Impact of Potential Organic Pesticides and Potential Fruit Crop Load Regulators on Photosynthesis and Growth of Apple

2004 Annual Report for GS04-033

Project Type: Graduate Student
Funds awarded in 2004: $10,000.00
Projected End Date: 12/31/2005
Grant Recipient: University of Arkansas
Region: Southern
State: Arkansas
Graduate Student:
Major Professor:
Dr. Curt Rom
University of Arkansas

Impact of Potential Organic Pesticides and Potential Fruit Crop Load Regulators on Photosynthesis and Growth of Apple


Alternative crop load regulators, also known as fruit thinners, and pesticides are needed for both certified organic and conventional fruit production. Prior to creating new management technologies, it is necessary to have an understanding of the physiological basis and impacts of the management treatments. Gas exchange and growth by plants are sensitive indicators of plant response to management or environment. For example, a transient reduction in photosynthesis (Pn) has proven to be an effective technique used for fruit thinning. Conversely, pesticides, which reduce Pn, may be detrimental to plant growth.

A three-year study was developed to measure plant response to foliar applications of various acids as potential organic pesticides or growth regulating treatments. Treatments were applied to vegetative apple trees under controlled environmental conditions to study effects on gas exchange and growth.

Gas exchange measurements included photosynthetic assimilation (A), evapotranspiration (Et), and stomatal conductance (gs). Growth measurements included stem diameter, stem length, stem weight, average leaf area, average leaf weight, and specific leaf weight. This model system for screening new compounds will establish a base for studying additional compounds that may have the potential to be effective organically certifiable pesticides or fruit thinning agents.

Five studies were conducted including treatments of fatty oils, sulfur compounds, organic acids, miscellaneous compounds, and essential oils.
-In Study I, fatty oils did not affect A and Et. Crocker’s fish oil significantly reduced gs eight days after treatment. No significant growth effect was observed.
-In Study II, sulfur compounds reduced A, Et, and gs. Potassium sulfate reduced A one day after treatment. Ferric sulfate increased A seven days after treatment. Copper sulfate, ammonium sulfate, and potassium sulfate significantly reduced Et seven days after treatment. Copper sulfate reduced gs one day after treatment, while potassium sulfate and ammonium sulfate reduced gs 23 days after treatment. No significance was observed for plant growth in this study.
-In Study III, organic acids resulted in a significant decrease in A for citric acid treatments three days after treatment. A trend for reduced Et was observed for all treatments one day after application; however, not significant. Salicylic acid significantly reduced stomatal conductance one and 15 days after treatment. Salicylic, citric, and acetic acids significantly reduced stem diameter.
-In Study IV, miscellaneous compounds reduced A, Et, and gs. Coumarin, chitosan, catechin, and thymol significantly reduced A one day after treatment. Catechin significantly reduced A five days after treatment. Thymol and chitosan reduced Et one day after treatment. Catechin reduced Et 12 days after treatment. Coumarin significantly increased gs one day after treatment. Plant growth was reduced for miscellaneous compounds. Thymol reduced stem height. Chitosan and thymol reduced average leaf area. Chitosan, thymol, and catechin reduced average leaf weight.
-In Study V, essential oil treatments showed no significant effect on Pn for treatments; however, clove oil was very phytotoxic and defoliated all trees in this study. Cedarwood oil significantly decreased Et and stomotal conductance one day after treatment. Differences in plant growth were not significantly different among essential treatments excluding clove oil.

Objectives/Performance Targets

  1. To evaluate 2% concentrations of potential alternative pesticides and growth regulating chemicals and the effects on gas exchange (photosynthetic assimilation, evapotranspiration, and stomatal conductance) of vegetative model apple trees grown in the greenhouse. To evaluate plant growth response following foliar applications of 2% concentrations of potential alternative pesticides and growth regulating chemicals.


Five studies were conducted. In these five studies, 21 chemicals were screened at 2% concentrations to determine the impact on gas exchange and growth. Out of these 21 compounds, 11 had significant effects on plant gas exchange. Six compounds will be chosen for further study of concentration effects and potential use as pesticides or plant growth regulators. These studies have established a basis for additional future work.

Impacts and Contributions/Outcomes

The next focus of this project will screen the best compounds from the 2004 studies. The same methodology will be used to screen individual compounds for the least and most effective concentration for potential use. Additional observations for phytotoxicity or burning of plant tissue will be necessary to establish the impact of various concentration effects on plant tissue. The overall goal is to provide growers additional alternatives for organically approved pesticides or crop growth regulators for tree fruit thinning. A subset of compounds will be used in a preliminary field-screening test.


Jason D. McAfee
University of Arkansas
316 Plant Science
Fayetteville, AR 72701
Office Phone: 4795757069