This study investigated the mechanisms which control the partitioning between roots and shoots in plants subjected to changes in environment. Two types of analyses were used: firstly, an examination of the cost and revenue associated with investment in different plant parts, and secondly, a test of the principle of functional equilibrium between roots and shoots, i.e. whether root dry matter × root specific activity balances shoot dry matter × shoot specific activity. Measurements were made on individual plants of Lolium perenne in sunlit controlled environments, grown from germination to canopy closure under optimal nitrogen supply. At the final harvest, increased air temperature (+4 °C above ambient) reduced whole-plant dry matter by 12% relative to the control, whereas elevated CO2 mole fraction (700 μmol mol−1) led to a 38% gain. The combined treatment yielded an intermediate result (+19%). Plants grown at +4 °C maintained balanced activity between roots and shoots throughout the experimental period, irrespective of CO2 concentration. This required enhanced allocation to roots in young plants to compensate for a strong negative effect of higher temperature on root specific activity, which suggests that plants conserve balanced activity by adjusting dry matter partitioning. The extra cost involved with the adjustment at +4 °C significantly enhanced the cost[ratio ]revenue ratio of plant investment. In ambient temperature, the balance between roots and shoots departed from equilibrium, slightly at ambient but substantially at elevated CO2: the plants accumulated excess carbon relative to nitrogen, and this imbalance increased with plant age. At elevated CO2, the cost[ratio ]revenue ratio increased in young plants but this was later reversed owing to loss of root specific activity, which explains the gradually declining CO2 stimulation with time. The strategies in equilibrating root and shoot functioning observed in the different treatments are discussed in the light of whole plant performance.