Leaf scorch on Pennisetum clandestinum dryland pastures induced by liquid urea ammonium nitrate fertilizer

Abstract

Transmission electron (TEM) and scanning electron microscopy (SEM) demonstrated that the application of liquid urea ammonium nitrate (UAN 32) fertilizer, in concentrated form, to dryland pastures resulted in leaf scorch and subsequent necrosis, irrespective of the level and concentration of the UAN solution applied. One week after UAN application, most treatments, except those which involved high application rates and high concentrations of UAN, snowed full cuticular and cellular recovery. Furthermore, chloroplasts and granal stacks recovered after such treatment. The addition of a urease inhibitor, ammonium thiosulphate (ATS 12), to the liquid fertilizer did not lower leaf cell damage, but may have aided the recovery process. The higher the application rate and the higher the concentration of the UAN, the greater the extent of damage to the outer waxy cuticle, guard cells and epidermal cells, and to the internal chloroplasts, grana and mesophyll cells. Guard and epidermal cells became flaccid and necrotic, while the cuticle readily peeled away from the leaf surface. Leaf browning was ascribed to the damage caused to the grana within the chloroplasts. Of the combinations tested, the least damaging to the leaf was the application of 69 kg N ha⁻¹ at a concentration of 8% N, combined with a 5% volume/volume (v/v) ATS.