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Estrategias en Florida para mitigar los efecto de la deficiencia de heirro, boro, y zinc en suelo calcáreos [Strategies in Florida to mitigate the effect of iron, boron, and zinc deficiencies in calcareous soil] Dr. Jonathan H. Crane, Tropical Fruit Crop Specialist University of Florida Institute of Food and Agricultural Sciences Tropical Research and Education Center Homestead, Florida, USA

Outline of presentation Characteristics of avocado anatomy and growth that impact boron, iron, and zinc nutrient uptake. Characteristics of avocado physiology that influence boron, iron, and zinc uptake. Characteristics of calcareous soils that influence boron, iron, and zinc availability for uptake by avocado trees. Boron, Iron, and Zinc management for avocado Plant uptake mechanisms roots and leaves. Preventing, correcting and maintaining sufficient boron, iron, and zinc for avocado trees. Suggested recommendations for chronic and temporary boron, iron, and zinc deficiencies.

Anatomical characteristics that affect avocado nutrient uptake Lack of root hairs. Suberized (brown) roots> apical non-suberized (white) root surfaces. Shallow root system. Thick waxy leaf cuticle. Lack of stomata on upper leaf surface. Short-lived leaves with annual leaf drop.

Physiological characteristics that affect nutrient uptake Rootstock effects Inconsistent rootstock effect for most nutrients. Some evidence of differences in Fe, K and B uptake. Evidence that rootstocks of West Indian origin are more tolerant of saline and highly calcareous soils than rootstocks of Mexican (intermediate tolerance) and Guatemalan (least tolerant) origin. Examples of rootstocks tolerant to high ph calcareous soils include: Moaz (VC43), En Harod (VC28), VC6, VC65, and Waldin seedlings. Sensitivity to ambient soil and atmospheric temperatures, e.g., clonal Velvick (Guatemalan) maintained a superior water status than Duke 7 (Mexican) under 13-28 C soil and 28 C air temperatures.

Relative effect of rootstock on scion leaf nutrient uptake Avocado race* Nutrient West Indian Guatemalan Mexican B - H L Fe H L M Zn H - L *, L, low; M, medium; H, high. Data from: Lahav, E. and A.W. Whiley. 2002. Irrigation and mineral nutrition. In: The avocado: botany, production and uses. A.W. Whiley, B. Schaffer, and B.N. Wolstenhome (editors). CABI Publishing, New York, NY. p.231-298.

Optimum temperature for Physiological characteristics that affect nutrient uptake Avocado race West Indian Guatemalan Mexican Root growth 20 C 18-28ºC Shoot growth 18-20 C 21/14 C-33/26 C (day/night) Carbon assimilation Pollination and fertilization 22 C? 15-30 C ~20-31 C

Characteristics of soils that influence B, Fe, and Zn uptake Soil type sandy, clay, loam. Soil ph. Soil drainage and moisture characteristics. Organic matter content. Mn, Cu, Ca, Mg, K and Zn content. Characteristics of calcareous soils High ph ( 7). High bicarbonate content ( 20%) and buffering capacity. Calcium and possibly other elements (e.g., P, Mn) dominate the cation exchange capacity. Irrigation water with high Ca and/or bicarbonate content.

Boron Needed in high concentrations in apical growing points anthers, stigmas, ovaries, maturing leaves and fruit. Two forms inside plants. bound-b to cell-walls B inside the cell (stored in vacuoles). Effects of B Increases effective pollination but effect on yields inconsistent. Shown to increase fruit size by about 15% and increase postharvest shelf-life by 4 to 6 days.

Leaf nutrient ranges for boron (B), iron (Fe), and zinc (Zn)* Element Florida California Compilation** South Africa B*** 40-50 50-100 40-60 70 Fe 40-94 50-200 50-200 - Zn 71-186 30-150 40-80 - *, mg/kg (ppm); **, Compilation Australia, Israel, + California; ***; inflorescence 50-75 mg/kg, peduncle 26 mg/kg.

Boron uptake mechanisms Root uptake is largely passive as a part of the transpiration stream in the xylem. Moves into the leaf cells by metabolic uptake. Boron is stored in leaf cell vacuoles. B moves via the phloem from mature leaves to inflorescences. Foliarly applied B moves into the leaf by diffusion and into the leaf cell by metabolic uptake.

Factors that affect B uptake Soil B is most available between 5-7 ph and >8.5. B less available below ph 5 and between 7.5-8.5. B leaches from sandy acid soils. In calcareous soils B complexes with Ca-bicarbonate. In clay soils B (borate) binds with clay particles. Low soil moisture decreases B availability. Excessively wet or compacted soils with low aeration decrease root activity and B uptake. Low soil temperatures suppress root activity.

Preventing, correcting, maintaining sufficient plant B content Rootstocks of Guatemalan origin are more efficient at B uptake than those of Mexican genotype. Improve soil drainage to optimize rhizosphere aeration. Determine and monitor soil B content and soil ph and leaf B content. Monitor soil and aerial temperatures. In chronically deficient orchards make 2 to 3 B applications to the soil per year until tissue samples indicate sufficient B, then reduce the rate by 75%. One of the soil B applications should be made 1 to 2 months prior to flower-bud formation. *If foliar applications are needed apply 1 time to early-stage of inflorescence emergence (cauliflower stage). *Florida

Preventing, correcting, maintaining sufficient plant B content Soil applications 4 to 6 kg/ha Borax split into 2 to 3 applications. Foliar application one application of 4 to 30 g Solubor per tree at cauliflower stage of flowering. Form % B Borax 11 Na- tetraborate 14 Na- pentaborate 18 Solubor 20.5 Boric acid 17

Temperatura media (C) Amount of development 30 25 Temperaturas de Cabildo Temperaturas óptimas de tierra Temperaturas suelo Temperaturas atmosphera 20 15 10 Root flush 5 0 May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr 2 nd app. Mes 1 st app. Flower bud development Flowering and fruit set Ground applied boron

Temperatura media (C) Amount of development Temperaturas de Cabildo 30 25 Temperaturas óptimas de atmosphera Temperaturas suelo 20 15 10 Temperaturas atmosphera 5 0 Flower bud development 1 app. May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr Flowering and fruit set Mes Foliar applied boron Vegetative flush

Iron Needed in high concentrations for chlorophyll synthesis and carbon assimilation in apical growing points anthers, stigmas, ovaries, maturing leaves and fruit. Two forms inside plants: bound-fe 3+ (ferric) to cell-walls in the apoplast free-fe 2+ (ferrous) available for cell uptake and stored as phytoferritin in the cell. Effects of Fe There is limited data to show leaf iron content is correlated with avocado bloom and yield [2 Guatemalan-West Indian cultivars in Florida]. Not mobilized from older to newer leaves and therefore required for new growth.

Factors that affect Fe uptake Soil Fe is most available in neutral to acid soils. less or not available at high ph >7.0 especially in calcareous, high bicarbonate content soils. Highly weathered sandy soils may be Fe deficient. In calcareous soils Fe 3+ forms complexes with Ca (calcium bicarbonate) and phosphates, and precipitates as hydrous iron oxides. Low soil moisture decreases iron availability. Excessive soil moisture (low soil O 2 content) increases available iron (Fe 2+ ) content but decreases root function or leads to root death. Compacted soils with low aeration also decreases root activity. Cool soil temperatures suppress root growth and activity. Soil Mn, Cu, Ca, Mg, K and Zn may compete with Fe for plant absorption.

Iron uptake mechanisms In soils with low available iron, avocado roots secrete H + to create a reduced rhizosphere conducive to ferrous (Fe 2+ ) iron formation and uptake. Root and leaf cell iron uptake is a metabolic process (Fe 3+ to Fe 2+ ). Root absorbed iron moves in the xylem and apoplast as Fe 3+ -citrate. Foliarly applied iron moves into the leaf by diffusion and into the leaf cell by active uptake.

Normal leaf-iron uptake Moderately acid ph Chlorophyll Fe 3+ +Citric acid Ferric chelate reductace Fe 2+ Respiration Enzyme systems Xylem/apoplast Cell wall Cell membrane Cell cytoplasm

Leaf-iron uptake in a high ph environment High ph Little to no Fe 3+ +Citric acid High ph Ferric chelate reductace activity depressed High ph Fe 2+ Chlorophyll Respiration Enzyme systems Xylem/apoplast Cell wall Cell membrane Cell cytoplasm

Iron paradox Trees growing in calcareous soils often have sufficient total (extractable) leaf iron. Iron deficient avocado trees have the same or higher leaf iron content. However the iron is in the ferric (Fe 3+ ) form and cannot enter the plant cells. Labs only assay for total iron.

Iron applications Soil applications non-chelated iron fertilizers do not prevent or alleviate iron deficiency. Foliar applications Non-chelated or chelated iron fertilizers alone do not work. Iron injections (chelated or non-chelated) have provided inconsistent results, multiple injection sites are necessary, and tree damage may result. EDDHA-chelated iron [(technical sodium ferric ethylenediamine di-(o-hydroxyphenylacetate)] as a soil drench works. However, it is very expensive.

Potential alternative solutions Krome very gravelly loam (Oolitic limestone) Acidify the soil profile with sulfur and or acid forming fertilizers. Depends upon buffering capacity of the soil. This does not work in oolitic limestone based soil. Why? ~70% CaCO 3 overwhelming buffering capacity.

Potential solutions Reducing the apoplastic ph with foliar dilute acids such as: Sulfuric (liquid, 36% conc.) Citric (dry) Ascorbic acid (dry) has been shown to decrease apoplastic ph and re-green leaves of kiwi, pear, orange, and peach.

EFFECT OF FOLIARLY-APPLIED ACIDS AND FERROUS SULFATE ON IRON NUTRITION OF AVOCADO TREES B. Schaffer, J.H. Crane, Y.C. Li, E.A. Evans, W. Montas, and C. Li University of Florida, IFAS, Tropical Research and Education Center, Homestead, Florida, USA

Objectives To test the effects of foliar applications of weak acids or weak acids plus iron sulfate on re-greening leaves on ferrous and total leaf iron content.

Materials and Methods A commercial avocado orchard. 11-year-old Donnie (West Indian) avocado trees grafted on Waldin (WI) seedling rootstock.

Materials and Methods 2006 + 2007 Treatments* Acid material Application method SA+Fe+Freeway Sulfuric acid (SA) Foliar CA+Fe+Freeway Citric acid (CA) Foliar AA+Fe+Freeway Ascorbic acid (AA) Foliar AA+Freeway Ascorbic acid Foliar Chelated Fe EDDHA-Fe Soil drench Control None None * All treatment (except Control) materials were diluted with water from a well and the adjuvant Freeway was mixed with all foliar treatments; Fe=ferrous sulfate.

Materials and Methods 2006 + 2007 Treatments Dates applied (2006) Dates applied (2007) SA+Fe+Freeway CA+Fe+Freeway AA+Fe+Freeway AA+Freeway Fall-early winter Sept. 15, 28; Oct. 12, 26; Nov. 11, 22 (6 total foliar applications at about 2-week interval) Summer-early fall July 18; Aug. 1, 24; Sept. 13; Oct. 23 (5 total foliar applications at 2-4 wk intervals) Soil applied chelated EDDHA Fe Feb. 6; Sept. 15; Oct. 12; Nov. 9 (4 applications) Control NA NA July 17; Aug. 28; Sept. 14 (3 applications)

Materials and Methods 5 to 6 trees per treatment. Measurements included: Leaf chlorophyll index leaf greeness (SPAD). Total iron and ferrous iron content (2006). Data were analyzed by ANOVA and Duncan- Waller multiple range test using SAS.

Total and ferrous leaf iron content Iron content (mg/kg dry wt.) Treatment Total Ferrous SA+Fe+Freeway 287.3a 27.0a CA+Fe+Freeway 188.8ab 22.0ab AA+Fe+Freeway 140.6bc 22.5ab AA+Freeway 58.2c 19.1bc Chelated Fe 69.8c 12.43c Control (no iron) 63.5c 11.6c

SPAD values recorded at 2-week intervals (from 27 October 2006 to 20 November 2006) and on 20 February 2007 for mature leaves of Donnie avocado trees. SPAD values 60 Control (no Fe) SA+Fe AA alone 50 Fe-EDDHA (soil drench) CA+Fe AA+Fe a a 40 ab ab ab ab b ab ab ab 30 ab b a a b a b b Little response during the late fall-winter a b bc cd d e 20 Pre-trt 10 Sept. Oct. Nov. Dec. Feb. Month

SPAD values recorded at about 2-week intervals from 13 July 2007 to 2 Oct 2007 for mature leaves of Donnie avocado trees. 35 Much greater response during the summerearly fall 30 a a ab a a ab SPAD 25 20 15 Pre-trt ab b b c c bc bc c d bc Fe-EDDHA CA+Fe SA+Fe AA+Fe Control (no Fe) AA 10 Jul Aug Sep Oct Nov a a a b c bc bc c Month

Chelated-Fe AA+Fe+Freeway CA+Fe+Freeway Control (no Fe) AA+Freeway SA+Fe+Freeway

Conclusions Conclusions 2006-2007 Foliarly applied Acids+Fe+Freeway may be an option or component of managing iron nutrition of avocado tress. Continued investigation 2008 Modified treatments Only 1 acid - ascorbic acid. Ferrous sulfate only. Freeway only. Investigating the effect of the number of iron applications, the timing of iron applications, and how this correlates with avocado phenology. Continuing to investigate the effect of solution ph and the effect of the adjuvant Freeway.

Preventing, correcting, maintaining sufficient plant Fe content Rootstocks of West Indian origin are more tolerant of calcareous soils than Guatemalan, Mexican or G-M hybrids. Improve soil drainage to optimize root aeration and function. Periodically determine soil Fe content and soil ph and leaf iron content. Monitor soil and aerial temperatures. *In deficient orchards make 2 to 6 chelated iron (EDDHA) applications of 30 to 400 g/tree as a soil drench or fertigation application per year. Time soil iron applications to root flushes and warm soil temperatures to optimize plant uptake. Foliar iron applications may be a component of maintaining leaf iron content in the future. *Florida

Temperatura media (C) Amount of development 30 25 Temperaturas de Cabildo Temperaturas óptimas de tierra Temperaturas suelo Temperaturas atmosphera 20 15 10 Root flush 5 0 May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr 1 st app. 2 nd Mes app. 3 rd app. Flower bud development Flowering and fruit set Ground applied chelated iron

Temperatura media (C) Amount of development Temperaturas de Cabildo 30 25 Temperaturas óptimas de atmosphera Temperaturas suelo 20 15 10 Temperaturas atmosphera 5 0 Flower bud development May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr Flowering and fruit set Vegetative flush Period Mes 1 Period 2 Tentative foliar acid/iron applications

Zinc Needed in moderate concentrations in apical growing points anthers, stigmas, ovaries, maturing leaves and fruit. Two forms of zinc: bound-zn to cell-walls Zn inside the cell for metabolic processes. Effects of Zn Deficiency reduces flower bud formation and may result in deformed fruit and reduced fruit size.

Zinc uptake mechanisms Root uptake is metabolic, forming a Zn-citrate complex and transported passively via the xylem (part of the transpiration stream). However, zinc-citrate is transported actively from cell to adjacent cell via the plasmodesmata. Only very small amounts of foliarly applied Zn are absorbed. Foliarly applied Zn moves into the leaf by diffusion and into the leaf cell by active uptake. Zinc inside leaf epidermal cells has been shown to translocate into adjacent parenchyma cells. Absorbed Zn has also been shown to move to leaves above and below the treated leaf.

Factors that affect Zn uptake Soil solution available Zn decreases with increasing ph, increasing calcium content, and with cool with soil temperatures. Zn is found in 3 soil fractions: Zn 2+ and soluble organic forms, adsorbed and exchangeable forms on clay and colloidal particles, and insoluble Zn-complexes and minerals. Acid sandy soils may be deficient in Zn due to leaching. Soils low in organic matter content may also be Zn deficient. Available Zn decreases in excessively wet soils due to binding with sulfides and bicarbonates and low root aeration decreases root activity. Prolonged dry soil conditions suppresses Zn uptake. High soil concentrations of N and P may depress Zn uptake.

Preventing, correcting, maintaining sufficient plant Zn content Some evidence rootstocks of West Indian genotype are more efficient at Zn uptake than Mexican genotype. Improve soil drainage to optimize rhizosphere aeration. Determine and monitor soil Zn content and soil ph and leaf Zn content. Monitor soil and aerial temperatures. Time soil applications to root flushes during warm soil conditions. *Foliarly applied zinc (e.g., zinc sulfate at 15 g/l) is useful for correcting or preventing Zn deficiency of the treated foliage. To treat chronic Zn deficiency. Apply zinc sulfate as a dry material or through fertigation at 4 to 6 kg/ha split into 2 to 3 applications per year. EDTA chelated Zn is not effective. *Florida

Temperatura media (C) Amount of development Temperaturas de Cabildo 30 25 Temperaturas óptimas de atmosphera Temperaturas suelo 20 15 10 Temperaturas atmosphera 5 0 Flower bud development May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr Flowering and fruit set Vegetative flush Period Mes 1 Period 2 Foliar zinc applications

Gracias por su atención Thank you for your attention

Temperatura media (C) 30 25 Temperaturas óptimas de tierra 20 15 10 5 0 May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr Mes Suelo Atmosfera

Temperatura media (C) 30 25 Temperaturas óptimas de atmosphera 20 15 10 5 0 May Jun Jul Ago Sep Oct Nov Dic Ene Feb Mar Abr Mes Suelo Atmosfera

Amount of development Flower bud development Flowering and fruit set Main harvest Fruit development Fruit drop Vegetative flush Root flush n s Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct May Jun July Aug Sept Oct Nov Dec Jan Feb Mar Apr