Rising atmospheric carbon dioxide (CO2) concentration and air temperature are of major concern when considering the possible effects of global climate change on vegetations. Although production has been found to increase in many cases, other experiments have also indicated increased hazards for plant growth because of the increased frequency of weather extremes, such as droughts, floods, and extreme temperatures. Thus at the same time elevated CO2 and the extreme climatic events, intra- and interannual climatic variability alone can be foreseen as an indirect constraint, which separately influences significantly the carbon cycling in ecosystems, too. In the shorter term the effect of CO2 is direct and is mediated by photosynthesis. In the longer term the effects of elevated CO2 became more and more indirect and its effects are mediated by source-sink interactions within plants, resources (nutrients,water), temperature, microbes, herbivores, and land-use management practice. In fact, the plants can make use of their general stress coping mechanisms to avoid or compensate possible negative effects of elevated CO2. One has to consider that all the classical abiotic, biotic, and anthropogenic stressors are threatening plant growth and development also under elevated CO2, although at possibly different doses compared to ambient CO2 concentrations. Therefore, the knowledge of the general stress coping, stress avoiding, and tolerance mechanisms is needed to understand the regulation of the plants' metabolism under normal and elevated CO 2 levels.