Effect of Petroleum Oil Spray on Photosynthesis and Respiration in Citrus Leaves

Various formulations of petroleum fractions in the light lubricating oil range provide efficient general insecticides for control of insect pests of citrus and have been used for this purpose for many years. The relatively low cost and high insecticidal efficiency of these materials make it probable that their widespread use in citrus culture will continue for some time. Many types of injury to citrus trees have been reported, however, as resulting from such sprays. The injurious effects of oil sprays on citrus trees have been summarized in reviews by EBELING (3) and RIEHL (12). Conclusive evidence has been presented to show that the application of oil sprays to citrus trees results in a reduction of the total percentage of soluble solids in the juice of citrus fruits. This decrease in the solids content of the juice was first noted by YOTHERS and McBRIDE in 1929 (16). SINCLAIR et al. (14), STOFBERG and ANDERSSEN (15), and BARTHOLOMEW et al. (2) have extended the results and have shown that, when fruits from oil-sprayed trees and from non-oilsprayed trees are compared, the use of oil spray in the usual commercial dosage causes a decrease of as much as 15% in the total soluble solids in the juice. The relatively large portion of the citrus crop marketed at present in the form of a frozen, concentrated juice having a uniform solids content makes this lowering of the solids in the juice, resulting from oil sprays, of more importance than formerly. Since the primary constituents of the soluble solids of citrus juice are sugars and citric acid, the reduction caused by oil sprays is probably due to an interference with the net production of photosynthate by the tree, or to a decrease in the translocation of elaborated foods from the leaves to the fruit, or to a combination of these factors. The present study was undertaken for the purpose of elucidating the effect of petroleum oils on photosynthesis and respiration of citrus leaves in the hope that knowledge concerning the mechanism of the toxic effect of oils would be useful in developing means of overcoming these effects in field practice. KNIGHT et al. (8) measured the effect of petroleum oils on the photosynthesis of citrus leaves by painting oil on both surfaces of the leaves of