Pollenkitt development and composition in Tilia platyphyllos (Tiliaceae) analysed by conventional and energy filtering TEM

Abstract

Advanced TEM techniques (improved chemical fixation protocols and new instrumental TEM features) can give new insight into old problems. Pollenkitt, the main cause of pollen stickiness in entomophilous angiosperms, was shown especially by thin-layer chromatography to be an inhomogeneous mixture of many, mostly neutral lipids. The production of pollenkitt in the tapetum starts after meiosis in the microspore stage. In Tilia platyphyllos besides plastids other organelles contribute substantially to pollenkitt components; this was proved by the employment of adequate fixation techniques, especially osmium ferrocyanide (''Os-FeCN'') used as a postfixative. SER-connected lipid droplets represent one type of pollenkitt precursors, and elaioplasts produce the second lipid/carotenoid pollenkitt precursor component. Both components are composed of inhomogeneous droplets with differing electron density. Mature pollenkitt adhering to the pollen is depicted by Conventional TEM (CTEM) mostly as a widely homogeneous, rather compact substance. Differing electron densities indicating variations in the chemical composition within pollenkitt droplets can only sometimes be visualized at CTEM level. An Energy Filtering Transmission Electron Microscope (EFTEM) improves by enhanced grey level discrimination the specific contrasts of lipid pollenkitt droplets. If comparing the sample in the zero loss imaging with images taken by contrast tuning this technique allows the finding of variations in electron density originating from variable chemical composition; so image analysis of lipid/carotenoid droplets is permitted. Thus at the CTEM level it is confirmed that also seemingly homogeneous droplets are structurally indeed inhomogeneous.