UNIVERSITY OF CALIFORNIA, RIVERSIDE. Botany. Department of. and. Plant Sciences.

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1 UNIVERSITY OF CALIFORNIA, RIVERSIDE Department of Botany and Plant Sciences

2 $Plant Growth Regulator $ Strategies and Avocado Phenology and Physiology $ $ Carol Lovatt Professor of Plant Physiology $ University of California $ $ $ Riverside carol.lovatt@ucr.edu $ $

3 Dr. Carol Lovatt and the California Avocado Commission remind you that no Plant Growth Regulator discu Dr. Carol Lovatt and the California Avocado Commission remind you that no Plant Growth Regulator discussed in this presentation is registered for use on avocado. The use of any unregistered Plant Growth Regulator in commercial avocado production is against the law and would place the entire California avocado industry in jeopardy. Provision of information on the use of Plant Growth Regulators in avocado production is NOT a recommendation and Dr. Carol Lovatt and the California Avocado Commission accept no liability for commercial or personal loss resulting from the use of an unregistered Plant Growth Regulator. mmercial avocado production is against the law and would place the entire California avocado industry in jeopardy. Provision of information on the use of Plant Growth Regulators in avocado production is NOT a recommendation and Dr. Carol Lovatt and the California Avocado Commission accept no liability for commercial or personal loss resulting from the use of an unregistered Plant Growth Regulator.

4 PGRs and Plant Hormones PGRs are synthetic analogs of naturally occurring hormones applied to mimic the effects of hormones Hormones are chemical compounds that act in very small amounts to coordinate the plant s physiology, growth, development and responses to the environment, including stress There are 5 classic groups of hormones: auxins, gibberellins, cytokinins, abscisic acid and ethylene

5 Growth-promoting Hormones Auxins Indole-3-acetic acid (IAA) cause growth by cell elongation; pollen tube growth; fruit set and reducing fruit drop; increase fruit size; inhibit bud break of lateral buds (floral or vegetative) 2,4-D (CitrusFix ) NAA (Tre-Hold A112 )

6 Growth-promoting Hormones Gibberellins Gibberellic acid (GA), more than 120 GAs cell enlargement; leaf expansion; break seed and bud dormancy; stimulate vegetative and floral shoot growth; fruit set and size GA 3 (ProGibb ) GA-biosynthesis inhibitors Prohexadione-calcium (Apogee )

7 Growth-promoting Hormones Cytokinins promote cell division; prevent senescence (aging); stimulate bud break to increase floral and vegetative shoot growth; increase fruit set and size; protect plants from abitotic stress 6-Benzyladenine (6-BA) (MaxCel )

8 Growth-inhibiting Hormones Abscisic acid (ABA) inhibits growth; causes dormancy of seeds and buds; causes bud abscission; protects plants from water-deficit stress by closing the stomates in the leaves to conserve the tree s water content S-ABA (ProTone SL )

9 Growth-inhibiting Hormones Ethylene fruit maturation and ripening; promotes senescence; causes abscission of flowers, fruit and leaves; involved in repairing wounded tissues, plays a role in plant defenses and responses to stress. Ethylene biosynthesis inhibitors Aminoethoxyvinylglycine (AVG, ReTain )

10 PGRs in Tree Crop Productivity PGRs are powerful tools for manipulating tree growth to increase yield, fruit size and grower profit. Each PGR has multiple effects depending: on its concentration relative to other hormones in the tree on its application time relative to tree phenology tree health climate

11 Phenology and Physiology Understanding the phenology and physiology of the tree is critical for identifying the hormone(s) regulating a physiological process for selecting the correct PGR and properly timing its application to elicit the desired result annually

12 When using PGRS Concentration is critical, but timing is everything! PGRs are effective over a very narrow range of concentrations The window of opportunity in tree phenology may be very small Not applying a PGR at the proper time or at the correct rate is why PGR treatments frequently fail

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14 Fruit Dry Wt (g/fruit) Total Flesh Peel Seed Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul

15 Avocado Produces Two Types of Floral Shoots Indeterminate floral shoot Determinate floral shoot

16 Faster growing fruit set and persist to harvest; slower growing fruit abscise early in their development.

17 Hormone Profiles of Faster-growing Fruit Flower development Stage I Cell division Stage II Cell expansion Stage III Fruit maturation (ripening) anthesis high IAA export for sink strength low ABA for growth IAA ABA high GA for growth GA low ethylene for fruit retention Ethylene high cytokinin for sink strength Cytokinins

18 Fruit Abscission in Tree Crops Four abscission periods: Flower abscission Bloom Early drop Stage I June drop Stage II Mature fruit drop (pre-harvest drop) Stage III

19 Hass Avocado Off-crop Year Flowers abscised (no./d) Immature fruit abscised (no./d) Apr May Jun Jul Aug Sep Oct Nov Temperature ( C)

20 Temperature ( C) Apr May Jun Jul Aug Sep Oct Nov Flowers abscised (no./d) Immature fruit abscised (no./d) Hass Avocado On-crop Year

21 CDFA-DPR Required Data At least two sites, representing the diversity of avocado acreage Treatment results must be statistically significant as annual, cumulative or when averaged across multiple years to compensate for alternate bearing; specific for the on- or off-crop Need to demonstrate dose response; the label rate must be statistically significantly greater than the untreated control

22 Goal of Our Research Is to demonstrate that one, or more, PGR is effective and reliable in increasing yield and/or fruit size and grower income Have Hass avocado added to an existing label for a commercial PGR registered for use in California

23 GA 3 25 mg/l cauliflower stage of inflorescence development or when fruit are mm in diameter increases total yield and yield of commercially valuable fruit of packing carton sizes as lbs and number per tree Greatest increase in on-crop year Slight increase in the off-crop year

24 Additional Benefits of GA 3 Application Effect on setting crop increases yield and size, stimulates leaf development, protects fruit from sunburn Effect on maturing crop reduces drop, increases size, delays fruit blackening with no effect on days to ripen or fruit quality

25 AVG and 2,4-D increase yield of fruit of packing carton sizes AVG 250 mg/l applied at full bloom or when fruit are mm in diameter 2,4-D 38 g ai/acre when fruit are mm in diameter On-crop year only; no reduction in yield

26 PGRs that increase yield of fruit of packing carton size Benzyladenine 25 mg/l applied at full bloom or when fruit are mm in diameter GA 3 25 mg/l when fruit are mm in diameter followed by 125 mg/l Prohexadione-Ca 30 days later On-crop year only; no reduction in yield

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30 Use of PGRs in Alternate Bearing Alternate bearing is a problem of economic consequence On crop = large crop of small fruit Off crop = good size, but too few fruit Loss of stable income Loss of market share Hinders development of value-added products Complicates orchard management

31 Initiation of Alternate Bearing Initiated by conditions that: limit crop production too little chilling for sufficient bloom adverse climate during flowering and fruit set excessive June drop lack of water inadequate fertilization excessive pruning cause excessive production optimal chilling optimal conditions during flower and fruit set no June drop Late harvest exacerbates alternate bearing

32 Perpetuation of Alternate Bearing Once initiated, alternate bearing is perpetuated by mechanisms within the tree that impact floral intensity. The goal of our research has been to identify these mechanisms for Hass avocado. Since climate is a major factor initiating alternate bearing, there is a recurring need for a strategy to mitigate it.

33 Effects of Crop Load starting at spring bloom Trees are setting an off-crop Trees are setting an on-crop Tagged shoots

34 Measurements Off-crop Summer vegetative shoot growth On-crop Tagged shoots

35 Off-crop Measurements Summer vegetative shoot growth On-crop Tagged shoots

36 Off-crop Measurements Summer and Fall vegetative shoot growth On-crop Tagged shoots

37 Return Bloom the Following Spring For both OFF- and ON-crop trees, On-crop trees produce an offcrop bloom Off-crop trees produce an oncrop bloom ~70% of return bloom and ~80% of determinate floral shoots are produced by the Summer vegetative shoots

38 Summer on-crop year Cytokinins IAA > Cytokinins Buds inhibited Cytokinins IAA Next spring on-crop trees produce an off-crop bloom Cytokinins Cytokinins IAA

39 Summer off-crop year Cytokinins IAA < Cytokinins Next spring off-crop trees produce an on-crop bloom

40 Spring following the on-crop year for late harvested cultivars Cytokinins IAA > Cytokinins ABA > Cytokinins Buds inhibited Cytokinins ABA

41 Overcoming Alternate Bearing Two PGR treatments are required during the on-crop year: Summer (June-July) to stimulate summer shoot growth Late winter (January and/or February) to stimulate bud break of floral shoots

42 Effect of January PGR trunk injections to increase spring bud break and floral shoot number of on-crop Hass avocado trees. Treatment Floral Floral (spring shoots) Floral (summer shoots) Determinate floral Floral (with fruit) Floral (without fruit) No. of shoots per 100 nodes On-crop Control Cpd X + Y P-value * NS * * * NS * Means within a column are significantly different by Fisher s Protected LSD Test P 0.10.

43 Alternate Bearing Research Currently testing synthetic and natural cytokinins and anti-auxins for their capacity to stimulate summer shoot growth, promote floral development, and/or increase spring bud break during the on-crop year to increase return bloom and yield the following year. The goal is a PGR strategy to mitigate alternate bearing without having to reduce crop load in the on-crop year.

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