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Summary The guard cell chloroplast is the site of perception of blue light and of photosynthetically active radiation, and of at least one of the mechanisms sensing CO2 in the guard cell. The guard cell chloroplast has been the focus of intense controversy over its capacity for light sensing and photosynthetic carbon fixation, and the osmoregulatory mechanisms mediating stomatal movements. It is argued here that a primary reason behind these long-lived controversies is the remarkable plasticity of the guard cell, which has resulted in responses being generalized as basic properties when opposite responses appear to be the norm under different environmental or experimental conditions. Examples of guard cell plasticity are described, including variation of chlorophyll fluorescence transients over a daily course, acclimation of the guard cell responses to blue light and CO2 , the shift from potassium to sucrose in daily courses of osmoregulation and the transduction of red light into different osmoregulatory pathways. Recent findings on the properties of the guard cell chloroplast are also presented, including the role of the chloroplastic carotenoid, zeaxanthin, in blue light photoreception, the blue-green reversibility of stomatal movements, and the involvement of phytochrome in the stomatal response to light in the orchid, Paphiopedilum . [ABSTRACT FROM AUTHOR]