Effects of air temperature, relative humidity (RH), and leaf age on penetration of urea through isolated leaf cuticles of `Marsh' grapefruit (Citrus×paradisi Macfad.) trees on `Carrizo' citrange (C. sinensis L. Osbeck × Poncirus trifoliata (L.) Raf. rootstock were examined. Intact cuticles were obtained from adaxial surfaces of `Marsh' grapefruit leaves of various ages. A finite dose diffusion system was used to follow movement of 14C-labeled urea from urea solution droplets across cuticles throughout a 4-day period. Within the first 4 to 6 hours after urea application, the rate of urea penetration increased as temperature increased from 19 to 28 °C, but there was no further increase at 38 °C. Increasing relative humidity increased urea penetration at 28 °C and 38 °C. Cuticle thickness, cuticle weight per area, and the contact angle of urea solution droplets increased as leaves aged. Cuticular permeability to urea decreased as leaf age increased from 3 to 7 weeks, but permeability increased in cuticles from leaves older than 9 weeks. Contact angles decreased with increased urea solution concentration on leaf surfaces that were 6 to 7 weeks old, but solution concentration had no effect on contact angle on cuticles from younger and older leaves. Changing urea solution pH from 8.0 to 4.0 could have an effect on the amount of urea penetrating the cuticle through the loss of urea from breakdown possibly due to hydrolysis. Results from this study define leaf age, environmental conditions, and formulation for maximum uptake of foliar-applied urea.
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