cation (ZnOH⫹) (7) Zinc is either bound to organic acids during long distance port in the xylem or may move as free divalent cations Zinc concentrations are fairly high in phloemsap where it is probably complexed to low-molecular-weight organic solutes (8) The metabolic func-tions of zinc are based on its strong tendency to form tetrahedral complexes with N-, O-, and partic-ularly S-ligands, and thus it plays a catalytic and structural role in enzyme reactions (9)

trans-Zinc is an integral component of enzyme structures and has the following three functions: alytic, coactive, or structural (9,10) The zinc atom is coordinated to four ligands in enzymes withcatalytic functions Three of them are amino acids, with histidine being the most frequent, fol-lowed by glutamine and asparagine A water molecule is the fourth ligand at all catalytical sites.The structural zinc atoms are coordinated to the S-groups of four cysteine residues forming a ter-tiary structure of high stability These structural enzymes include alcohol dehydrogenase, and theproteins involved in DNA replication and gene expression (11) Alcohol dehyrogenase containstwo zinc atoms per molecule, one with catalytic reduction of acetaldehyde to ethanol and the otherwith structural functions Ethanol formation primarily occurs in meristematic tissues under aero-bic conditions in higher plants Alcohol dehyrdrogenase activity decreases in zinc-deficient plants,but the consequences are not known (7) Flooding stimulates the alcohol dehydrogenase twice asmuch in zinc-sufficient compared with zinc-deficient plants, which could reduce functions in sub-merged rice (12)

cat-Carbonic anhydrase (CA) contains one zinc atom, which catalyzes the hydration of carbondioxide (CO2) The enzyme is located in the chloroplasts and the cytoplasm Carbon dioxide is thesubstrate for photosynthesis in C3 plants, but no direct relationship was reported between CAactivity and photosynthetic CO2assimilation in C3plants (13) The CA activity is absent when zinc

is extremely low, but when even a small amount of zinc is present, maximum net photosynthesiscan occur Photosynthesis by C4metabolism is considerably different (14,15) than that occurring

in C3plants For C4metabolism, a high CA activity is necessary to shift the equilibrium in favor

of HCO3⫺for phosphoenolpyruvate carboxylase, which forms malate for the shuttle into the dle sheath chloroplasts, where CO2is released and serves as substrate of ribulosebisphosphate car-boxylase

bun-15.3 ZINC DEFICIENCY

Zinc deficiency is common in plants growing in highly weathered acid or calcareous soils (16).Roots of zinc-deficient trees often exude a gummy material Major zinc-deficient sites are old barn-yards or corral sites, where an extra heavy manure application accumulated over the years Zincions become tied to organic matter to the extent that zinc is not available to the roots of peach trees(17,18) Zinc deficiency initially appears in all plants as intervenial chlorosis (mottling) in whichlighter green to pale yellow color appears between the midrib and secondary veins (Figure 15.1 andFigure 15.2) Developing leaves are smaller than normal, and the internodes are short Popularnames describe these conditions as ‘little leaf’ and ‘rosette’ (19,20) Pecan trees in particular suffer

FIGURE 15.1 Zinc deficiency of peaches (Prunus persica Batsch) is expressed as developing leaves that are

smaller than normal and the internodes are shorter causing leaves to be closer to each other and thence the ular names which describes the terminal branches as ‘little leaf’ (Photograph by J.B Storey.) (For a color pres- entation of this figure, see the accompanying compact disc.)

pop-FIGURE 15.2 Zinc-deficient pecan (Carya illinoinensis K Koch) leaves (left) can contain less than 30 mg

Zn per kg compared to over 80 mg Zn per kg Zn in healthy leaves (right) The zinc-de ficient leaves have small

crinkled leaves that are mottled with yellow Healthy zinc-su fficient leaves are dark green Actual zinc

con-centration of each leaf is shown in the photograph (Photograph by J.B Storey.) (For a color presentation of this figure, see the accompanying compact disc.)

from shortened internodes (rosette) (Figure 15.3) Shoot apices die (shoot die-back) under severezinc deficiency, as in a tree in Comanche county, Texas (Figure 15.4) Forest plantations in Australiahave shown similar symptoms (21) Citrus often show diffusive symptoms (mottle leaf) (Figure 15.5).The ideal time to demonstrate citrus trace element deficiency symptoms is in winter months when the

FIGURE 15.3 Zinc-deficient pecan (Carya illinoinensis K Koch) trees have shorter internodes so that the

leaves are closer together forming a rosette of poorly formed crinkled, chlorotic leaves (Photograph by J.B Storey.) (For a color presentation of this figure, see the accompanying compact disc.)

FIGURE 15.4 If the rosetted pecan (Carya illinoinensis K Koch) trees are not treated, the terminals die

fol-lowed by death of the entire tree Dieback can occur on young or old trees (Photograph by J.B Storey.) (For a color presentation of this figure, see the accompanying compact disc.)

soil is relatively cold Treatment with zinc fertilizers is not necessary if the symptoms disappear when

the soil temperature rises in the spring Sorghum (Sorghum bicolor Moench) that is deficient in zincforms chlorotic bands along the midrib and red spots on the leaves (22) Shoots are more inhibited

by zinc deficiency than roots (23) For most plants, the critical leaf zinc deficiency levels range from

10 to 100 mg kg⫺1depending on species (Table 15.1)

15.4 ZINC TOLERANCE

Zinc is the heavy metal most often in the highest concentrations in wastes arising in industrializedcommunities (21) Zinc exclusion from uptake, or binding in the cell walls, does not seem to con-

tribute to zinc tolerance (24,25) Zinc exclusion might exist in Scots pine (Pinus sylvestris L.), where

certain ectomycorrhizal fungi retain most of the zinc in their mycelia, resulting in the ability of theplant to tolerate zinc (26) Infections with ectomycorrhizal fungi are beneficial for the growth anddevelopment of pecan (27) These fungi are highly specialized parasites that do not cause root disease.They are symbiotic, thus gaining substance from the root and contributing to the health of the root.Tolerance is achieved through sequestering zinc in the vacuoles, and zinc remains low in thecytoplasm of tolerant plants, whereas zinc is stored in the cytoplasm of non-tolerant clones (28).Positive correlation between organic acids such as citrate and malate with zinc in tolerant plants

indicates a mechanism of zinc tolerance (29,30) Zinc tolerance in tufted hair grass (Deschampsia caespitosa Beauvois) was increased in plants supplied with ammonium as compared to nitrate nutri-

tion This effect apparently is caused by greater accumulation of asparagine in the cytoplasm ofammonium-fed plants, which form stable complexes with asparagines and zinc (31)

Foliar application of chelates is inefficient because of poor absorption of the large organic ecules through cuticles (32,33) Foliar ZnSO4treatments are toxic to peach leaves (34) and to manyother species, probably because sulfur accumulates on leaves and results in salt burn A zinc nitrate-ammonium nitrate-urea fertilizer (NZNTM; 15% N, 5% Zn; Tessenderlo Kerley Group, Phoenix,

mol-AZ, U.S.A.) did not burn peach leaves Apparently, NZN-treated peach leaves do not suffer fromsalt burn because the nitrate in NZN is readily absorbed in response to the need of leaves for nitro-gen in protein synthesis thus not accumulating on the surface to cause leaf burn (34)

FIGURE 15.5 Mottled leaf symptoms characterize zinc deficiency symptoms in citrus (Citrus spp L.).

(Photograph by J.B Storey.) (For a color presentation of this figure, see the accompanying compact disc.)

Experience with trunk injections of zinc has been disappointing in all cases despite rumors of cess It would seem logical that placement of any form of zinc in the secondary xylem of an activelytranspiring tree would utilize the xylem vessels to rapidly transport the zinc to the actively growingmeristems However, many researchers including Millikan and Hanger (35,36) have proven thatzinc transport is more complex than injecting zinc in any form into tree trunks Millikan and Hanger(36) reported that 65Zn moved from the injection point only when zinc was injected into the bark of2-year-old apple trees Supplying ethylenediaminetetraacetic acid (EDTA) enhanced 65Zn move-ment in an acropetal (upward) direction only The 65Zn was distributed to spurs and laterals on thedistal side of the injection point Millikan and Hanger (36) also reported that 65Zn accumulated atthe nodes on lateral branches and in the petioles, midrib, and major veins of the leaves Wadsworth(37) reported no significant effect of ZnEDTA applied via injection into the secondary xylem ofmature ‘Western’ or ‘Burkett’ pecan tree leaves on nut quality or yield He suggested that the vol-ume of zinc was inadequate to influence such a large tree The possibility of home owners using thismeans of applying zinc to their large pecan landscape trees, which would otherwise require largespray machines, was discounted by the danger of small children pulling them out of the trunks andinserting them in their mouths The direct application of zinc chelates to the secondary xylem viainjection was unsuccessful primarily because of the small volume of zinc injected (37).

suc-15.6 ZINC IN SOILS

Zinc has a complete 3d104s2outer electronic configuration and, unlike the other d block trients such as such as manganese, molybdenum, copper, and iron, has only a single oxidation stateand hence a single valence of II The average concentration of zinc in the crust of the Earth, granitic,and basaltic igneous rock is approximately 70, 40, and 100 mg kg⫺1, respectively (38), whereas sed-imentary rocks like limestone, sandstone, and shale contain 20, 16, and 95 mg kg⫺1, respectively(39) The total zinc content in soils varies from 3 to 770 mg kg⫺1with the world average being

micronu-64 mg kg⫺1(40)

There are five major pools of zinc in the soil: (a) zinc in soil solution; (b) surface adsorbed andexchangeable zinc; (c) zinc associated with organic matter; (d) zinc associated with oxides and car-bonates; and (e) zinc in primary minerals and secondary alumino-silicate materials (41)

There is evidence that Zn2 ⫹ activities in the soil solution may be controlled by franklinite(ZnFe2O4), whose equilibrium solubility is similar to that of soil-held zinc over pH values of 6 to 9(42,43) The mineral will precipitate whenever zinc concentration in the soil solution exceeds theequilibrium solubility of the mineral and will dissolve whenever the opposite is true This processprovides a zinc-buffering system

Zinc may be associated with soil organic matter, which includes water-soluble and organic pounds Zinc is bound via incorporation into organic molecules, exchange, chelation, or by specificand nonspecific adsorption (41)

com-Zinc is associated with hydrous oxides and carbonates via adsorption, surface complex tions, ion exchange, incorporation into the crystal lattice, and co-precipitation (41) Some of thesereactions fix zinc rather strongly and are believed to be instrumental in controlling the amount ofzinc in the soil solution (44) Zinc is complexed with CaCO3in alkaline (pH 8.2) soils in the west-ern half of Texas where most of the pecans are grown in the state (45–47) Soil-incorporated ZnSO4

forma-at 91 kg per pecan tree did not bring the zinc content of the soils to an adequforma-ate level because thezinc was transferred from the sulfate form to sparingly soluble ZnCO3(48)

Five rates of ZnSO4and three rates of S were supplied to pecan trees in March 1966 in a singleapplication to soil (deep Tivoli sand, pH 8.2; mixed thermic, Typic ustipamments) in Dawsoncounty, Texas (south plains) (49) In the absence of applied sulfur, adding of ZnSO4in excess of

20 kg per tree was required to raise zinc concentrations in leaflets in June or September 1966 above

to reach 60 mg kg⫺1to 18.8 kg per tree with 4.5 kg S per tree and to 16.2 kg per tree with 11.9 kg Sper tree Leaflets collected in September 1967 contained more than 60 mg Zn kg⫺1if ZnSO4wasapplied in March 1966 at rates greater than 4.8 kg per tree However, in 1967, at any given rate ofZnSO4(above 1.4 kg per tree), leaflet zinc concentration was reduced by the addition of sulfur, butthe concentrations of zinc in the leaflets remained above the minimum optimum level This studyindicates that leaflet zinc of pecan trees in calcareous soils can be increased by soil applications ofZnSO4, but that a larger increase will occur if S is applied with ZnSO4 On the other hand, soil appli-cations seemed impractical considering the fact that with a planting of 86 trees per ha, an applica-tion of 120 kg of ZnSO4ha⫺1would be required In acid soils of the southeastern United States, highrates of soil-applied zinc may be responsible for the elusive mouse-ear symptom in the acid soils ofthe southeastern United States (50) These results agree with Sommers and Lindsay (51), whoreported that in soils with high concentrations of heavy metals, nickel will compete with zinc forchelation in acid soils and that cadmium and lead will do the same in alkaline soils.

15.7 PHOSPHORUS–ZINC INTERACTIONS

The higher phosphorus content in zinc-deficient plants supplied with high phosphorus can to somedegree be attributed to a concentration effect (52) However, the main reason for the high concen-tration in the leaves is that zinc deficiency enhances the uptake rate of phosphorus by the roots andtranslocation to the shoots (53) This enhancement effect is specific for zinc deficiency and is notobserved when other micronutrients are deficient Enhanced phosphorus uptake in zinc-deficientplants can be part of an expression of higher passive permeability of the plasma membranes of rootcells or impaired control of xylem loading Zinc-deficient plants also have a high phosphoruscontent because the retranslocation of phosphorus is impaired

15.8 TRYPTOPHAN AND INDOLE ACETIC ACID SYNTHESIS

The most distinct zinc deficiency symptoms are ‘little leaf’ and ‘rosette’ in pecans and peaches(Figure 15.1 and Figure 15.2) These symptoms have long been considered to represent problems

in indole acetic acid (IAA, auxin) metabolism However, the mode of action of zinc in auxin olism is unidentified Retarded stem elongation in zinc-deficient tomato (Lycopersicon esculentum

metab-Mill.) plants was correlated with a decrease in IAA level, but resumption of stem elongation andIAA content occur after zinc is resupplied Increased IAA levels preceded elongation growth uponresupply of zinc (54), which would be expected if growth was a response of increased supply ofauxin caused by application of zinc Low levels of IAA in zinc-deficient plants are probably theresults of inhibited synthesis of IAA (55) There is an increase in tryptophan content in the dry mat-

ter of rice (Oryza sativa L.) grains by zinc fertilization of plants grown in calcareous soil (56) The

lower IAA content in zinc-deficient leaves may be due to the biosynthesis of IAA tryptophan (57).Lower IAA contents may be the result of enhanced oxidative degradation of IAA caused by super-oxide generation enhanced under conditions of zinc deficiency (55)

15.9 ROOT UPTAKE

Zinc absorbed by pecan seedlings was translocated predominately to the youngest, physiologicallyactive tissue, in agreement with the results of Millikan and Hanger (35), who worked with

subterranean clover (Trifolium subterraneum L.) Autoradiograph and radio assays revealed

varia-tion between seedlings of open pollinated pecans with respect to rate of Zn absorpvaria-tion (37) Forexample, one set of seedlings absorbed extremes from 0.7 to 91 mg Zn kg⫺1if roots were exposed

to 65Zn in a beaker of water for 96 h