The increasing atmospheric CO2 concentration resulting as a consequence of economic development generally leads to increased plant biomass production. However, little attention has been paid to the effects of combined factors, such as CO2, temperature or water availability, on plant regrowth after cutting or grazing, which represent the usual methods of managing forage legumes like alfalfa. It has been demonstrated that nitrogen pools in alfalfa taproot, especially vegetative storage proteins (VSP), condition new regrowing shoots. The aim of our study was to determine the effect Of CO2 (ambient, around 350 mu mol mol(-1) versus 700 mu mol mol(-1)), temperature (ambient versus ambient + 4 degrees C) and water availability (well-irrigated versus partially irrigated) on taproot N accumulation, especially VSP, in nodulated alfalfa before defoliation and after I month of regrowth. At the end of vegetative normal growth, elevated CO2 enhanced dry matter production only in plants grown under high temperature and irrigated at field capacity. The taproot VSP content was increased by drought during this period, and this increase may explain the reduction in production differences between well-watered and drought plants at the end of cutting/regrowth cycle. After I month of regrowth, drought increased the VSP level again and it could be expected a greater forage production in drought treatments under equal regrowth conditions during the next cutting-regrowth cycle. (c) 2007 Elsevier Ireland Ltd. All rights reserved.