ESCA STUDIES OF CUTICULAR PASSIVATION IN LEAF SENESCENCE OF THE SILVER MAPLE (ACER NEGUNDO)

High-resolution X-ray photoelectron spectroscopy (XPS), also known as ESCA (for Electron Spectroscopy for Chemical Analysis), has been used to follow the chemical evolution of the surface of the leaves of the s ilver maple (deer negundo) at various stages of senescence. The elemen tal C(ls), O(ls), N(ls), O(2s) and Si(2p) core level peaks were monito red as were loss lines and valence band spectra, providing information on the type, quantity and chemical status of these elements on the ou termost surface of the leaf. The thickness and integrity of the layer of cuticle wax found on all leaf surfaces vary substantially before th e leaves detach, with regions of the surface also suffering encroachme nts by amino acids from the extracellular part of the epidermis and ox idation induced inward by air and water. Enhanced epidermal exposure a nd oxidation characterize the last stages in the ''life struggle'' of the dying leaf. Detailed analysis of the O(ls) spectra reveals that ai r-induced oxidation results in passivation of the outer surface of lea ves. The presence of excess cuticle material accompanies the post-deta chment degradation of the leaves which continues inexorably with time off the tree. Cuticle wax is more extensive at the lower surface of th e leaves where the gas and water-exchanging stomata are primarily loca ted. In brown (''dead'') leaves, the cuticle build-up ceases, suggesti ng that the surface of the leaf is a chemically evolving entity. Chemi cal changes continue during senescence, even after the leaf has been r emoved from the tree.