Foliar-applied surfactants and urea temporarily reduced carbon assimilation of grapefruit leaves

Although urea can be an effective adjuvant to foliar sprays, we examined ef fects of additional surfactants on urea penetration through leaf cuticles a long with the effect of urea with and without surfactants on net gas exchan ge of leaves of 'Marsh' grapefruit (Citrus paradisi Macf.) trees budded to Carrizo citrange (C. sinensis IL. Osbeck x Poncirus trifoliata L.. Raf,) ro otstock. various combinations of urea, a nonionic surfactant (X-77), and an organosilicone surfactant (L-77), were applied to grapefruit leaves and al so to isolated adaxial cuticles. When compared to X-77, L-77 exhibited supe rior surfactant features with smaller contact angles of droplets deposited on a teflon slide. Both L-77 and X-77 initially increased penetration rate of urea through cuticles, but the effect of X-77 was sustained for a longer period of time. The total amount of urea which penetrated within a 4-day p eriod, however, was similar after addition of either surfactant, Solutions of either urea, urea + L-77, urea + X-77, or L-77 alone decreased net assim ilation of CO2 (A(CO2)) for 4 to 24 hours after spraying onto grapefruit le aves, A solution of X-77 alone had no effect on A(CO2) over the 4-day perio d. Although reductions in A,,, were similar following sprays of urea formul ated with two different surfactants, the underlying mechanisms may not have been the same. For the urea + X-77 treatment, X-77 increased the inhibitor y effects of urea on A(CO2) indirectly by increasing penetration of urea in to Leaves. For the urea + L-77 formulation, effects of L-77 on A(CO2) were 2-fold, direct by inhibiting A(CO2) and indirect by increasing urea penetra tion, One hour after application, scanning electron microscopy (SEM) of lea f surfaces treated with X-77 revealed that they were heavily coated with th e residue of the surfactant, whereas leaves treated with L-77 looked simila r to nontreated leaves with no apparent residues on their surfaces. The amo unt of X-77 residue on the leaves was lower 24 hours after application than after 1 hour as observed by SEM.