Plant Growth Regulators
Table of Contents
What is a plant growth regulator?
Plant growth regulators (PGRs) are chemicals used to modify plant growth such as increasing branching, suppressing shoot growth, increasing return bloom, removing excess fruit, or altering fruit maturity. Numerous factors affect PGR performance including how well the chemical is absorbed by the plant, tree vigour and age, dose, timing, cultivar, and weather conditions before, during, and after application.
Plant growth regulators can be grouped into five classes: compounds related to auxins, gibberellins and inhibitors of gibberellin biosynthesis, cytokinins, abscisic acid and compounds affecting the ethylene status. Products that block the biosynthesis of plant hormones are also available (Apogee, Retain)
The five groups of plant growth regulators used in fruit crops include:
Auxins: These are growth promoting substances that contribute to the elongation of shoots, but at high concentrations they can inhibit growth of lateral buds. In addition to being used as plant growth regulators, auxins can also be herbicides (2, 4-D etc.). In apple production napthaleneacetic acid (NAA) is a synthetic auxin that can be used to thin fruit and prevent fruit drop shortly before harvest. For more information on the use of products for thinning see Ontario.ca/apples and find Thinning of Tree Fruit.
Gibberellins: Gibberellins (GA) promote cell elongation, shoot growth, and are involved in regulating dormancy. Promalin®/Perlan® (containing GA4+7 and 6-benzyladenine) have been used to improve fruit size and reduce russetting in apples. ProGibb 40SB and Falgro (containing GA3) are used to delay ripening, improve fruit firmness and extend the harvest period in sweet cherries. Gibberellins are used in tart cherries to manage flowering to avoid over production. Apogee® (Prohexidione-calcium) inhibits the biosynthesis of gibberellins. Apogee is used to modify the morphology of trees (apple and cherries) and to control runner production in strawberries.
Cytokinins: Cytokinins promote cell division. Cytokinins are involved in branching and stimulating bud initiation. They are used as fruit thinners (Maxcel® or Cilis Plus® 6-BA) in apples. For more information on the use of these products for thinning see Ontario.ca/apples and find Thinning of Tree Fruit.
Absicisic Acid: Absicsic acid controls the dormancy of buds and seeds, inhibits shoot growth and is involved in regulating water loss from plants.
Ethylene: Ethylene promotes abscission of leaves and fruits, inhibits shoot elongation and inhibits lateral bud development. In apples and cherries, ethylene is involved in the transition of fruit from being physiologically mature to ripe. Ethephon (Ethrel®) is a synthetic compound that releases ethylene upon application. Retain interferes with ethylene biosynthesis and allows fruit to hang on trees longer and lengthens storage life.
There are a variety of plant growth regulators registered for use in fruit crops in Ontario.
Improving fruit shape
Promalin® and Perlan®
Promalin and Perlan (a combination of Benzyladenine and Gibberellins A4A7) are plant growth regulators used to improve the shape of apples through elongation and development of more prominent calyx lobes on apples that have a natural typiness like Red Delicious, Golden Delicious, Ambrosia and Gala. Perlan is only registered on Red Delicious. For satisfactory results, time the application carefully, follow label directions and ensure complete spray coverage.
Precautions: If Promalin or Perlan are applied at higher rates or volumes than those recommended on the label or where blooms are weak or frost-injured, fruit thinning may occur. Promalin or Perlan use may also increase the amount of thinning achieved with subsequent blossom thinning sprays. Apples may not respond to Promalin or Perlan if spur vigour is low or the king blossoms have been damaged by frost. Do not apply Promalin or Perlan if rain is expected within 6 hours. Do not apply when air temperatures are lower than 24°C or greater than 32°C.
Russeting can occur on susceptible varieties (Golden Delicious) during cool, wet or humid weather at bloom and early stages of fruit development. Promalin can reduce russeting by increasing the epidermal cell density in the skin. Promalin cannot reduce russeting caused by frost damage, disease, herbicide drift, or phytotoxicity.
Precautions: Do not apply Promalin when air temperatures are below freezing or greater than 32°C. Rainfall or overhead irrigation within 6 hours after application will often reduce the activity of Promalin. A pH range between 7.0 and 8.5 will provide optimum results.
Feathered nursery trees are important for most high density apple planting systems. Nursery trees should ideally have 10 to 15 feathers per tree to allow for adequate production in the second and third years. Promalin can be used to stimulate lateral bud break and additional branch growth on young trees, resulting in improved branch angles, and providing a better tree structure for early cropping.
Precautions: Refer to precautions listed above under Reduce russeting in apples.
Apogee (Prohexadione-calcium) is a plant growth regulator that inhibits the synthesis of gibberellins. Trees treated with Apogee often have the same number of shoots as untreated trees, but shoots from treated trees are thicker or greater in diameter and are shorter from having compressed internodes. Apogee does not reduce the number of leaves or fruit size. When used properly in apple orchards, Apogee can:
Note: Apogee has no activity against the fire blight bacteria or blossom blight.
- = No activity
Precaution: Do not tank-mix Apogee with calcium sprays like calcium chloride. In the presence of calcium, Apogee will precipitate in the tank, clog nozzles and screens, and reduce tree response.
In some instances, Apogee may increase fruit set and make thinning more difficult. This response is not consistent but is more likely at concentrations above 125 ppm (45 g per 100 L). Apogee-treated trees may require more aggressive thinning to reduce the crop load to the desired level.
Apogee can cause severe cracking on Empire and Stayman cultivars. The cause is unclear, but may be related to environmental conditions. Apogee may result in decreased yield and marketable yield of Cortland. Despite this, clear benefits of the use of Apogee on tip-bearing cultivars such as Cortland and Northern Spy have been observed in areas where the shortened internodes of Apogee-treated trees have produced a more compact tree habit.
NAA (napthaleneacetic acid) is an auxin plant growth regulator that will effectively promote return bloom when used at the thinning stage in trees that are biennial bearing. For biennial bearing trees, a year of full bloom and heavy fruit set would be considered an "on-year" and an "off-year" is where the bloom and fruit set is light. Application of NAA will be the most beneficial in an "on-year" to promote return bloom for the following season in the "off-year".
Precaution: Do not apply during the same growing season as MaxCel or Cilis Plus or pygmy fruit may result.
Ethrel (ethephon) can be used to enhance bloom on non-bearing trees.
Promoting fruit colouring
Ethrel (ethephon) can be used to stimulate ripening and improve fruit colour on apples that will be marketed immediately. Apples treated with Ethrel will not store for long and fruit may drop prematurely.
Blush (Prohydrojasmon) is used to increase anthocyanin production and colour development in bi-colour apples like Fuji, Gala, Honeycrisp and McIntosh. It can also help to overcome colour delay in ReTain treated apples. Blush enhances chlorophyll degradation, as well as anthocyanin and carotene accumulation which contribute to a redder apple. Improvement in colour may not be seen if the strain of the variety is known to produce a good colour, or your orchard gets good colour development due to environmental conditions.
Application: Make 1-2 applications of Blush, 7-28 days before anticipated harvest at a rate of 190-381 mL/100 L of water (100-200 ppm). If making two applications, apply the first application 21 to 28 days before harvest followed by a second application 7-14 days later. The first application should be made once red colour starts to develop. Results are variable and can be impacted by variety and environmental conditions. Do not apply to trees that are injured or under stress from excessive heat, drought, disease or nutrient deficiency. For best results, apply during slow drying conditions, and avoid application during the hottest part of the day.
Chemical control of preharvest drop of apples
Cultivars differ in their tendency to drop apples before harvest. McIntosh and Honeycrisp are the cultivars of greatest concern in Ontario. Conditions associated with premature fruit drop include:
Products such as Fruitone-L ® and Fruit Fix Concentrate contain NAA (naphthaleneacetic acid) and are registered to control drop on apples.
Precaution: NAA inhibits fruit drop but the fruit continues to mature at an accelerated rate. The higher the concentration used and the greater the number of applications, the greater the ripening effect. One application of 10 ppm has little effect on direct ripening.
Apples treated with NAA for preharvest drop will not keep well in long-term storage, especially McIntosh. Apples treated twice with NAA should be sold immediately. Do not apply NAA closer than 5 days before harvest. Do not apply more than 2 times per season.
ReTain (Aviglycine hydrochloride) is an effective preharvest drop control agent for apples such as Gala, Honeycrisp, McIntosh, Red Delicious and Northern Spy. It reduces the production of ethylene in maturing apples. Apples treated with ReTain also develop less stem-end cracking and skin greasiness.
Precautions: Do not apply a sun protectant product 3 days before or after a ReTain application. When you mix ReTain with the surfactant, keep agitation to a minimum because Xiameter OFX-0309 has a tendency to foam. To minimize foam, add Xiameter OFX-0309 last, when the tank is full. Compatibility and performance data for ReTain with antifoam products are not available. Do not use surfactants other than Xiameter OFX-0309 because they are not registered with ReTain in Canada. If the rate of ReTain is reduced, maintain the Xiameter rate at a 0.05-0.1% v/v ratio (5-10 L/ 1,000 L water).
Discard any unused spray material at the end of each day.
Do not tank-mix with other products as combination effects and potential interactions between ReTain and NAA or ethephon products have not been thoroughly evaluated.Use of these products on ReTain-treated blocks may negate or reduce the benefits of ReTain.
Do not use overhead irrigation for 8 hours after application.
Adjust spray water volumes based on tree size and spacing. Do not apply to the point of runoff but wet the tree under slow drying conditions.
Use tree-row volume (TRV) reductions or Crop Adapted Spraying cautiously and test on a few rows or small blocks to gain a level of comfort with this technique. For many orchards, 1,000 L water per ha is adequate for size-controlling rootstocks.
For best results, have spray water at a pH between 6 and 8.
To maximize the benefits of using ReTain, segregate treated fruit from untreated fruit in storage to reduce exposure to ethylene.
Only spray trees that are healthy and not under severe drought, insect, disease or nutritional stress. Apply treatments to Gala strains, Golden Delicious types and Honeycrisp with caution. These cultivars appear to be more responsive to ReTain compared to McIntosh. Using ReTain on these sensitive cultivars can significantly impede crop maturity and development.
The harvest window for treated cultivars is shorter but delayed and results in fewer pickings. This can increase harvest efficiency. ReTain is not likely to compensate for the effects of an aggressive calcium spray program to control bitter pit. Calcium sprays can advance apple maturity by accelerating ethylene production in maturing fruit.
Harvista (1-Methylcyclopropene or 1-MCP) inhibits sensitivity to ethylene which slows down ripening. The active ingredient, 1-MCP, is also in SmartFresh which is applied post-harvest to apples in a sealed storage room to improve storage performance. Harvista can delay harvest 7-14 days from the anticipated harvest date. It may also reduce preharvest drop, reduce fruit ethylene production, allow for additional time to develop colour and increase size, maintain fruit firmness, delay starch hydrolysis, delay onset or reduce incidence of watercore and enhance storage potential. It can impact or delay red colour development on bi-colour apples.
Precaution: Do not apply when temperatures are above 35?C. Do not allow the product to come into contact with copper.
Modifying tree growth
Precaution: Promalin mixed with latex paint and applied directly to buds is not registered for pears and only registered for apples (nursery and orchard) and sweet cherries (orchard only). Refer to precautions listed under reducing russeting in apples.
Runner suppression in strawberries
Strawberry plants treated with Apogee (Prohexadione-calcium) have significantly shorter runners than untreated plants. Apogee is recommended when runners are no longer needed to increase plant density.
Precaution: Apogee is a relatively new tool for Ontario growers. Test Apogee on a small scale and leave untreated checks in order to evaluate the effectiveness on different varieties and production systems.
Always tank-mix Apogee with an equal amount of ammonium sulphate by weight. Ammonium sulphate (AMS) can improve efficacy as Apogee is impacted by hard water. Use a high-quality, greenhouse grade AMS to avoid plugging of nozzles. Do not tank-mix Apogee with calcium sprays like calcium chloride, which will reduce Apogee's effectiveness.
ProGibb® 40 SG and Falgro®
ProGibb 40 SG and Falgro (Gibberellic acid) delays fruit ripening 4-5 days and thus extends the picking period, which may delay the susceptibility to rain cracks. The treatment also increases fruit size, firmness and resistance to postharvest disorders.
Vegetative growth control
Apogee (Prohexidione-calcium) can be used to reduce terminal growth in sweet cherries. Reduction in terminal growth should help reduce the time required to dormant prune as well as open up the tree canopy, leading to improved spray coverage and reduced disease pressure. For sweet cherries, growers can expect a significant reduction in terminal shoot growth - ranging from 20-50% in a given season by using Apogee at the proper timing and concentration.
Precautions: A spray adjuvant (Agral 90, LI-700) should be included to improve plant uptake of Apogee. In addition, where a high calcium or magnesium water source (hard water) is used, it is important to include an equal amount of ammonium sulphate (AMS) fertilizer by weight with Apogee. Effects of Apogee on fruit set, fruit size and yield are inconsistent based on results reported in the literature. Several studies have reported a resurgence in late-season growth in some situations when marked suppression of growth was achieved early in the season. Furthermore, some resurgence in growth the year following application has been observed. Therefore, fine-tuning of treatment timings and concentrations may need to be considered.
Ethephon (Ethrel) can be used in tart cherry orchards to promote fruit loosening, uniform maturity and to facilitate mechanical harvest.
Precaution: Trees respond more quickly to Ethrel applications in higher temperatures. For this reason, apply Ethrel only in the temperature range of 18-30ºC. Tree vigour also affects the effectiveness of Ethrel treatment. Do not spray trees that have low vigour or are severely stressed by drought, disease or winter injury, which is indicated by gumming on the trunk and scaffold limbs. Treat only trees that are vigorous and in good health.
Moderating early production
ProGibb® 40 SB and Falgro®
ProGibb 40 SB and Falgro (Gibberellic acid) are applied in the
fourth year to moderate early production. This allows flowering
in year 5 at a reasonable level rather than allowing heavy bloom
and production. Overproduction can significantly reduce growth in
Precaution: Do not harvest within 21 days of application.
Apogee (Prohexidione-calcium) can be used to reduce terminal growth in tart cherries. Reduction in terminal growth should help reduce the time required to dormant prune, as well as open up the tree canopy leading to improved spray coverage and reduced disease pressure. Limited research is available on the response of Montmorency tart cherries to Apogee.
Precautions: Do not harvest within 20 days of application. Calcium sprays should not be tank-mixed with Apogee. Refer to precautions listed under vegetative control of tart cherries.
Plant growth regulators play an important role in the production of high quality trees and fruit. Read the product label and follow all safety precautions. For additional information on the timing and use of plant growth regulators refer to the crop calendars in OMAFRA Publication 360 Guide to Fruit Production.
This information was written by Kathryn Carter and Erica Pate, Fruit Specialists, OMAFRA; and Amanda Green, Tree Fruit Specialist, OMAFRA.
Thanks to Dr. John Cline, University of Guelph, Simcoe, Leslie Huffman and Margaret Appleby (formerly of OMAFRA), and Dr. Julia Reekie, Agriculture and Agri-Food Canada-Kentville for their collaboration.
For more information:
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