Leaf surface ultrastructure and susceptibility to ammonium nitrate injury

Abstract

Ammonium nitrate solution (20% N) can burn the foliage of many broadleaf weeds without significant injury to cole (broccoli, Brussels sprouts, cabbage, and cauliflower) and onion crops. Scanning electron microscopy showed that a heavy crystalline epicuticular wax layer composed of tubules, dendrites, and (or) platelets was present on the adaxial leaf surface of ammonium nitrate tolerant species. Susceptible species had little or no wax on their leaf surface. Removal of epicuticular wax by cellulose acetate stripping from tolerant cabbage leaves increased ammonium nitrate retention and salt injury. We conclude that epicuticular wax protects the foliage of tolerant species by repelling the salt solution. The solution, however, did not wet the stripped leaves evenly and leaf kill was incomplete, indicating that amorphous cuticular wax offers some protection to these leaves. The foliage of susceptible species, which lacked epicuticular wax, was damaged because it retained the solution. The presence of surface structures such as trichomes on common groundsel (Senecio vulgaris L.), lady's thumb (Polygonum persicaria L.), and shepherd's purse (Capsella bursa-pastoris (L.) Medic.) may further increase solution retention and salt injury. Key words: epicuticular wax, cole crops, onion, weed, leaf surface, trichome.