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.