Net CO2 Assimilation of Apple following Application of Soybean Oil

Abstract

Soybean oil can be used as an alternative pesticide for fruit trees. Two separate studies were conducted to determine the effects of oil concentration on leaf phytotoxicity and net CO2 assimilation (ACO2). In one study, concentrations of 0%, 2%, 4%, and 6% soybean oil in water were applied to individual shoots with a hand-held mist bottle. In the second study, 0%, 1.0%, and 1.5% were applied to whole trees with an airblast sprayer. Petroleum oil was applied as a separate treatment. Net CO2 assimilation was measured on single leaves. Oil residue was removed from the leaf with chloroform, dried, and weighed. Chlorosis and defoliation occurred with applications of 4% and 6% soybean oil. No visible phytotoxicity occurred with 2% or less oil. Net CO2 assimilation decreased as the rate of soybean oil increased from 0% to 4% oil, but there was no difference between 4% and 6%. Net CO2 assimilation decreased with increasing oil concentration from 0% to 1.5% and recovered to the rate of the control on day 7. Net CO2 assimilation was negatively related to oil residue. At an equivalent oil residue, there was no difference in ACO2 between petroleum and soybean oil. Below a residue of 0.15 mg·cm–2, foliar phytoxicity did not occur. Reductions in ACO2 were small and did not last longer than 7 days if residues were ≤0.10 mg·cm–2.