Description:
<jats:title>SUMMARY</jats:title><jats:p>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 <jats:italic>Lolium perenne</jats:italic> 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 CO., mole fraction (700 μmol mol <jats:sup>−1</jats:sup>) 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 CO<jats:sub>2</jats:sub> 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:revenue ratio of plant investment. In ambient temperature, the balance between roots and shoots departed from equilibrium, slightly at ambient but substantially at elevated CO<jats:sup>2</jats:sup>: the plants accumulated excess carbon relative to nitrogen, and this imbalance increased with plant age. At elevated CO<jats:sup>2</jats:sup>, the cost: revenue ratio increased in young plants but this was later reversed owing to loss of root specific activity, which explains the gradually declining CO<jats:sup>2</jats:sup>, 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.</jats:p>