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Background and aimsGiven that environmental factors like atmospheric CO2, temperature, and water availability will likely change simultaneously, it is difficult to make accurate predictions about future crop production. Effects of elevated CO2 or drought on aboveground plant growth are relatively well studied, while effects on the rhizosphere are rarely dealt with. The present work investigates the root exudation pattern of different osmotic protectants such as ions and sugars of two cultivars of barley grown under different water and CO2 levels.MethodsBarley (Hordeum vulgare L.) plants were grown in plant containers in climate chambers with an optimal and reduced water supply and two CO2 concentrations (380 and 550ppm) and harvested at the stem elongation growth stage and when the inflorescences emerged.ResultsThe responses of root exudation to CO2 enrichment, watering level, and cultivar and their interactions varied with the developmental stage. While K+/Na+ ratio in root exudates remained stable at the stem elongation stage (DC30), it was higher at the booting stage (DC49) in the cv. Bambina than in cv. Golden Promise. At DC30, in general the exudation of sugars, dissolved organic carbon (DOC) and their ratio (SUG/DOC) were lower at e[CO2] compared to ambient CO2 at normal watering conditions. Under reduced watering conditions, plants exuded on average 55% more sucrose than under well-watered conditions. The modern cv. BA exuded on average 53% more fructose than the older cv.GP. In contrast, at DC49, sucrose, DOC and SUG/DOC exudation were not affected by any factor. At e[CO2] plants exuded on average 39% less fructose than under ambient CO2. A water and CO2 interaction on glucose exudation was found at this stage. Under reduced water conditions, plants growing at ambient CO2 exuded less glucose but at e[CO2] exuded similar (GP) or even higher (BA) glucose levels than under normal water supply. Although drought and cultivar factors affected the root exudation of barley, the strongest effects in the exudation pattern were caused by CO2.ConclusionsThis study presents a non-destructive percolation method for the collection of root exudates which can be used to give an insight into the complex interaction between global warming-associated environmental factors that cause yield losses and changes in crop quality and components of the belowground plant metabolism.