41:	Urea penetration through the isolated cuticles of citrus leaves.

Authors:	Orbovic, Vladimir(A)Petracek, Peter(B)Syvertsen, James, P.(A)
Affiliations:	(A): Univ. of Florida-Citrus Res. & Edu. Center (B): Florida Department of Citrus
Presenter:	Orbovic, Vladimir , orbovic@lal.ufl.edu

Foliar application of urea N to citrus leaves increases leaf N and can enhance flowering. Since penetration through the thick, waxy cuticle of citrus leaves can be problematic, we wanted to define conditions which maximize the safe uptake of urea. Citrus leaf cuticles were enzymatically isolated from the leaves, dried, and used to examine the penetration of radioactively labeled urea. When droplets of urea were applied onto the cuticles, penetration, evaporation, and volatilisation occured together. Penetration and evaporation rate determine concentration gradients and thus, how much urea penetrates through cuticles. At 28oC, about 40% of amount of applied urea penetrated within first 6 hours and after 4 days, a total of 55% had penetrated the cuticles of 3 months-old (mature) leaves. Similar trends were recorded for urea penetration through the cuticles of 1 month-old (immature) leaves except that the total amount penetrated was about 40% after 4 days. On higher temperature (38oC), only about 40% and 20% of applied urea penetrated through the cuticles of mature and immature leaves, respectively, during the 4 day period. The kinetics of urea penetration at 38oC resemble those at 28oC. The difference between penetration through mature and immature cuticles was probably due to evaporation losses. Since the evaporation rate from surfaces of immature leaves was higher than that from mature leaves, the resulting penetration through the cuticles of immature leaves is lower. The effect of humidity on evaporative losses and urea penetration will be discussed in detail.