Plant Growth Regulators: Factors Affecting Plant Response
The effectiveness of a PGR application is determined by (1) how much of the active ingredient is absorbed by the plant and reaches the appropriate tissue or cells, and (2) how sensitive the plant is to the PGR. Environmental conditions at the time of application, formulation of the material, and method of application all affect plant absorption. Tree age, tree vigor, dosage, timing and cultivar all interact to affect plant sensitivity. By understanding the role of each of these variables, you will be better equipped to adjust PGR applications, within product label allowances, to compensate for year-to-year and block-to-block variation.
Weather conditions before, during, and after applications affect response to PGRs. Warm temperatures, slow drying conditions, and healthy foliage will enhance absorption and increase plant response. Cool temperatures, fast drying conditions, and damaged trees or foliage will decrease plant response. Because of low humidity and high temperatures typical of Intermountain West weather, evening or night-time applications are typically best, to allow for proper drying time and avoid volatilization and photo-degradation of PGRs.
Tree Vigor and Age
Weak trees and young trees are more responsive to PGRs. Stresses caused by lack of water, low nitrogen, or plant or leaf injury also increase sensitivity. Doses should be decreased or application eliminated for trees that are stressed. Although damaged foliage can reduce absorption, weak trees can still be oversensitive to PGR applications.
Compared to crop protectants (insecticides and fungicides) PGRs have a relatively narrow acceptable dose range, where overdose can result in negative side effects. Conversely, when the dose is too low, none of the desired response will be achieved. Therefore, PGRs are applied in very precise and low concentrations. Extreme care must be taken to mix and apply these chemicals accurately to avoid incorrect dosage. PGRs should be applied as dilute high volume sprays (200 + gallons per acre) to ensure uniform coverage and because applying the correct dose is more difficult in low-volume sprays. Growers that only have access to low-volume sprays should use the highest volume possible.
Spray Tank Considerations
Surfactants are a class of spray tank additives that affect the surface tension properties of the spray solution, which increases leaf wetting in order to increase absorption. Read the PGR product label carefully before including a surfactant. Many of the commercial PGRs come formulated with surfactants to provide the proper solution characteristics for leaf wetting. Adding more surfactant to the spray tank solution may not appreciably improve PGR delivery, and may actually decrease retention of the spray solution on the surface of the plant.
Many of the PGRs are sensitive to spray solution pH, where the stability and activity of the PGR molecules are best in slightly acidic solution. With the alkaline water typical of orchards in the Intermountain West, a spray tank buffer will improve both the stability and uptake of the PGR. PGRs should not be tank mixed with pesticides.
PGRs can cause different effects when applied at different times during the season. For a predictable response to occur, PGRs must be applied in a relatively narrow time period, usually within a few days.
Different cultivars display varying degrees of responsiveness to PGR application. This is especially true for chemical thinning.
Evaluating and Monitoring Plant Response
To evaluate the effectiveness of PGR treatments, leave some trees untreated for comparison. Keeping detailed accurate records of application rates, weather, and plant response will help in making adjustments during future years to achieve the optimal response.
Effect of Sunlight on Thinning Agents
While the precise mode of action of PGR thinning agents is not clearly understood, evidence is accumulating that the carbohydrate status of the tree plays a key role in plant sensitivity. Carbohydrate status is affected by light levels (more light = more photosynthesis = improved carbohydrate levels) and by overall tree health. This effect occurs both at the whole-tree and the individual-branch level. Overcast conditions and internal shading both make the fruit more sensitive to PGR thinning applications. Likewise, well-exposed branches under full sunlight conditions tend to be more difficult to thin.