Plant growth regulator for fruit crops
What is a plant growth regulator?Plant growth regulators 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.
Characteristics of Plant Growth Regulators
As the plants require oxygen, water, sunlight, and nutrition to develop and grow, they do require certain chemical substances to manage their growth and development. These chemical substances are known as Plant Growth Regulators and are naturally produced by the plants itself.
These are simple organic molecules having several chemical compositions. They are also described as phytohormones, plant growth substances, or plant growth hormones.
They can accelerate as well as retard the rate of growth in plants.
Plants growth hormones or plant growth regulators exhibit the following characteristics:
Differentiation and elongation of cells.
Formation of leaves, flowers, and stems.
Wilting of leaves.
Ripening of fruit.
Seed dormancy, etc.
Generally, there are five types of plant hormones, namely, auxin, gibberellins (GAs), cytokinins, abscisic acid (ABA) and ethylene. In addition to these, there are more derivative compounds, both natural and synthetic, which also act as plant growth regulators.
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.
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 or Kudos 27.5 WDG® (Prohexidione-calcium) inhibits the biosynthesis of gibberellins. Apogee or Kudos 27.5 WDG are 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.
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.
Plant growth regulator use in apples
Improving fruit shape
Promalin and Perlan (a combination of 6-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.
Application:
For optimum results, the first application of Promalin or Perlan should be applied between early king bloom and early petal fall. Applications earlier or later are likely to produce unsatisfactory response.
For Promalin, apply a single application of 1.2-2.3 L/ha. Alternatively, if a prolonged bloom occurs, apply two split applications of 0.6-1.2 L/ha, first one between early king bloom to early petal fall to side blooms and the second application to follow 3-21 days later (or when the rest of the canopy is in bloom).
For Perlan, apply a single application of 1.2-2.3 L/ha. If a prolonged bloom occurs, apply 1.2 L/ha first and reapply 5-7 days later.
High relative humidity and slow drying conditions favour maximum absorption. It is preferable to apply in the morning or evening.
Optimal temperatures for application are between 24-32°C.
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.
Improving Fruit Size
Fruit size can be improved by thinning apples to a desirable crop load through chemical fruitlet thinning and/or hand thinning. The biggest factor for improving fruit size would be early thinning
MaxCel or Cilis Plus (1.9% or 2.0%, respectively, 6-benzyladenine(6-BA)) improves fruit size by increasing cell division shortly after flowering in the early stages of fruit growth and development.
Application:
Make 2-4 applications at 10-50 ppm, beginning at petal fall and repeating every 3-10 days. Apply in a spray volume to get complete coverage of leaves and fruit (ie. 1000 L/ha)
Fruit thinning may occur in some easy-to-thin cultivars and/or if conditions are favourable for thinning.
Best results are obtained when maximum temperatures are above 18°C on the day of application and the 2-3 days after application. Avoid spraying when temperatures are above 30°C
Do not exceed 446 g of 6-BA (22.5 L of MaxCel, 21.3 L of Cilis Plus) per ha for all uses of 6-BA in a growing season.
Do not apply within 86 days of harvest with MaxCel and within 28 days of harvest with Cilis Plus.
Reduce russeting in apples
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.
Application:
Apply 250-500 mL/ha of Promalin at 7-12 day intervals for a maximum of 4 applications beginning at full bloom to petal fall.
During conditions that favour russet development (long cold and wet periods during bloom) apply Promalin in shorter intervals and at the higher rate.
Apply during periods of slow drying conditions to maximize efficacy.
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.
Plant growth regulator use in pears
Modifying tree growth
Application:
For nursery stock and non-bearing pears, foliar applications should be made after the trees have reached a terminal height at which lateral branching is desired (e.g. 75 cm from the ground).
For non-bearing orchard trees, foliar applications are made when new terminal growth is approximately 2.5-8 cm long (approximately king bloom to 1 week after petal fall).
Apply at a rate of 125-500 mL Promalin per 10 L of spray solution (250-1000 ppm) for non-bearing nursery and orchard trees.
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).
Plant growth regulator use in berries
Runner suppression in strawberries
Strawberry plants treated with Apogee or Kudos 27.5 WDG (Prohexadione-calcium) have significantly shorter runners than untreated plants. Apogee or Kudos 27.5 WDG arerecommended when runners are no longer needed to increase plant density.
Application:
Apply prior to the beginning of runner initiation.
Make subsequent applications at 14-21-day intervals, up to a maximum of three applications per season, if required.
In day-neutral production systems, plant as early as possible in spring, and apply Apogee or Kudos 27.5 WDG after the first flower trusses are removed but before runners develop.
In June-bearing varieties grown on plastic mulch, use Apogeeor Kudos 27.5 WDG after planting in the year before harvest.
Avoid use of Apogee or Kudos 27.5 WDG when buds or blooms are present. Apogee can reduce flower truss length and make harvest more difficult.
Do not apply later than 21 days prior to harvest.
The effects of Apogee or Kudos 27.5 WDG last for two to three weeks after application. Afterward, the plant produces gibberellin normally and typical growth continues.
Use 45 g/100 L water (Do not exceed 135 g/ha of Apogee or Kudos 27.5 WDG ).
Use adequate water to insure thorough coverage.
Always tank-mix Apogee or Kudos 27.5 WDG with Agral 90 at a rate of 50 mL/100 L of water. Agral 90 improves uptake of Apogee or Kudos 27.5 WDG into the plant.
Precaution: Apogee or Kudos 27.5 WDG are relatively new tools for Ontario growers. Test Apogee or Kudos 27.5 WDG on a small scale and leave untreated checks in order to evaluate the effectiveness on different varieties and production systems.
Always tank-mix Apogee or Kudos 27.5 WDG with an equal amount of ammonium sulphate by weight. Ammonium sulphate (AMS) can improve efficacy as Apogee or Kudos 27.5 WDG are 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 or Kudos 27.5 WDG effectiveness.
Plant growth regulator use in sweet cherry
Fruit quality
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.
Application:
Dissolve 20 Falgrotablets in 1,000 L of water to produce a 20 ppm solution and apply as a foliar spray to runoff.
Apply 21 days before normal harvest when the fruit is at the straw-coloured stage.
Harvest when fruits are at the desired shade of red but not within 21 days of application.
Vegetative growth control
Apogee or Kudos 27.5 WDG (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 or Kudos 27.5 WDG at the proper timing and concentration.
Application:
Terminal shoot growth in Ontario proceeds rapidly usually during the first 6 weeks of the season.
Since 2 weeks are required for Apogee or Kudos 27.5 WDG to slow growth effectively, it is essential to make the first application when terminal shoots are no longer than 2.5-5 cm. This typically coincides with late bloom, when sufficient leaf area has developed for Apogee or Kudos 27.5 WDG to be translocated absorbed into the leaf.
Apply 45 g/100 L of dilute spray (which equates to 125 ppm or 125 mg/L of active ingredient) and just 'spray' dilute (up to 3000 L/ha). A repeat application must be made 14-21 days later.
Sprays are limited to a maximum of 2 sprays per season and a total of 2.7 kg of formulated product per ha per season.
Based on the 125 ppm rate and a tree-row volume dilute rate of 2,000 L/ha, 1.8 kg of product per hectare (728 g/acre) will be required.
Calcium sprays should not be tank-mixed with Apogee or Kudos 27.5 WDG .
Precautions: A spray adjuvant (Agral 90, LI-700) should be included to improve plant uptake of Apogee or Kudos 27.5 WDG . 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 or Kudos 27.5 WDG .
Potential adverse effects of plant growth regulator applications
Potential adverse effects of PGR applications include leaf yellowing, plant stunting, delayed flowering and petal bleach, all of which will negatively impact final plant quality.
Phytotoxicity
Phytotoxic effects have been noted on some plant species following application of PGRs, including foliage discolouration, leaf crinkling, growing point distortion and stunting. Often symptoms can be related to incorrect rates or water volumes, or poor application technique.
In terms of individual active ingredients, chlormequat can give rise to leaf edge yellowing, leaf spotting and crinkling when applied at high rates. Chlorosis can be minimised through the use of an approved non-ionic wetter. Daminozide phytotoxicity is not seen often, but symptoms include flower stem stunting and a delay in crop development. Phytotoxic effects resulting from paclobutrazol include leaf necrosis and crinkling, hardening of the growing point and stunting, particularly if applied to sensitive plants, such as Begonia. Stunting can be permanent, particularly where paclobutrazol is applied to young plants too soon.
Flowering
Delayed flowering was noted in some species following daminozide and paclobutrazol application in AHDB project HNS 103b. Late application of PGRs can also result in reduced flower size and reduced flower stem length, so that the flowers are no longer held clearly above the leaf canopy.
Petal bleach
Petal bleach has been observed in Begonia, Dahlia, Lavandula and Verbena following application of Moddus or Regalis. The effect is due to their impact on anthocyanin formation. Care should be taken when applying these products, earlier applications may limit the bleaching effect.
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