THE PHYLLOPLANE METABOLITES - OCCURRENCE AND ROLE IN PLANT-INSECT RELATIONSHIPS

The phylloplane (leaf surface) is the interface between the plant and its environment. It is thus of great ecological interest. Our aim was to determine precisely the influence of its biochemical composition on insect host plant selection for oviposition. The plants considered in this study were maize, sunflower and leek. Plants were grown in hydro ponic cultures in green houses to different phenological stages. A rep roducible collecting method for water soluble substances such as solub le carbohydrates, free amino acids and organic acids was adapted. This improved technique was devised for microlocalization of monosaccharid es. A major difficulty was the very low quantities of measured chemica ls. These varied from 10(-5) to 10(-6) moles m-2 of leaf surface. Rati os of free amino acids were very constant and characteristic of each p lant species. The quantities of soluble carbohydrates found within the plant tissues bore a direct relationship to those found on the phyllo plane. The first results concerning the kinetic release of soluble com pounds upon water leaching revealed that this phenomenon was relativel y slow and varied according to the type of molecule. The monosaccharid es came out at the epidermal cell junction, above the apolastic zone a nd/or from deposits within craters beneath the cuticule. The insect fe males (Lepidoptera O. nubilalis Hbn.) were sensitive to detected level s of soluble carbohydrate on the phylloplane. Oviposition and host sel ection were linked to the carbohydrate status of the phylloplane and i n particular to the fructose level.