Air temperature, humidity, and leaf age affect penetration of urea throughgrapefruit leaf cuticles

Effects of air temperature, relative humidity (RH), and leaf age on penetra tion of urea through isolated leaf cuticles of 'Marsh' grapefruit (Citrus x paradisi Macfad,) trees on 'Carrizo' citrange (C, sinensis L, Osbeck x Ponc irus trifoliata (L,) Raf, rootstock were examined. Intact cuticles were obt ained from adasial surfaces of 'Marsh' grapefruit leaves of various ages. A finite dose diffusion system was used to follow movement of C-14-labeled u rea 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 pene tration increased as temperature increased from 19 to 28 degreesC, but ther e was no further increase at 38 degreesC. Increasing relative humidity incr eased urea penetration at 28 degreesC and 38 degreesC, Cuticle thickness, c uticle weight per area, and the contact angle of urea solution droplets inc reased as leaves aged. Cuticular permeability to urea decreased as leaf age increased from 3 to 7 weeks, but permeability increased in cuticles from l eaves older than 9 weeks. Contact angles decreased with increased urea solu tion concentration on leaf surfaces that were 6 to 7 weeks old, but solutio n concentration had no effect on contact angle on cuticles from younger and older leaves, Changing urea solution pll from 8.0 to 4.0 could have an eff ect on the amount of urea penetrating the cuticle through the loss of urea from breakdown possibly due to hydrolysis. Results from this study define l eaf age, environmental conditions, and formulation for maximum uptake of fo liar-applied urea.