Effect of a California Spray Oil on Transpiration of Citrus1

Abstract

The effect of transpiration in plants of several varieties of citrus, of a representative California foliage spray oil applied according to present-day practice for control of citrus pests in the field was investigated in laboratory experiments. Plants of Eureka lemon, Bearss lime, navel orange, and Valencia orange were grown in pots in the glasshouse; temperature control permitted a maximum of 92[degree]F. during the day and a minimum of 66[degree]F. during the night. Transpiration was measured by determining the loss in total weight of the potted plant during a period of 7 hours from morning to afternoon; the surface area of the foliage was measured and transpiration was expressed as milligrams of water transpired per square centimeter of leaf surface per hour. On the first day after the application of 1.75% spray oil in aqueous mixture, transpiration of the oil-sprayed plants was reduced to one-third that of untreated plants. Oil-sprayed plants recovered with time, and analyses showed that the correlation with the log 10 of time is highly significant. The available evidence indicates that reduction in transpiration in citrus foliage after the application of oil spray is due to physical interference by the spray oil on or in the leaf tissue and that recovery of transpiration occurs with dissipation of oil from the leaves. Transpiration was restored to original levels in the oil-sprayed plants in 3 to 5 weeks after the spray was applied.