© Springer International Publishing Switzerland 2016. Longan trees are irrigated in Thailand as fruit growth takes place during the dry season. Due to the scarcity of water resources, ways for water saving irrigation are investigated. As deficit irrigation was found to have a high water saving potential, the focus was on the investigation of plant stress responses to drought, which can be used for optimizing the deficit irrigation regime. Five split-root potted longan trees in sand culture were subjected to partial rootzone drying (PRD), and during six months the CO 2 concentration in the rootzone was measured by rootzone probes and compared to a well-watered control. The CO 2 efflux from the rootzone was found to be well correlated to the moisture regime in the substrate. However, it was necessary to correct the measured values by the values from the control to obtain a significant correlation between CO 2 concentration and soil moisture. The main observed external factor influencing CO 2 was the ambient temperature. It was shown that the CO 2 efflux from the soil can be used as a non-destructive method for drought stress monitoring, but continuous measurement will be necessary to externalize disturbing environmental effects.