The suitability of root zone-CO 2 measurements (WR-CO 2) for irrigation scheduling was investigated and evaluated. The basic assumption was that plants reduce root respiration as a reaction to water deficit. A study with apple trees proved that measured WR-CO 2 originates in large parts (56-72 %) from plant specific CO 2 production (mainly root respiration). Furthermore , WR-CO 2 reacts to irrigation as well as correlates with soil water content. Since WR-CO 2 variations were also reflected in

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... hanges of plant water status, basic potential of WR-CO 2 for irrigation demand assessment of the investigated apple trees is apparent. The transferability to other crops will be investigated. Keywords root respiration, plant water status, soil water content Accurate irrigation planning is an important part of modern agriculture. In the practice, it is mostly based on soil moisture determination or climatic water balance calculations (Krüger et al. 1999, Allen et al. 1998). Soil moisture measurements give information about the amount of water stored in the soil (volumetric soil water content) and/or its plant availability (soil water potential). Climatic water balance calculations estimate the potential evapotranspiration of a cropped area based on weather data and the possible and required amount of irrigation necessary to fill the soil water storage. Both approaches detect the irrigation demand of plants indirectly based on weather or soil data, but not directly at the plant itself. In the framework of a research and development project, funded by the Federal Office for Agriculture and Food (BLE), root zone-CO 2 measurements and their suitability for irrigation scheduling in agriculture were investigated. The basic assumption was that plants reduce root respiration due to water limitation, which results in a WR-CO 2 decrease. Thus, irrigation water requirement should be directly measured at the plant by detecting a physiological response to water shortage. One advantage of this approach is the possible plant water status determination, enabling both stress avoiding irrigation and controlled deficit irrigation to improve fruit quality or water use efficiency. Another advantage is the non-destructive and continuous measurement, which bears all options for an automated irrigation.