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“Introduction Progress in photosynthesis research has been driven to a large extent by the development of new measuring techniques and methodology. Outstanding examples are Pierre Joliot’s pioneering developments in amperometric techniques for oxygen detection (Joliot 1956, 1968) and in absorption spectrophotometry (Joliot et al. 1980, 2004), which have led to numerous important discoveries and have been stimulating generations of photosynthesis researchers. Our present contribution describes a new instrument for continuous measurements of the electrochromic absorbance shift in vivo, i.e., a topic that has been close to the heart of Pierre Joliot for at least 40 years. We dedicate this paper to him and to Govindjee on the occasion of their

80th birthdays. During the past 50 years the major mechanisms involved in the complex process of photosynthesis have been elucidated by basic research using isolated chloroplasts

or membrane fragments (with substantial contributions MI-503 cell line by both Pierre Joliot and Govindjee). Some important open questions have remained, in particular regarding the regulation of the highly complex in vivo process in response to environmental factors, which limit the rate of CO2-assimilation and consequently plant growth. Obtaining reliable information on the intact system, as close as possible in its natural state, is complicated not only by the much higher degree of complexity, but also by various aggravating factors affecting the quality of optical probes. diglyceride While measurements of the overall rate of CO2-uptake or O2-evolution in intact leaves are relatively simple and straightforward, specific absorbance changes due to various electron transfer steps are covered by much larger broadband absorbance changes due to electrochromic pigment absorbance shifts and light scattering changes. Furthermore, leaf transmittance in the visible spectral region is low due to high Chl content and the strongly increased path length of measuring light (ML) by multiple scattering. Another complicating factor is the need to keep the time-integrated intensity of the ML to a minimum, so that its actinic effect does not change the state of the sample. Therefore, in vivo optical spectroscopy in the visible range is a challenging task.