Anatomical and ultrastructural changes associated with ethylene-induced defoliation of guayule explants

Explants of guayule,Parthenium argentatum Gray, were treated with concentrations of ethephon varying from 0.5 to 10gl^-1 for six days. The cut ends of the shoots were immersed in the solutions and allowed to stand so that the ethylene-releasing agent entered the explant via the transpiration stream. With higher concentrations of ethephon, defoliation of the explants commenced after one day and was 100% effective after six days. Lower concentrations were less effective. Examination of the petiole bases of treated explants at the light microscope level revealed enhanced development of abscission layers and hydrolytic degradation of the tissue immediately distal to these layers. This led to separation of the leaf which had become senescent. Food reserves appeared to have been mobilised from the senescent leaves. Histochemical staining and ultrastructural observations indicated loss of insoluble polysaccharides and cellulose from the induced separation layer. Pectic substances were lost to a lesser extent.The financial support of Cooperative Scientific Programmes of the C.S.I.R. and technical assistance of the Electron Microscope Unit of the University of Natal, Pietermaritzburg is gratefully acknowledged.