Relation of Oil Type, Deposit, and Soaking to Effects of Spray Oils on Photosynthesis in Citrus Leaves1

Abstract

This investigation concerns the effects of varying hydrocarbon composition and molecular size of spray oils on inhibition of photosynthesis in citrus leaves and their recovery with time after application. Effects were measured by determining regressions of inhibition of photosynthesis on amount of oil applied. Preselected known deposits of oil were applied to potted, single-leaf, rooted stem cuttings by means of a precision vertical spraying tower. Photosynthesis was measured by the Warburg procedure using a modified apparatus equipped with lights and temperature control bycmeans of either refrigeration or heat. No sonsistent inhibition of photosynthesis of Eureka lemon or Bearss lime leaves occurred with California light-medium or medium-grade spray oils at the deposit level of 150 µg. oil/sq. cm. of leaf surface, an amount sufficient for the control of California reel scale and citrus red mite. Since the plants in the laboratory experiments were kept in the glasshouse they were not exposed to the ultra-violet components of sunlight. Data taken with leaves from trees sprayed in the field support the conclusion that ultra-violet light is not an important factor in the effect of spray oil on the photosynthesis of citrus leaves following application of amounts of oil necessary to control citrus pests. A definite relation between inhibition of photosynthesis and increasing oil deposit was found in the deposit range of 300 to 600 µg. oil/sq. cm. of leaf surface; however, the data showed that the response to small increments of oil would be slight. Recovery of photosynthesis occurred in less time in plants treated with napthenic oils than in those treated with paraffinic oils. The principal effect on photosynthesis occurs in the tissue of the leaf marked by the dark discoloration known as oil soaking. During the first week after application, photosynthesis was inhibited 50 to 60% in the oil-soaked tissue. Tests with the tetrazolium reaction showed that the cells of the discolored tissue arc not killed. Inhibition of photosynthesis seems to be the result of interference with gaseous exchange caused by the presence of the spray oil. Dissipation of the oil is accompanied by recovery of photosynthesis with a log10relation to time.