Methods and techniques to measure molecular oxygen in plants

Designing and developing sensors for molecular oxygen (O2) has turned into a large, interdisciplinary field of research, with significant progress seen in the past decades. Until the early 1980s, the field of O2 sensing was dominated by polarographic electrode sensors, among which the most popular Clark-type electrode found wide application in plant science. Nevertheless, the great demand for more sophisticated, intracellularly applicable O2 sensors for real-time measurements in plants cannot be satisfied by the predominant techniques. Thus, optical sensors applying an O2-specific reduction of luminescent probes or dyes provide novel, promising tools and open new perspectives on the cellular or even subcellular level of O2 measurements. This chapter aims to give a comprehensive overview on the variety of methods and systems available in the field of O2 sensing with respect to application in plant tissue. Different types of the earlier polarographic electrode technique as well as emerging alternatives will be discussed, including fluorescent proteins as potential, genetically encoded intracellular O2 sensors. Due to the tremendous variety of materials and formats, the young field of optical O2 sensing will receive particular attention directing the focus towards the progress that has been made in developing new probes and dyes. Moreover, the current state of fluorescence measurements will be explored, particularly novel, plant-specific measurement modalities that mask plant autofluorescence. For the potential user, important practical aspects are also presented, revealing the limitations of the existing methods and further encouraging more interdisciplinary research in O2 sensing